The Structure and Life-history of the Cockroach - Studies in Comparative Anatomy

By Author	[ A B C D E F G H I J K L M N O P Q R S T U V W X Y Z \| Other Symbols ]
By Title	[ A B C D E F G H I J K L M N O P Q R S T U V W X Y Z \| Other Symbols ]
By Language

Download this book: [ ASCII | HTML | PDF ]

Look for this book on Amazon

We have new books nearly every day.
If you would like a news letter once a week or once a month fill out this form and we will give you a summary of the books for that week or month by email.

Title: The Structure and Life-history of the Cockroach - Studies in Comparative Anatomy—III
Author: Denny, Alfred, Miall, L. C.
Language: English
As this book started as an ASCII text book there are no pictures available.

*** Start of this LibraryBlog Digital Book "The Structure and Life-history of the Cockroach - Studies in Comparative Anatomy—III" ***

Transcriber’s notes:

Except for the spelling corrections listed below, the text of this
book has been preserved as in the original, including inconsistent
punctuation, hyphenation and accents.

Lepidotera → Lepidoptera
coccoon → cocoon
subtances → substances
Bütchsli → Bütschli

In this plain-text version, paired underscores denote _italicised
text_, paired asterisks denote *bold text*, a ^ (caret) indicates
^superscripted text and an underscore denotes _subscripted text.

Footnotes have been positioned below the relevant paragraphs and
illustration captions adjacent to the relevant text.

THE COCKROACH

An Introduction to the Study of Insects

STUDIES IN COMPARATIVE ANATOMY--III

THE STRUCTURE AND LIFE-HISTORY

THE COCKROACH

(_PERIPLANETA ORIENTALIS_)

An Introduction to the Study of Insects

L. C. MIALL

PROFESSOR OF BIOLOGY IN THE YORKSHIRE COLLEGE, LEEDS

AND

ALFRED DENNY

LECTURER ON BIOLOGY IN THE FIRTH COLLEGE, SHEFFIELD

LONDON: LOVELL REEVE & CO.
LEEDS: RICHARD JACKSON

1886

STUDIES IN COMPARATIVE ANATOMY.

I.--THE SKULL OF THE CROCODILE. A Manual for Students. By Professor
L. C. MIALL. 8vo, 2_s._ 6_d._

II.--THE ANATOMY OF THE INDIAN ELEPHANT. By Professor L. C. MIALL
and F. GREENWOOD. 8vo, 5_s._

III.--THE COCKROACH: An Introduction to the Study of Insects. By
Professor L. C. MIALL and A. DENNY. 8vo, 7_s._ 6_d._

IV.--MEGALICHTHYS; A Ganoid Fish of the Coal Measures. By Professor
L. C. MIALL (_In preparation_).

MAY BE HAD OF
LOVELL REEVE & CO., LONDON;
RICHARD JACKSON, LEEDS.

PREFACE.

That the thorough study of concrete animal types is a necessary
preliminary to good work in Zoology or Comparative Anatomy will now
be granted by all competent judges. At a time when these subjects,
though much lectured upon, were rarely taught, Döllinger, of Würzburg,
found out the right way. He took young students, often singly, and
made them master such animal types as came to hand, thereby teaching
them how to work for themselves, and fixing in their minds a nucleus
of real knowledge, around which more might crystallise. “What do you
want lectures for? Bring any animal and dissect it here,” said he to
Baer, then a young doctor longing to work at Comparative Anatomy.[1]
It was Döllinger who trained Purkinje, Pander, Baer, and Agassiz,
and such fame cannot be heightened by words of praise. In our own
time and country Döllinger’s methods have been practised by Professor
Huxley, whose descriptive guides, such as the Elementary Biology and
the delightful little book on the Crayfish, now make it easy for
every teacher to work on the same lines. From the description of the
Cockroach in Huxley’s Anatomy of Invertebrated Animals came the impulse
which has encouraged us to treat that type at length. It may easily
turn out that in adding some facts and a great many words to his
account, we have diluted what was valuable for its concentration. But
there are students--those, namely, who intend to give serious attention
to Entomology--who will find our explanations deficient rather than
excessive in detail. It is our belief and hope that naturalists will
some day recoil from their extravagant love of words and names, and
turn to structure, development, life-history, and other aspects of the
animal world which have points of contact with the life of man. We have
written for such as desire to study Insects on this side.

[1] Baer’s account of Döllinger is to be found in the Leben und
Schriften von K. E. von Baer, § 8.

Whoever attempts to tell all that is important about a very common
animal will feel his dependence upon other workers. Much of what is
here printed has been told before. The large number of new figures is,
however, some proof that we have worked for ourselves.

It is a pleasant duty to offer our thanks for friendly help received.
Professor Félix Plateau, of Ghent; Mr. Joseph Nusbaum, of Warsaw;
and Mr. S. H. Scudder, of Cambridge, Massachusetts, have very kindly
consented to treat here of those parts of the subject which they have
specially illustrated by their own labours.[2] Mr. E. T. Newton,
of the Jermyn Street Museum, has lent us the wood blocks used to
illustrate one of his papers on the Brain of the Cockroach. A number
of the figures have been very carefully and faithfully drawn for us
by Miss Beatrice Boyle, a student in the Yorkshire College. We are
much indebted to Dr. Murie, the Librarian of the Linnean Society, for
procuring us access to the extensive literature of Insect Anatomy, and
for answering not a few troublesome questions.

[2] Prof. Plateau’s chief communications will be found on pp. 131 and
159; Mr. Nusbaum has furnished the account of the Development of the
Cockroach, pp. 180 to 195; and Mr. Scudder the Geological History of
the Cockroach, chap. xi.

Five articles on the Cockroach were contributed by us to Science Gossip
in 1884, and some of the figures were then engraved and published.

In issuing a book which has been long in hand, but which can never hope
to be complete, we venture to adopt the words already used by Leydig
concerning his Lehrbuch der Histologie:--“Die eigentlich nie fertig
wird, die man aber für fertig erklären muss, wenn man nach Zeit und
Umständen das Möglichste gethan hat.”

CONTENTS.

CHAP. PAGE

I.--WRITINGS ON INSECT ANATOMY 1

II.--THE ZOOLOGICAL POSITION OF THE COCKROACH 9

III.--THE NATURAL HISTORY OF THE COCKROACH 17

IV.--THE OUTER SKELETON 28

V.--THE MUSCLES; THE FAT-BODY AND CŒLOM 71

VI.--THE NERVOUS SYSTEM AND SENSE ORGANS 86

VII.--THE ALIMENTARY CANAL AND ITS APPENDAGES 113

VIII.--THE ORGANS OF CIRCULATION AND RESPIRATION (including
a section on the Respiratory Movements of Insects,
by Prof. Félix Plateau, of Ghent) 133

IX.--REPRODUCTION 167

X.--DEVELOPMENT (including a section on the Embryonic
Development of the Cockroach, by Joseph Nusbaum,
of Warsaw) 181

XI.--THE COCKROACH OF THE PAST, by S. H. Scudder, of
the U.S. Geological Survey 205

APPENDIX:--

PARASITES OF THE COCKROACH.

SENSE OF SMELL IN INSECTS.

⁂ Where the species is not named, it is to be understood that the
figures are drawn from the Cockroach.

LEEDS:
MCCORQUODALE & CO. LIMITED,
BASINGHALL STREET.

STUDIES IN COMPARATIVE ANATOMY.--No. III.

THE COCKROACH.

CHAPTER I.

WRITINGS ON INSECT ANATOMY.

Marcello Malpighi. 1628–1694.
Jan Swammerdam. 1637–1680.
Pierre Lyonnet. 1707–1789.
Hercule Straus-Dürckheim. 1790–1865.

The lovers of minute anatomy have always been specially attracted to
Insects; and it is not hard to tell why. No other animals, perhaps,
exhibit so complex an organisation condensed into so small a body.
We possess, accordingly, a remarkable succession of memoirs on the
structure of single Insects, beginning with the revival of Anatomy in
the 17th century and extending to our own times. The most memorable
of these Insect-monographs bear the names of Malpighi, Swammerdam,
Lyonnet, and Straus-Dürckheim.

_Malpighi on the Silkworm._

Malpighi’s treatise on the Silkworm (1669) is an almost faultless essay
in a new field. No Insect--hardly, indeed, any animal--had then been
carefully described, and all the methods of work had to be discovered.
“This research,” says Malpighi, “was extremely laborious and tedious”
(it occupied about a year) “on account of its novelty, as well as the
minuteness, fragility, and intricacy of the parts, which required a
special manipulation; so that when I had toiled for many months at this
incessant and fatiguing task, I was plagued next autumn with fevers
and inflammation of the eyes. Nevertheless, such was my delight in
the work, so many unsuspected wonders of nature revealing themselves
to me, that I cannot tell it in words.” We must recall the complete
ignorance of Insect-anatomy which then prevailed, and remember that
now for the first time the dorsal vessel, the tracheal system, the
tubular appendages of the stomach, the reproductive organs, and the
structural changes which accompany transformation were observed, to
give any adequate credit to the writer of this masterly study. Treading
a new path, he walks steadily forward, trusting to his own sure eyes
and cautious judgment. The descriptions are brief and simple, the
figures clear, but not rich in detail. There would now be much to
add to Malpighi’s account, but hardly anything to correct. The only
positive mistakes which meet the eye relate to the number of spiracles
and nervous ganglia--mistakes promptly corrected by Swammerdam. Had the
tract De Bombycibus been the one work of its author, this would have
kept his memory bright, but it hardly adds to the fame of the anatomist
who discovered the cellular structure of the lung, the glandular
structure of the liver and kidney, and the sensory papillæ of the skin,
who first saw the blood-corpuscles stream along a vessel, who studied
very early and very completely the minute structure of plants and the
development of the chick, and whose name is rightfully associated with
the mucous layer of the epidermis, the vascular tufts of the kidney,
and the follicles of the spleen, as well as with the urinary tubules of
Insects.

All that we know of Malpighi commands our respect. Precise and rapid
in his work, keen to discover points of real interest, never losing
himself in details, but knowing when he had done enough, he stands
pre-eminent in the crowd of minute anatomists, who are generally
faithful in a few things, but very unfit to be made rulers over many
things. The last distinct glimpse which we get of him is interesting.
Dr. Tancred Robinson, writing to John Ray, from Geneva, April 18th,
1684, tells how he met Malpighi at Bologna. They talked of the origin
of fossils, and Malpighi could not contain himself about Martin
Lister’s foolish hypothesis that fossils were sports of nature. “Just
as I left Bononia,” he continues, “I had a lamentable spectacle of
Malpighi’s house all in flames, occasioned by the negligence of his
old wife. All his pictures, furniture, books, and manuscripts were
burnt. I saw him in the very heat of the calamity, and methought I
never beheld so much Christian patience and philosophy in any man
before; for he comforted his wife, and condoled nothing but the loss of
his papers, which are more lamented than the Alexandrian Library, or
Bartholine’s Bibliothece, at Copenhagen.”[3]

[3] Correspondence of John Ray, p. 142.

_Swammerdam on the Honey Bee._

Swammerdam’s great posthumous work, the Biblia Naturæ, contains about a
dozen life-histories of Insects worked out in more or less detail. Of
these the May-fly (published during the author’s life-time, in 1675) is
the most famous; that on the Honey Bee the most elaborate. Swammerdam
was ten years younger than Malpighi, and knew Malpighi’s treatise on
the Silkworm--a not inconsiderable advantage. His working-life as a
naturalist comes within the ten years between 1663 and 1673; and this
short space of time was darkened by anxiety about money, as well as by
the religious fanaticism, which in the end completely extinguished his
activity. The vast amount of highly-finished work which he accomplished
in these ten years justifies Boerhaave’s rather rhetorical account
of his industry. Unfortunately, Boerhaave, whom we have to thank
not only for a useful sketch of Swammerdam’s life, but also for the
preservation of most of his writings, was only twelve years old when
the great naturalist died, and his account cannot be taken as personal
testimony. Swammerdam, he tells us, worked with a simple microscope and
several powers. His great skill lay in his dexterous use of scissors.
Sometimes he employed tools so fine as to require whetting under the
microscope. He was famous for inflated and injected preparations. As
to his patience, it is enough to say that he would spend whole days
in clearing a single caterpillar. Boerhaave gives us a picture of
Swammerdam at work which the reader does not soon forget. “His labours
were superhuman. Through the day he observed incessantly, and at night
he described and drew what he had seen. By six o’clock in the morning
in summer he began to find enough light to enable him to trace the
minutiæ of natural objects. He was hard at work till noon, in full
sunlight, and bareheaded, so as not to obstruct the light; and his head
streamed with profuse sweat. His eyes, by reason of the blaze of light
and microscopic toil, became so weakened that he could not observe
minute objects in the afternoon, though the light was not less bright
than in the morning, for his eyes were weary, and could no longer
perceive readily.”

Comparing Swammerdam’s account of the Bee with the useful and amply
illustrated memoir of Girdwoyn (Paris, 1876), it is plain that two
centuries have added little to our knowledge of the structure of this
type. Much has been made out since 1675 concerning the life-history
of Bees, but of what was to be discovered by lens and scalpel,
Swammerdam left little indeed to others. It is needless to dwell
upon the omissions of so early an explorer. Swammerdam proved by
dissection that the queen is the mother of the colony, that the drones
are males, and the working-bees neuters; but he did not find out that
the neuters are only imperfect females. In this instance, as in some
others, Swammerdam’s authority served, long after his death, to delay
acceptance of the truth. It is far from a reproach to him that in the
Honey Bee he lit upon an almost inexhaustible subject. In the 17th
century no one suspected that the sexual economy of any animal could be
so complicated as that which has been demonstrated, step by step, in
the Honey Bee.

_Lyonnet on the Goat Moth._

In Lyonnet’s memoir on the larva of the Goat Moth (Traité Anatomique
de la Chenille qui ronge le bois de Saule, 1760[4]) we must not look
for the originality of Malpighi, nor for the wide range of Swammerdam.
One small thing is attempted, and this is accomplished with unerring
fidelity and skill. There is something of display in the delineation
of the four thousand and forty-one muscles of the Caterpillar, and the
author’s skill as a dissector is far beyond his knowledge of animals,
whether live or dead. The dissections of the head are perhaps the most
extraordinary feat, and will never be surpassed. Modern treatises on
Comparative Anatomy continue to reproduce some of these figures, such
as the general view of the viscera, the structure of the leg, and the
digestive tract. Nearly the whole interest of the volume lies in the
plates, for the text is little more than a voluminous explanation of
the figures.

[4] Copies dated 1762 have a plate representing the microscope and
dissecting instruments used by the author.

It is not without surprise that we find that Lyonnet was an amateur,
who had received no regular training either in anatomy or engraving,
and that he had many pursuits besides the delineation of natural
objects. He was brought up for the Protestant ministry, turned to the
bar, and finally became cipher-secretary and confidential translator
to the United Provinces of Holland. He is said to have been skilled in
eight languages. His first published work in Natural History consisted
of remarks and drawings contributed to Lesser’s Insect Theology (1742).
About the same time, Trembley was prosecuting at the Hague his studies
on the freshwater Polyp, and Lyonnet gave him some friendly help in
the work. Those who care to turn to the preface of Trembley’s famous
treatise (Mémoires pour servir à l’histoire des Polypes d’eau douce,
1744) will see how warmly Lyonnet’s services are acknowledged. He made
all the drawings, and engraved eight of them himself, while Trembley
is careful to note that he was not only a skilful draughtsman, but an
acute and experienced observer. When the work was begun, Lyonnet had
never even seen the operation of engraving a plate. Wandelaar, struck
by the beauty of his drawings, persuaded him to try what he could do
with a burin. His first essay was made upon the figure of a Dragon-fly,
next he engraved three Butterflies, and then, without longer
apprenticeship, he proceeded to engrave the plates still required to
complete the memoir on Hydra.

Lyonnet tells us that the larva of the Goat Moth was not quite his
earliest attempt in Insect Anatomy. He began with the Sheep Tick, but
suspecting that the subject would not be popular, he made a fresh
choice for his first memoir. Enough interest was excited by the Traité
Anatomique to call for the fulfilment of a promise made in the preface
that the description of the pupa and imago should follow. But though
Lyonnet continued for some time to fill his portfolio with drawings
and notes, he never published again. Failing eyesight was one ground
of his retirement from work. What he had been able to finish, together
with a considerable mass of miscellaneous notes, illustrated by
fifty-four plates from his own hand, was published, long after his
death, in the Mémoires du Muséum (XVIII.–XX.).

_Straus-Dürckheim on the Cockchafer._

In beauty and exact fidelity Straus-Dürckheim’s memoir on the
Cockchafer (Considérations Générales sur l’Anatomie Comparée des
Animaux Articulés, auxquelles on a joint l’Anatomie Descriptive du
Melolontha vulgaris, 1828) rivals the work of Lyonnet. Insect Anatomy
was no longer a novel subject in 1828, but Straus-Dürckheim was able
to treat it in a new way. Writing under the immediate influence of
Cuvier, he sought to apply that comparative method, which had proved so
fertile in the hands of the master, to the Articulate sub-kingdom. This
conception was realised as fully as the state of zoology at that time
allowed, and the Considérations Générales count as an important step
towards a complete comparative anatomy of Arthropoda. Straus-Dürckheim
had at command a great mass of anatomical facts, much of which had
been accumulated by his own observations. He systematically compares
Insects with other Articulata, Coleoptera with other Insects, and the
Cockchafer with other Coleoptera. Perhaps no one before him had been
perfectly clear as to the morphological equivalence of the appendages
in all parts of the body of Arthropods, and here he was able to extend
the teaching of Savigny. His limitations are those of his time. If in
certain sections we find his collection of facts to be meagre, and his
generalisations nugatory, we must allow for the progress of the last
sixty years--a progress in which Straus-Dürckheim has his share. It is
the work of science continually to remake its syntheses, and no work
becomes antiquated sooner than morphological generalisation.

It is therefore no reproach to Straus-Dürckheim that his treatise
should now be chiefly valuable, not as “Considérations Générales,”
but as the anatomy of the Cockchafer. Long after his theories and
explanations have ceased to be instructive, when the morphology
and physiology of 1828 have become as obsolete as the Ptolemaic
astronomy, the naturalist will study these exquisite delineations
of Insect-structure with something of the pleasure to be found in
examining for the hundredth time a delicate organism familiar to many
generations of microscopic observers.

The fidelity and love of anatomical detail which characterise
the description of the Cockchafer are not less conspicuous in
Straus-Dürckheim’s Anatomie Descriptive du Chat (1846). Both treatises
have become classical.

We have seen how, in Straus-Dürckheim’s hands, Insect anatomy
became comparative. New studies--histology, embryonic development,
and palæontology--have since arisen to complicate the task of the
descriptive anatomist, and it appears to be no longer possible for
one man to complete the history of any animal of elaborate structure
and ancient pedigree. As a method of research the monograph has had
its day. The path of biological discovery now follows an organ or a
function across all zoological boundaries, and it is in the humbler
office of biological teaching that the monograph finds its proper use.

_Later Insect Anatomists._

It is impossible even to glance at the many anatomists who have
illustrated the structure of Insects by studies, less simple in plan,
but not less profitable to science, than those of the monographers. If
we attempt to select two or three names for express mention, it is with
a conviction that others are left whom the student is bound to hold in
equal honour.

Dufour[5] laboured, not unsuccessfully, to construct a General Anatomy
of Insects, which should combine into one view a crowd of particular
facts. The modern reader will gratefully acknowledge his industry and
the beauty of his drawings, but will now and then complain that his
sagacity does not do justice to his diligence.

[5] Dufour. Rech. anat. et phys. sur les Hémiptères (1833) les
Orthoptères, les Hymenoptères et les Neuroptères (1841), et les
Diptères (1851). Mém. de l’Institut, Tom. IV., VII., XI. Also many
memoirs in Ann. des Sci. Nat.

Newport,[6] a naturalist of greater weight and interest, is memorable
for his skill in minute dissection, for his many curious observations
upon the life-history of Insects (see, for example, his memoir on the
Oil-beetle), and especially for his early appreciation of the value of
embryological study.

[6] Newport. Art. “Insecta,” in Cycl. of Anat. and Phys. (1839),
besides many special memoirs in the Phil. and Linn. Trans.

Leydig[7] was the first to occupy fully the new field of Insect
histology, and point out its resources to the physiologist. In all
his works the student finds beauty and exactness of delineation,
suggestiveness in explanation. Leydig’s contributions to Insect anatomy
and physiology, valuable as they are to the specialist, are not
isolated researches, but form part of a new comparative anatomy, based
upon histology. Incomplete so vast a work must necessarily remain, but
it already extends over considerable sections of the animal kingdom.

[7] Leydig. Vom Bau des Thierischen Körpers (1864), Tafeln zur vergl.
Anatomie (1864), Untersuchungen zur Anat. und Histologie der Thiere
(1883), &c., besides many special memoirs in Müller’s Archiv., Zeits.
f. wiss. Zool., Nova Acta, &c.

CHAPTER II.

THE ZOOLOGICAL POSITION OF THE COCKROACH.

Sub-kingdom ARTHROPODA.

Class I. Crustacea.
" II. Arachnida.
" III. Myriopoda.
" IV. Insecta.

Order 1. Thysanura.
" 2. _Orthoptera._
" 3. Neuroptera.
" 4. Hemiptera.
" 5. Coleoptera.
" 6. Diptera.
" 7. Lepidoptera.
" 8. Hymenoptera.

The place of the Cockroach in the Animal Kingdom is illustrated by the
above table. It belongs to the sub-kingdom Arthropoda, to the class
Insecta, and to the order Orthoptera.

_Characters of Arthropoda._

Arthropoda are in general readily distinguished from other animals by
their jointed body and limbs. In many Annelids the body is ringed, and
each segment bears a pair of appendages, but these appendages are soft,
and never articulated. The integument of an Arthropod is stiffened
by a deposit of the tough, elastic substance known as Chitin, which
resembles horn in appearance, though very different in its chemical
composition. In marine Arthropoda, as well as in many Myriopoda and
Insects, additional firmness may be gained by the incorporation of
carbonate and phosphate of lime with the chitin. However rigid the
integument may be, it is rendered compatible with energetic movements
by its unequal thickening. Along defined, usually transverse lines
it remains thin, the chitinous layer, though perfectly continuous,
becoming extremely flexible, and allowing a certain amount of
deflection or retraction (fig. 1). The joints of the trunk and limbs
may thus resemble stiff tubes. Muscles are attached to their inner
surface, and are therefore enclosed by the system of levers upon which
they act (fig. 2B). In Vertebrate animals, on the contrary, which
possess a true internal skeleton, the muscles clothe the levers (bones)
to which they are attached (fig. 2A). The whole outer surface of an
Arthropod, including the eyes, auditory membrane (if there is one),
and surface-hairs, is chitinised. Chitin may also stiffen the larger
tendons, internal ridges and partitions, and the lining membrane of
extensive internal cavities, such as the alimentary canal, and the
air-tubes of Insects.

[Illustration: Fig. 1.--Diagram of Arthropod limb extended, retracted,
and flexed. Graber has given a similar figure (Insekten, fig. 8).]

[Illustration: Fig. 2.--Vertebrate and Arthropod joints. A, Vertebrate
joint, the skeleton clothed with muscles. B, Arthropod joint, the
skeleton enclosing the muscles.]

In most Arthropoda the body is provided with many appendages. In
Crustacea there are often twenty pairs, but some Myriopoda have not far
from two hundred pairs. Some of these may be converted to very peculiar
functions; in particular, several pairs adjacent to the mouth are
usually appropriated to mastication. One or more pairs of appendages
are often transformed into antennæ.

The relative position of the chief organs of the body, viz.:--heart,
nerve-cord, and alimentary canal, is constant in Arthropoda. The heart
is dorsal, the nerve-cord ventral, the alimentary canal intermediate.
(See fig. 3.) The œsophagus passes between the connectives of the
nerve-cord. Not a few other animals, such as Annelids and Mollusca,
exhibit the same arrangement.

Arthropoda are not known to be ciliated in any part of the body, or in
any stage of growth. Another histological peculiarity, not quite so
universal, is the striation of the muscular fibres throughout the body.
In many Invertebrates there are no striated muscles at all, while in
Vertebrates only voluntary muscles, as a rule, are striated.

The circulatory organs of Arthropoda vary greatly in plan and degree of
complication, but there is never a completely closed circulation.

The development of Arthropoda may be accompanied by striking
metamorphosis, _e.g._, in many marine Crustacea, but, as in other
animals, the terrestrial and fluviatile forms usually develop directly.
Even in Insects, which appear to contradict this rule flatly, the
exception is more apparent than real. The Insect emerges from the egg
as a fully formed larva, and so far its development is direct. It is
the full-grown larva, however, which corresponds most nearly to the
adult Myriopod, while the pupa and imago are stages peculiar to the
Insect. It is not by any process of embryonic development, but by a
secondary metamorphosis of the adult that the Insect acquires the power
of flight necessary for the deposit of eggs in a new site.

[Illustration: Fig. 3.--Longitudinal section of Female Cockroach, to
show the position of the principal organs. _Oe_, œsophagus; _S.gl_,
salivary gland; _S.r_, salivary reservoir; _Cr_, crop; _G_, gizzard;
_St_, chylific stomach; _R_, rectum; _Ht_, heart; _N.C_, nerve-cord. ×
7.]

_Characters of Insects._

Insects are distinguished from other Arthropoda by the arrangement
of the segments of the body into three plainly marked regions--head,
thorax, and abdomen; by the three pairs of ambulatory legs carried
upon the thorax; by the single pair of antennæ; and by the tracheal
respiration. Myriopods and Arachnida have no distinct thorax. Most
Crustacea have two pairs of antennæ, while in Arachnida antennæ are
wanting altogether. Crustacea, if they possess special respiratory
organs at all, have branchiæ (gills) in place of tracheæ (air-tubes).
In Arachnida, Myriopoda, and Crustacea there are usually more than
three pairs of ambulatory legs in the adult.

The appendages of an Insect’s head (antennæ, mandibles, maxillæ) are
appropriated to special senses, or to the operations of feeding,
and have lost that obvious correspondence with walking legs which
they still retain in some lower Arthropoda (_Peripatus_, _Limulus_,
_Arachnida_). The thorax consists of three[8] segments, each of which
carries a pair of ambulatory legs. No abdominal legs are found in any
adult insect. The middle thoracic segment may carry a pair of wings or
wing-covers, and the third segment a pair of wings.

[8] In some Insects there are traces of a fourth thoracic segment.

The lower or less-specialised Insects, such as the Cockroach, have
nearly as many nerve-ganglia as segments, and the longitudinal
connectives of the nerve-cord are double. In the adult of certain
higher Insects[9] (_e.g._, many Coleoptera, and some Diptera) the
nerve-ganglia are concentrated, reduced in number, and restricted to
the head and thorax; while all the connectives, except those of the
œsophageal ring, may be outwardly single.

[9] So also in some larvæ (_Calandra_, _Œstrus_, &c.).

The heart, or dorsal vessel, is subdivided by constrictions into a
series of chambers, from which an aorta passes forwards to the head.

Air is usually taken into the body by stigmata or breathing-pores,[10]
which lie along the sides of the thorax and abdomen. It circulates
through repeatedly-branching tracheal tubes, whose lining is
strengthened by a spiral coil. Air-sacs (dilated portions of the
air-tubes) occur in Insects of powerful flight.

[10] In some aquatic Insects the exchange of gases is effected by
“pseudobranchiæ,” and the tracheal system is closed.

The generative organs are placed near the hinder end of the body.[11]
Most Insects are oviparous.[12] The sexes are always distinct; but
imperfect females (“neuters”) occur in some kinds of social Insects.
Agamogenesis (reproduction by unfertilised eggs) is not uncommon.

[11] Dragon-flies have the male copulatory apparatus, but not the
genital aperture, in the fore part of the abdomen.

[12] Aphis and Cecidomyia are at times viviparous, and a viviparous
Moth has been observed by Fritz Müller (Trans. Entom. Soc. Lond.,
1883).

_Orders of Insects._

The orders of Insects are usually defined with reference to the
degree of metamorphosis and the structure of the parts of the mouth.
Five of the orders (3, 5–8) in the table on page 9 undergo complete
metamorphosis, and during the time of most rapid change the insect
is motionless. In the remaining orders (1, 2, 4) there is either no
metamorphosis (_Thysanura_), or it is incomplete--_i.e._, the insect
is active in all stages of growth. Among these three orders we readily
distinguish the minute and wingless Thysanura. Two orders remain,
in which the adult is commonly provided with wings; of these, the
Orthoptera have biting jaws, the Hemiptera, jaws adapted for piercing
and sucking.

The name of Black Beetle, often given to the Cockroach, is therefore
technically wrong. True Beetles have a resting or chrysalis stage, and
may further be recognised in the adult state by the dense wing-covers,
meeting along a straight line down the middle of the back, and by the
transversely folded wings. Cockroaches have no resting stage, the
wing-covers overlap, and the wings fold up fan-wise.

_Further Definition of Cockroaches._

In the large order of Orthoptera, which includes Earwigs, Praying
Insects, Walking Sticks, Grasshoppers, Locusts, Crickets, White Ants,
Day-flies, and Dragon-flies, the family of Cockroaches is defined as
follows:--

Family _Blattina_. Body usually depressed, oval. Pronotum shield-like.
Legs adapted for running only. Wing-covers usually leathery, opaque,
overlapping (if well developed) when at rest, anal area defined by
a furrow (fig. 4). Head declivent, or sloped backwards, retractile
beneath the pronotum. Eyes large, ocelli rudimentary, usually two,
antennæ long and slender.

[Illustration: Fig. 4.--Generalised sketch of Cockroach wing-cover.]

About eight hundred species of Cockroaches have been defined, and to
facilitate their arrangement, three groups have been proposed, under
which the different genera are ranked.[13]

Group 1. Both sexes wingless (_Polyzosteria_).

Group 2. Males winged, females wingless (_Perisphæria_, _Heterogamia_).

Group 3. Both sexes with more or less developed wings (about 7 genera).

[13] For descriptions of the species Fischer’s Orthoptera Europæa
(1853) or Brunner von Wattenwyl’s Nouveau Système des Blattaires
(1865) may be consulted. The classification adopted by the last-named
author is here summarised.

BLATTARIÆ.

A.--Femora spinous (_Spinosæ_).

Fam. 1.--_Ectobidæ._ Seventh abdominal sternum undivided in female.
Sub-anal styles absent in male. Wings with triangular apical area.
_Ectobia_, including _E. lapponica_ (_Blatta_) and other genera.

Fam. 2.--_Phyllodromidæ._ Seventh abdominal sternum undivided
in female. Sub-anal styles usual in male (0 or rudimentary
in _Phyllodromia_). Wings without triangular apical area.
_Phyllodromia_, including _P. germanica_ (_Blatta_) and other
genera.

Fam. 3.--_Epilampridæ._

Fam. 4.--_Periplanetidæ._ Seventh abdominal sternum divided in
female. Sub-anal styles conspicuous in male. _Polyzosteria_,
_Periplaneta_, &c.

B.--Femora not spinous (_Muticæ_).

Families.--_Chorisoneuridæ_, _Panchloridæ_, _Perisphæridæ_,
_Corydidæ_, _Heterogamidæ_, _Blaberidæ_, _Panesthidæ_.

Many useful references will be found in Scudder’s Catalogue of N.
American Orthoptera, Smiths. Misc. Coll., viii. (1868).

In Group 3 occur the only two genera which we shall find it necessary
to describe--viz., _Blatta_, which includes the European Cockroaches,
and _Periplaneta_, to which belong the Cockroaches of tropical Asia and
America.

Genus _Blatta_. A pulvillus between the claws of the feet. The
seventh sternum of the abdomen entire in both sexes; sub-anal styles
rudimentary in the male.

Genus _Periplaneta_. Readily distinguished from Blatta by the divided
seventh abdominal sternum of the female, and the sub-anal styles of
the male.

Two species of Periplaneta have been introduced into Europe. These are--

1. _P. orientalis_ (Common Cockroach, Black Beetle). Wing-covers and
wings not reaching the end of the abdomen in the male; rudimentary in
the female.

2. _P. americana_ (American Cockroach). Wing-covers and wings longer
than the body in both sexes.

CHAPTER III.

THE NATURAL HISTORY OF THE COCKROACH.

_SPECIAL REFERENCES._

HUMMEL. Essais Entomologiques, No. 1 (1821).

CORNELIUS. Beiträge zur nähern Kenntniss von Periplaneta orientalis
(1853.)

GIRARD. La domestication des Blattes. Bull. Soc. d’Acclimatisation,
3^e Sér., Tom. IV., p. 296 (1877).

_Range._

The common Cockroach is native to tropical Asia,[14] and long ago made
its way by the old trade-routes to the Mediterranean countries. At the
end of the sixteenth century it appears to have got access to England
and Holland, and has gradually spread thence to every part of the world.

[14] Linnæus was certainly mistaken in his remark (Syst. Nat., 12th
ed.) that this species is native to America, and introduced to the
East--“Habitat in America: hospitatur in Oriente.” He adds, “Hodie in
Russiæ adjacentibus regionibus frequens; incepit nuperis temporibus
Holmiæ, 1739, uti dudum in Finlandia.”

Perhaps the first mention of this insect in zoological literature
occurs in Moufet’s Insectorum Theatrum (1634), where he speaks of the
Blattæ as occurring in wine cellars, flour mills, &c., in England. It
is hard to determine in all cases of what insects he is speaking, since
one of his rude woodcuts of a “Blatta” is plainly _Blaps mortisaga_;
another is, however, recognisable as the female of _P. orientalis_; a
third, more doubtfully, as the male of the same species. He tells how
Sir Francis Drake took the ship “Philip,”[15] laden with spices, and
found a great multitude of winged Blattæ on board, “which were a little
larger, softer, and darker than ours.” Perhaps these belonged to the
American species, but the description is obscure. Swammerdam also was
acquainted with our Cockroach as an inhabitant of Holland early in the
seventeenth century. He speaks of it as “insectum illud Indicum, sub
nomine Kakkerlak satis notum,” and very properly distinguishes from
it “the species of Scarabæus” (_Blaps_), which Moufet had taken for a
Blatta.[16]

[15] This must have been the “San Felipe,” a Spanish East Indiaman,
taken in 1587. See Motley, United Netherlands, Vol. II., p. 283.

[16] Biblia Naturæ, Vol. I., p. 216.

The American Cockroach is native to tropical America, but has
now become widely spread by commerce. An Australian species also
(_P. australasiæ_) has begun to extend its native limits, having
been observed in Sweden,[17] Belgium, Madeira, the East and West
Indies,[18] Florida,[19] &c. In Florida it is said to be the torment of
housekeepers.

[17] De Borck. Skandinaviens rätvingade Insekters Nat. Hist., I.,
i., 35.

[18] Brunner. N. Syst. d. Blattaires, p. 234.

[19] Scudder. Proc. Boston Soc. N.H., Vol. XIX., p. 94.

To the genus _Blatta_ belong a number of small European species, which
mostly lurk in woods and thickets. Some of these are found in the south
of England. _B. lapponica_ is one of the commonest and most widely
distributed. It is smaller than the common Cockroach, and both sexes
have long wings and wing-cases. The males are black and the females
yellow. It is found on the mountains of Norway and Switzerland as high
as shrubs extend, and when sheltered by human dwellings, can endure
the extreme cold of the most northern parts of Europe. This is the
insect of which Linnæus tells, that in company with _Silpha lapponica_
it has been known to devour in one day the whole stock of dried but
unsalted fish of a Lapland village. _B. germanica_ also has the wings
and wing-cases well developed in both sexes. Two longitudinal stripes
on the pronotum, or first dorsal plate of the thorax, are the readiest
mark of this species, which is smaller and lighter in colour than the
common Cockroach. It is plentiful in most German towns, and has been
introduced from Germany into many other countries;[20] but it appears
to be native, not to Germany alone, but to Asia and all parts of
central and southern Europe. Where and how it first became domesticated
we do not know.

[20] For example, the Russians often call it _Proussaki_, the
Prussian Cockroach, and believe that their troops brought it home
with them after the Seven Years’ War. The native Russian name
is _Tarakan_. In Finland and Sweden the same species is called
_Torraka_, which appears to be a corruption of the Russian word, and
confirms the account of Linnæus quoted above.

_B. germanica_ is found in the United States from the Atlantic to the
Pacific. It is generally known as the Croton Bug, because in New York
it is often met with about the water pipes, which are supplied from
the Croton River (Dr. Scudder).

The other species of Cockroaches which have been met with in Europe are
_Panchlora maderæ_, said by Stephens to be occasionally seen in London,
and _Blabera gigantea_ the Drummer of the West Indies, which has often
been found alive in ships in the London Docks.

_Blatta germanica_, _Periplaneta orientalis_, and _P. americana_, are
so similar in habits and mode of life as to be interchangeable, and
each is known to maintain itself in particular houses or towns within
the territory of another species, though usually without spreading.

_Orientalis_ is, for example, the common Cockroach of England, but
_germanica_ frequently gets a settlement and remains long in the same
quarters. H. C. R., in Science-Gossip for 1868, p. 15, speaks of it as
swarming in an hotel near Covent Garden, where it can be traced back as
far as 1857. In Leeds, one baker’s shop is infested by this species; it
is believed to have been brought by soldiers to the barracks, after the
Crimean war, and to have been carried to the baker’s in bread-baskets.
We have met with no instance in which it has continued to gain ground
at the expense of _orientalis_. _Americana_ also seems well established
in particular houses or districts in England. H. C. R. (loc. cit.)
mentions warehouses near the Thames, Red Lion and Bloomsbury Squares,
and the Zoological Gardens, Regent’s Park. It frequents one single
warehouse in Bradford, and is similarly local in other towns with
foreign trade.

Many cases are recorded in which _germanica_ has been replaced by
_orientalis_, as in parts of Russia and Western Germany, but detailed
and authenticated accounts are still desired. On the whole _orientalis_
seems to be dominant over both _germanica_ and _americana_.

The slow spread of the Cockroaches in Europe is noteworthy, not as
exceptional among invading species, but as one more illustration of
the length of time requisite for changes of the equilibrium of nature.
It took two centuries from the first introduction of _orientalis_ into
England for it to spread far from London. Gilbert White, writing, as
it would appear, at some date before 1790, speaks of the appearance of
“an unusual insect,” which proved to be the Cockroach, at Selborne,
and says: “How long they have abounded in England I cannot say; but
have never observed them in my house till lately.”[21] It is probable
that many English villages are still clear of the pest. The House
Cricket, which the Cockroaches seem destined to supplant, still dwells
in our houses, often side by side with its rival, sharing the same
warm crannies, and the same food. The other imported species, though
there is reason to suppose that they cannot permanently withstand
_orientalis_, are by no means beaten out of the field; they retreat
slowly where they retreat at all, and display inferiority chiefly in
this, that in countries where both are found, they do not spread, while
their competitor does. It may yet require some centuries to settle the
petty wars of the Cockroaches.

[21] Bell’s Edition, Vol. I., p. 454.

It is also worth notice that in this, as in most other cases, the
causes of such dominance over the rest as one species enjoys are
very hard to discover. We cannot explain what peculiarities enable
Cockroaches to invade ground thoroughly occupied by the House Cricket,
an insect of quite similar mode of life: and it is equally hard to
account for the superiority of _orientalis_ over the other species. It
is neither the largest nor the smallest; it is not perceptibly more
prolific, or more voracious, or fonder of warmth, or swifter than
its rivals, nor is it easy to see how the one conspicuous structural
difference--viz., the rudimentary state of the wings of the female, can
greatly favour _orientalis_. Some slight advantage seems to lie in
characteristics too subtle for our detection or comprehension.

_Food and Habits._

As to the food of Cockroaches, we can hardly except any animal or
vegetable substance from the long list of their depredations. Bark,
leaves, the pith of living cycads, paper, woollen clothes, sugar,
cheese, bread, blacking, oil, lemons, ink, flesh, fish, leather, the
dead bodies of other Cockroaches, their own cast skins and empty
egg-capsules, all are greedily consumed. Cucumber, too, they will eat,
though it disagrees with them horribly.

In the matter of temperature they are less easy to please. They
are extremely fond of warmth, lurking in nooks near the oven, and
abounding in bakehouses, distilleries, and all kinds of factories which
provide a steady heat together with a supply of something eatable. Cold
is the only check, and an unwarmed room during an English winter is
more than they can endure. They are strictly nocturnal, and shun the
light, although when long unmolested they become bolder. The flattened
body enables the Cockroach to creep into very narrow crevices, and
during cold weather it takes refuge beneath the flags of a kitchen
floor, or in other very confined spaces.

The Cockroach belongs to a miscellaneous group of animals, which may be
described as in various degrees parasitic upon men. These are all in a
vague sense domestic species, but have not, like the ox, sheep, goat,
or pig, been forcibly reduced to servitude; they have rather attached
themselves to man in various degrees of intimacy. The dog has slowly
won his place as our companion; the cat is tolerated and even caressed,
but her attachment is to the dwelling and not to us; the jackal and
rat are scavengers and thieves; the weasel, jackdaw, and magpie are
wild species which show a slight preference for the neighbourhood of
man. All of these, except the cat, which holds a very peculiar place,
possess in a considerable degree qualities which bring success in the
great competitive examination. They are not eminently specialised,
their diet is mixed, their range as natural species is wide. Apart from
man, they would have become numerous and strong, but those qualities
which fit them so well to shift for themselves, have had full play
in the dwellings of a wealthy and careless host. Of these domestic
parasites at least two are insects, the House-fly and the Cockroach;
and the Cockroach in particular is eminent in its peculiar sphere
of activity. The successful competition of Cockroaches with other
insects under natural conditions is sufficiently proved by the fact
that about nine hundred species have already been described,[22] while
their rapid multiplication and almost worldwide dissemination in the
dwellings of man is an equally striking proof of their versatility and
readiness to adapt themselves to artificial circumstances. In numerical
frequency they probably exceed all domestic animals of larger size,
while in geographical range the five species, _lapponica_, _germanica_,
_orientalis_, _americana_, and _australasiæ_, are together comparable
to the dog or pig, which have been multiplied and transported by man
for his own purposes, and which cover the habitable globe.

[22] British Museum Catalogue of Blattariæ (1868) and Supplement
(1869). It is probable that the number is over-estimated in this
catalogue, the same species being occasionally renamed.

_The Cockroach a persistent type._

The Cockroach is historically one of the most ancient, and structurally
one of the most primitive, of our surviving insects. Its immense
antiquity is shown by the fact that so many Cockroaches have been found
in the Coal Measures, where about eighty species have been met with.
The absence of well-defined stages of growth, such as the soft-bodied
larva or inactive pupa, the little specialised wings and jaws, the
simple structure of the thorax, the jointed appendages carried on the
end of the abdomen, and the unconcentrated nervous system, are marks
of the most primitive insect-types. The order Orthoptera is undeniably
the least specialised among winged insects at least, and within this
order none are more simple in structure, or reach farther back in the
geological record than the Cockroaches. The wingless Thysanura are even
more generalised, but their geological history is illegible.[23]

[23] Brongniart has just described a Carboniferous Insect which he
considers a Thysanuran (_Dasyleptus Lucasi_), though it has but one
anal appendage. See C. R. Soc. Ent., France, 1885.

_Life-History._

The eggs of the Cockroach are laid sixteen together in a large
horny capsule. This capsule is oval, with roundish ends, and has a
longitudinal serrated ridge, which is uppermost while in position
within the body of the female. The capsule is formed by the secretion
of a “colleterial” gland, poured out upon the inner surface of a
chamber (vulva) into which the oviducts lead. The secretion is at first
fluid and white, but hardens and turns brown on exposure to the air. In
this way a sort of mould of the vulva is formed, which is hollow, and
opens forwards towards the outlet of the common oviduct. Eggs are now
passed one by one into the capsule; and as it becomes full, its length
is gradually increased by fresh additions, while the first-formed
portion begins to protrude from the body of the female. When sixteen
eggs have descended, the capsule is closed in front, and after an
interval of seven or eight days, is dropped in a warm and sheltered
crevice. In _Periplaneta orientalis_ it measures about ·45 in. by
·25 in. (fig. 5). The ova develop within the capsule, and when ready
to escape are of elongate-oval shape, resembling mummies in their
wrappings. Eight embryos in one row face eight others on the opposite
side, being alternated for close packing. Their ventral surfaces,
which are afterwards turned towards the ground, are opposed, and their
rounded dorsal surfaces are turned towards the wall of capsule; their
heads are all directed towards the serrated edge. The ripe embryos
are said by Westwood to discharge a fluid (saliva?) which softens the
cement along the dorsal edge, and enables them to escape from their
prison. In _Blatta germanica_ the female is believed to help in the
process of extrication.[24] The larvæ are at first white, with black
eyes, but soon darken. They run about with great activity, feeding upon
any starchy food which they can find.

[24] Hummel, loc. cit.

[Illustration: Fig. 5.--Egg-capsule of _P. orientalis_ (magnified). A,
external view; B, opened; C, end view.]

The larvæ of the Cockroach hardly differ outwardly from the adult,
except in the absence of wings. The tenth tergum is notched in both
sexes, as in the adult female. The sub-anal styles of the male are
developed in the larva.

Cornelius, in his Beiträge zur nähern Kenntniss von Periplaneta
orientalis (1853), gives the following account of the moults of the
Cockroach. The first change of skin occurs immediately after escape
from the egg-capsule, the second four weeks later, the third at the end
of the first year, and each succeeding moult after a year’s interval.
At the sixth moult the insect becomes a pupa,[25] and at the seventh
(being now four years old) it assumes the form of the perfect Insect.
The changes of skin are annual, and like fertilisation and oviposition,
take place in the summer months only. He tells us further that the
ova require about a year for their development. These statements are
partly based upon observation of captive Cockroaches, and are the
only ones accessible; but they require confirmation by independent
observers, especially as they altogether differ from Hummel’s account
of the life-history of _Blatta germanica_, and are at variance with the
popular belief that new generations of the Cockroach are produced with
great rapidity.

[25] The use of the term _pupa_ to denote the last stage before the
complete assumption of wings in the Cockroach, is liable to mislead.
There is no resting-stage at all; wings are developed gradually,
and are nearly as conspicuous in the last larval state as in the
so-called pupa. There seems no reason for speaking of pupæ in this
case.

It is preferable to designate as “nymphs” young and active Insects,
immature sexually, but with mouth-parts like those of the adult. See
Lubbock, Linn. Trans., 1863, and Eaton, Linn. Trans., 1883.

[Illustration: Fig. 6.--Young nymph (male). × 6.]

The antennæ of the male nymph resemble those of the adult female. Wings
and wing-covers appear first in the later larval stages, but are then
rudimentary, and constitute a mere prolongation of the margins of the
thoracic rings. Cornelius says that the round white spot internal to
the antenna first appears plainly in the pupa, but we have readily
found it in a very young larva. The Insect is active in all its stages,
and is therefore, with other Orthoptera, described as undergoing
“incomplete metamorphosis.” After each moult it is for a few hours
nearly pure white. Of the duration of life in this species we have no
certain information, and there is great difficulty in procuring any.

[Illustration: Fig. 7.--Older nymph (male) with rudiments of wings.
× 2-1/2.]

_Sexual Differences._

Male Cockroaches are readily distinguished from the females by the
well-developed wings and wing-covers. They are also slighter and weaker
than the females; their terga and sterna are not so much thickened;
their alimentary canal is more slender, and they feed less greedily
(the crop of the male is usually only half-full of food). They stand
higher on their legs than the females, whose abdomen trails on the
ground. The external anatomical differences of the sexes may be
tabulated thus:--

_Parasites._

We have before us a long list of parasites[26] which infest the
Cockroach. There is a conferva, an amœba, several infusoria, nematoid
worms (one of which migrates to and fro between the rat and the
Cockroach), a mite, as well as hymenopterous and coleopterous Insects.
The Cockroach has a still longer array of foes, which includes monkeys,
hedgehogs, pole-cats, cats, rats, birds, chamæleons, frogs, and wasps,
but no single friend, unless those are reckoned as friends which are
the foes of its foes.

[26] See Appendix.

_Names in common use._

A few lines must be added upon the popular and scientific names of
this insect. Etymologists have found it hard to explain the common
English name, which seems to be related to _cock_ and _roach_, but
has really nothing to do with either. The lexicographers usually hold
their peace about it, or give derivations which are absurd. Mr. James
M. Miall informs us that “_Cockroach_ can be traced to the Spanish
_cucarácha_, a diminutive form of _cuco_ or _coco_ (Lat. _coccum_,
a berry). _Cucarácha_ is used also of the woodlouse, which, when
rolled up, resembles a berry. The termination _-ácha_ (Ital. _-accio_,
_-accia_) signifies _mean_ or _contemptible_. The Spanish word has also
taken a French form; at least _coqueraches_ has some currency (see,
for example, Tylor’s Anahuac, p. 325).” In provincial English _Black
Clock_ is a common name. The German word _Schabe_, often turned into
_Schwabe_, means perhaps _Suabian_, as Moufet, quoting Cordus, seems
to explain.[27] _Franzose_ and _Däne_ are other German words for the
insect, applied specially to _Blatta germanica_; and all seem to imply
some popular theory as to the native country of the Cockroach.[28]
This etymology of _Schabe_ is not free from suspicion, particularly
as the same term is commonly applied to the clothes-moth. _Kakerlac_,
much used in France and French-speaking colonies, is a Dutch word of
unknown signification. _P. Americana_ is usually named _Kakerlac_ or
_Cancrelat_ by the French; while _orientalis_ has many names, such
as _Cafard_, _Ravet_, and _Bête noire_.[29] The name _Blatta_ was
applied by the ancients to quite different insects, of which Virgil and
Pliny make mention; _Periplaneta_ is a modern generic term, coined by
Burmeister.

[27] Insectorum Theatrum, p. 138. The name _Schwabe_ is frequent in
Franconia, where it is believed to have taken origin. Suabia adjoins
Franconia, to the south.

[28] Compare the Swedish name (_supra_, p. 18).

[29] A fuller list of vernacular names is given by Rolland, Faune
Populaire de la France, Vol. III., p. 285. See also Nennich,
Polyglotten Lexicon, Vol. I., p. 620.

_Uses._

Of the uses to which Cockroaches have been put we have little to
say. They constitute a popular remedy for dropsy in Russia, and both
cockroach-tea and cockroach-pills are known in the medical practice of
Philadelphia. Salted Cockroaches are said to have an agreeable flavour
which is apparent in certain popular sauces.

CHAPTER IV.

THE OUTER SKELETON.

_SPECIAL REFERENCES._

KRUKENBERG. Vergleichend-Physiologische Vorträge. IV.--Vergl.
Physiologie der Thierischen Gerüstsubstanzen. (1885.) [Chemical
Relations of Chitin.]

GRABER. Ueber eine Art fibrilloiden Bindegewebes der Insectenhaut.
Arch. f. mikr. Anat. Bd. X. (1874.) [Minute Structure of
Integument.] Also,

VIALLANES. Recherches sur l’Histologie des Insectes. Ann. Sci.
Nat., Zool. VI^e Série, Tom. XIV. (1882).

AUDOUIN. Recherches anatomiques sur le thorax des Insectes, &c.
Ann. Sci. Nat. Tom. I. (1824.) [Theoretical Composition of Insect
Segments.] Also,

MILNE-EDWARDS. Leçons sur la Physiologie et l’Anatomie Comparée.
Tom. X. (1874.)

SAVIGNY. Mémoires sur les animaux sans vertèbres. Partie I^{e.}
Théorie des organes de la bouche des Crustacées et des Insectes.
(1816.) [Comparative Anatomy of the Mouth-parts.]

MUHR. Ueber die Mundtheile der Orthopteren. Prag. 1877.
[Mouth-parts of Orthoptera.]

_Chitin._

When the skin of an Insect is boiled successively in acids,
alkalies, alcohol, and ether, an insoluble residue known as Chitin
(C_{15}H_{26}N_{2}O_{10}) is obtained. It may be recognised and
sufficiently separated by its resistance to boiling liquor potassæ.
Chitin forms less than one-half by weight of the integument, but it is
so coherent and uniformly distributed that when isolated by chemical
reagents, and even when cautiously calcined, it retains its original
organised form. The colour which it frequently exhibits is not due to
any essential ingredient; it may be diminished or even destroyed by
various bleaching processes. The colouring-matter of the chitin of the
Cockroach, which is amber-yellow in thin sheets and blackish-brown in
dense masses, is particularly stable and difficult of removal. Its
composition does not appear to have been ascertained; it is white
when first secreted, but darkens on exposure to air. Fresh-moulted
Cockroaches are white, but gradually darken in three or four hours.
Lowne[30] observes that in the Blow-fly the pigment is “first to be met
with in the fat-bodies of the larvæ. These are perfectly white, but
when cut from the larva, and exposed to the air, they rapidly assume
an inky blackness.... When the perfect insect emerges from the pupa,
and respiration again commences, the integument is nearly white, or a
faint ashy colour prevails. This soon gives place to the characteristic
blue or violet tint, first immediately around those portions most
largely supplied with air vessels.” Professor Moseley[31] tells us
that, thinking it just within the limits of possibility that the brown
coloration of the Cockroach might be due to the presence of silver, he
analysed one pound weight of Blatta. He found no silver, but plenty
of iron, and a remarkable quantity of manganese. That light has some
action upon the colouring matter seems to be indicated by the fact that
in a newly-moulted Cockroach the dorsal surface darkens first.

[30] Anatomy of the Blow-fly, p. 11.

[31] Q. J. Micr. Sci., 1871, p. 394.

Chitin is not peculiar to Insects, nor even to Arthropoda. The pen of
cuttle-fishes and the shell of Lingula contain the same substance,[32]
which is also proved, or suspected, to occur in many other animals.

[32] Krukenberg. Vergl. Physiologische Vorträge, p. 200. Halliburton,
Q. J. Micr. Sci., 1885, p. 173.

The chemical stability of chitin is so remarkable that we might well
expect it to accumulate like the inorganic constituents of animal
skeletons, and form permanent deposits. Schlossberger[33] has, however,
shown that it changes slowly under the action of water. Chitin kept
for a year under water partially dissolved, turned into a slimy mass,
and gave off a peculiar smell. This looks as if it were liable to
putrefaction. The minute proportion of nitrogen in its composition may
explain the complete disappearance of chitin in nature.

[33] Ann. d. Chem. u. Pharm., Bd. 98.

_The Chitinous Cuticle._

The chitinous exoskeleton is rather an exudation than a true tissue. It
is not made up of cells, but of many superposed laminæ, secreted by
an underlying epithelium, or “chitinogenous layer.” This consists of
a single layer of flattened cells, resting upon a basement membrane.
A cross-section of the chitinous layer, or “cuticle,” examined with a
high power shows extremely close and fine lines perpendicular to the
laminæ. The cells commonly form a mosaic pattern, as if altered in
shape by mutual pressure. The free surface of the integument of the
Cockroach is divided into polygonal, raised spaces. Here and there an
unusually long, flask-shaped, epithelial cell projects through the
cuticle, and forms for itself an elongate chitinous sheath, commonly
articulated at the base; such hollow sheaths form the hairs or setæ of
Insects--structures quite different histologically from the hairs of
Vertebrates.

[Illustration: Fig. 8.--Diagram of Insect integument, in section. _bm_,
basement membrane; _hyp_, hypodermis, or chitinogenous layer; _ct_,
_ct_′, chitinous cuticle; _s_, a seta.]

The polygonal areas of the cuticle correspond each to a chitinogenous
cell. Larger areas, around which the surrounding ones are radiately
grouped, are discerned at intervals, and these carry hairs, or give
attachment to muscular fibres.

Viallanes (loc. cit.) has added some interesting details to what was
previously known of Insect-hairs. There are, he points out, two kinds
of hairs, distinguished by their size and structure. The smaller
spring from the boundary between contiguous polygonal areas, and have
no sensory character. The larger ones occupy unusually large areas,
surmount chitinogenous cells of corresponding size, and receive a
special nervous supply. The nerve[34] expands at the base of the hair
into a spindle-shaped, nucleated mass (bipolar ganglion-cell), from
which issues a filament which traverses the axis of the hair, piercing
the chitinogenous cell, whose protoplasm surrounds it with a sheath
which is continued to the tip of the hair. Such sensory hairs are
abundant in parts which are endowed with special sensibility.

[34] Previously observed by Leydig in _Corethra_.

[Illustration: Fig. 9.--Nerve-ending in skin of Stratiomys larva. _h_,
hairs; _b_, their chitinous base; _c_, nucleus of generating cell; _g_,
ganglion cell. × 250. Copied from Viallanes.]

[Illustration: Fig. 10.--Diagram of sensory hair of Insect. _Cc_,
chitinous cuticle; _h_, hair; _c_, its generating cell; _g_, ganglion
cell; _bm_, basement-membrane.]

The chitinous cuticle is often folded in so as to form a deep pit,
which, looked at from the inside of the body, resembles a lever, or a
hook. Such inward-directed processes serve chiefly for the attachment
of muscles, and are termed _apodemes_ (_apodemata_). A simple example
is afforded by the two glove-tips which lie in the middle line of the
under-surface of the thorax (p. 58, and fig. 27). In other cases the
pit is closed from the first, and the apodeme is formed in the midst
of a group of chitinogenous cells distant from the superficial layer,
though continuous therewith. Many tendons of insertion are formed in
this way. The two forked processes in the floor of the thorax (p. 58,
and fig. 27) are unusually large and complex structures of the same
kind. In the tentorium of the head (p. 39, and fig. 17) a pair of
apodemes are supposed to unite and form an extensive platform which
supports the brain and gullet.

[Illustration: Fig. 11.--Nymph (in last larval stage) escaping from old
skin. × 2-1/2.]

Like other Arthropoda, Insects shed their chitinous cuticle from time
to time. A new cuticle, at first soft and colourless, is previously
secreted, and from it the old one gradually becomes detached. The setæ
probably serve the same purpose as the “casting-hairs” described by
Braun in the crayfish, and by Cartier in certain reptiles,[35] that is,
they mechanically loosen the old skin by pushing beneath it. In many
soft-bodied nymphs the new skin can be gathered up into a multitude of
fine wrinkles, which facilitate separation, but we have not found such
wrinkles in the Cockroach, except in the wings. The integument about
to be shed splits along the back of the Cockroach, from the head to
the end of the thorax,[36] and the animal draws its limbs out of their
discarded sheaths with much effort. It is remarkable that the long,
tapering, and many-jointed antennæ are drawn out from an entire sheath.
At the same time the chitinous lining of the tracheal tubes is cast,
while that of the alimentary canal is broken up and passed through the
body.

[35] A condensed and popular account of these researches will be
found in Semper’s Animal Life, p. 20.

[36] Prof. Huxley (Anat. Invert. Animals, p. 419) states that the
integument splits along the abdomen also, but this is a mistake.

[Illustration: Fig. 12.--Cast skin of older nymph (“pupa”). × 2-1/2.]

Prolonged boiling in caustic potash, though it dissolves the viscera,
does not disintegrate the exoskeleton. This shows that the segments
of the integument are not separate chitinous rings, but thickenings
of a continuous chitinous investment. Nevertheless, their constancy
in position and their conformity in structure often enable us to
trace homologies between different segments and different species as
certainly as between corresponding elements of the osseous vertebrate
skeleton.

_Parts of a Somite._

Audouin’s laborious researches into the exoskeleton of Insects[37]
resulted in a nomenclature which has been generally adopted. He divides
each somite (segment) into eight pieces, grouped in pairs--viz.,
_terga_ (dorsal plates), _sterna_ (ventral plates), _epimera_ (adjacent
to the terga), and _episterna_ (adjacent to the sterna).

[37] Audouin. Rech. anat. sur le thorax des Insectes, &c. (Ann. Sci.
Nat., Tom I., p. 97. 1824.)

While admitting the usefulness of these terms, we must warn the reader
not to suppose that this subdivision is either normal or primitive. The
eight-parted segment exists in no single larval or adult Arthropod.
Lower forms and younger stages take us further from such a type,
instead of nearer to it; and Audouin’s theoretical conception is most
fully realised in the thorax of an adult Insect with well-developed
legs and wings.

The morphologist would derive all the varieties of Arthropod segments
from the very simple and uniform chitinous cuticle found in Annelids
and many Insect-larvæ. This becomes differentiated by unequal
thickening and folding in, and a series of rings connected by flexible
membranes is produced. Locomotive and respiratory activity commonly
lead to the definition of terga and sterna, which are similarly
attached to each other by flexible membranes. A pair of limbs may
next be inserted between the terga and sterna, and the simple segment
thus composed occurs so extensively in the less modified regions and
in early stages that it may well be considered the typical Arthropod
somite.

Special needs may lead to the division of the sterna into lateral
halves, but this is purely an adaptive change. The third thoracic
sternum of the male Cockroach, and the second and third of the female
are thus divided, as is also the hinder part of the seventh abdominal
sternum of the female.

In an early stage every somite has its tergal region divided into
lateral halves, owing to the late completion of the body on this side.
Traces of this division may survive even in the imago. There is often
a conspicuous dorsal groove in the thoracic terga, and at the time of
moult the terga split along an accurately median line (see fig. 12).

Additional pieces may be developed between the terga and sterna, and
these have long been termed _pleural_.[38] There may be, for example,
single stigmatic plates, as in the abdomen of the Cockroach, pieces to
support the thoracic legs, and pieces to support the wings; but the
number and position of these plates depends upon their immediate use,
and their homologies become very uncertain when Insects of different
orders are compared. In general, the pleural elements of the segment
are late in development, variable, and highly adaptive.

[38] This application of the word to denote parts intermediate
between terga and sterna has become general since its adoption
by Audouin. It appears also in the older and deservedly obsolete
nomenclature of Kirby and Spence. Professor Huxley has unfortunately
disturbed the consistent use of this term by giving the name _pleura_
to the free edges of the terga in Crustacea.

_Somites of the Cockroach._

The exoskeleton of the Cockroach is divisible into about seventeen
segments, which are grouped into three regions, as follows:--

Head {Procephalic lobes
{Post-oral segments 3

Thorax 3[39]

Abdomen 11
--
17
--

[39] Where the thorax apparently consists of four somites, as in
some Hymenoptera, Hemiptera, Coleoptera, and Lepidoptera, the first
abdominal segment has become blended with it.

It is a strong argument in favour of this estimate that many Insects,
at the time when segmentation first appears, possess seventeen
segments.[40] The procephalic lobes, from which a great part of the
head, including the antennæ, is developed, are often counted as an
additional segment.[41]

[40] Balfour. Embryology, Vol. I., p. 337.

[41] _E.g._, by Graber. Insekten, Vol. II., p. 423.

The limbs, which in less specialised Arthropoda are carried with
great regularity on every segment of the body, are greatly reduced
in Insects. Those borne by the head are converted into sensory and
masticatory organs; those on the abdomen are either totally suppressed,
or extremely modified, and only the thoracic limbs remain capable of
aiding in locomotion.

The primitive structure of the Arthropod limb is adapted to
locomotion in water, and persists, with little modification, in
most Crustacea. Here we find in most of the appendages[42] a basal
stalk (_protopodite_), often two-jointed, an inner terminal branch
(_endopodite_), and an outer terminal branch (_exopodite_), each of
the latter commonly consisting of several joints. It does not appear
that the appendages of Insects conform to the biramous Crustacean type,
though the ends of the maxillæ are often divided into an outer and an
inner portion.

[42] See, for example, Huxley on the Crayfish.

We shall now proceed to describe, in some detail, the regions of the
body of the adult Cockroach.

_Head; Central Parts._

The head of the Cockroach, as seen from the front, is pear-shaped,
having a semi-circular outline above, and narrowing downwards. A
side-view shows that the front and back are flattish, while the top and
sides are regularly rounded. In the living animal the face is usually
inclined downwards, but it can be tilted till the lower end projects
considerably forward. The mouth, surrounded by gnathites or jaws,
opens below. On the hinder surface is the occipital foramen, by which
the head communicates with the thorax. A rather long neck allows the
head to be retracted beneath the pronotum (first dorsal shield of the
thorax) or protruded beyond it.

[Illustration: Fig. 13.--Front of Head. × 10.]

On the front of the head we observe the clypeus, which occupies a large
central tract, extending almost completely across the widest part of
the face. It is divided above by a sharply bent suture from the two
epicranial plates, which form the top of the head as well as a great
part of its back and sides. The labrum hangs like a flap from its lower
edge. A little above the articulation of the labrum the width of the
clypeus is suddenly reduced, as if a squarish piece had been cut out of
each lower corner. In the re-entrant angle so formed, the ginglymus, or
anterior articulation of the mandible, is situated.

The labrum is narrower than the clypeus, and of squarish shape,
the lower angles being rounded. It hangs downwards, with a slight
inclination backwards towards the mouth, whose front wall it forms.
On each side, about halfway between the lateral margin and the middle
line, the posterior surface of the labrum is strengthened by a vertical
chitinous slip set with large setæ. Each of these plates passes above
into a ring, from the upper and outer part of which a short lever
passes upwards, and gives attachment to a muscle (_levator menti_).

[Illustration: Fig. 14.--Top of Head. _ep_, epicranial plate; _oc_,
eye; _ge_, gena. × 10.]

The top and back of the head are defended by the two epicranial
plates, which meet along the middle line, but diverge widely as they
descend upon the posterior surface, thus enclosing a large opening,
the occipital foramen. Beyond the foramen, they pass still further
downwards, their inner edges receding in a sharp curve from the
vertical line, and end below in cavities for the articulation of the
mandibular condyles.[43]

[43] One of the few points in which we have to differ from the
admirable description of the Cockroach given in Huxley’s Comparative
Anatomy of Invertebrated Animals, relates to the articulation of the
mandible, which is there said to be carried by the gena.

[Illustration: Fig. 15.--Side of Head. _oc_, eye; _ge_, gena; _mn_,
mandible. × 10.]

The sides of the head are completed by the eyes and the genæ. The large
compound eye is bounded above by the epicranium; in front by a narrow
band which connects the epicranium with the clypeus; behind, by the
gena. The gena passes downwards between the eye and the epicranial
plate, then curves forwards beneath the eye, and just appears upon
the front of the face, being loosely connected at this point with the
clypeus. Its lower edge overlaps the base of the mandible, and encloses
the extensor mandibulæ.

The occipital foramen has the form of an heraldic shield. Its lateral
margin is strengthened by a rim continuous with the tentorium, or
internal skeleton of the head. Below, the foramen is completed
by the upper edge of the tentorial plate, which nearly coincides
with the upper edge of the submentum (basal piece of the second
pair of maxillæ); a cleft, however, divides the two, through which
nerve-commissures pass from the sub-œsophageal to the first thoracic
ganglion. Through the occipital foramen pass the œsophagus, the
salivary ducts, the aorta, and the tracheal tubes for the supply of air
to the head.

[Illustration: Fig. 16.--Back of Head. _ca_, cardo; _st_, stipes; _ga_,
galea; _la_, lacinia; _pa_, palp; _sm_, submentum; _m_, mentum; _pg_,
paraglossa. × 10.]

The internal skeleton of the head consists of a nearly transparent
chitinous septum, named _tentorium_ by Burmeister, which extends
downwards and forwards from the lower border of the occipital foramen.
In front it gives off two long crura, or props, which pass to the
ginglymus, and are reflected thence upon the inner surface of the
clypeus, ascending as high as the antennary socket, round which they
form a kind of rim. Each crus is twisted, so that the front surface
becomes first internal and then posterior as it passes towards the
clypeus. The form of the tentorium is in other respects readily
understood from the figure (fig. 17). Its lower surface is strengthened
by a median keel which gives attachment to muscles. The œsophagus
passes upwards between its anterior crura, the long flexor of the
mandible lies on each side of the central plate; the supra-œsophageal
ganglion rests on the plate above, and the sub-œsophageal ganglion
lies below it, the nerve-cords which unite the two passing through the
circular aperture. A similar internal chitinous skeleton occurs in the
heads of other Orthoptera, as well as in Neuroptera and Lepidoptera.
Palmén[44] thinks that it represents a pair of stigmata or spiracles,
which have thus become modified for muscular attachment, their
respiratory function being wholly lost. In Ephemera he finds that the
tentorium breaks across the middle when the skin is changed, and each
half is drawn out from the head like the chitinous lining of a tracheal
tube.

[44] Morphologie des Tracheen-systems, p. 103.

[Illustration: Fig. 17.--Fore-half of Head, with tentorium, seen from
behind. × 12.]

_Antennæ; Eyes._

A pair of antennæ spring from the front of the head. In the male of
the common Cockroach they are a little longer than the body; in the
female rather shorter. From seventy-five to ninety joints are usually
found, and the three basal joints are larger than the rest. Up to about
the thirtieth, the joints are about twice as wide as long; from this
point they become more elongate. The joints are connected by flexible
membranes, and provided with stiff, forward-directed bristles. The
ordinary position of the antennæ is forwards and outwards.

[Illustration: Fig. 18.--Base of Antenna of Male (to left) and Female
(to right). × 24.]

Each antenna is attached to a relatively large socket (fig. 15), which
lies between the epicranium and clypeus, to the front and inner side of
the compound eyes. A flexible membrane unites the antenna to the margin
of the socket, from the lower part of which a chitinous pin projects
upwards and supports the basal joint.

It is well known that in many Crustacea two pairs of antennæ are
developed, the foremost pair (antennules) bearing two complete
filaments. Some writers have suggested that both pairs may be present
in Insects, though not simultaneously, the Crustacean antennule being
found in the larva, and the Crustacean antenna in the adult. This view
was supported by the familiar fact that in many larvæ the antennæ are
placed further forward than in the adult. The three large joints at the
base of Orthopterous antennæ have been taken to correspond with those
of Crustacean antennules, and it has been inferred that in Insects
with incomplete metamorphosis, only antennules or larval antennæ are
developed.[45] This reasoning was never very cogent, and it has been
impaired by further inquiry. Weismann has shown that in _Corethra
plumicornis_, the adult antenna, though inserted much further back than
that of the larva, is developed within it,[46] and Graber has described
a still more striking case of the same thing in a White Butterfly.[47]
There is, therefore, no reason to suppose that Insects possess more
than one pair of antennæ, which is probably preoral, not corresponding
with either of the Crustacean pairs.

[45] Zaddach, Entw. des Phryganiden Eies, p. 86; Rolleston, Forms of
Animal Life, p. 75, &c.

[46] Zeits. f. wiss. Zool., Bd. XVI., pl. vii., fig. 33.

[47] Insekten, Vol. II., p. 508.

We have already noticed (p. 26) the superficial points in which the
antenna of the male Cockroach differs from that of the female.

The eyes of some Crustacea are carried upon jointed appendages, but
this is never the case in Insects, though the eye-bearing surface
may project from the head, as in _Diopsis_ and _Stylops_. Professor
Huxley[48] supposes that the head of an Insect may contain six somites,
the eyes representing one pair of appendages. The various positions
in which the eyes of Arthropoda may be developed weakens the argument
drawn from the stalk-eyed Crustacea. Claus and Fritz Müller go so far
on the other side as to deny the existence of an eye-segment even in
Crustacea.

[48] Anat. Invert. Animals, p. 398.

_Mouth-parts of the Cockroach._

Before entering upon a full description of the mouth-parts of the
Cockroach, which present some technical difficulties, the beginner in
Insect anatomy will find it useful to get a few points of nomenclature
fixed in his memory. Unfortunately, the terms employed by entomologists
are at times neither convenient nor philosophical.

There are three pairs of jaws, disposed behind the labrum, as in the
diagram:--

LABRUM.
1st pair of Jaws (MANDIBLES).
2nd " (MAXILLÆ).
3rd " (LABIUM, or 2nd pair of Maxillæ).

[Illustration: Fig. 19.--Diagram of Cockroach Jaws, in horizontal
section.]

The mandible is undivided in all, or nearly all, Insects. Each maxilla
may consist of

A _palp_ on the outer side,
A _galea_ (hood),
A _lacinia_ (blade), on the inner side.

The galea (hood) of the 3rd pair of jaws is sometimes called the
_paraglossa_.

A tongue-like process may be developed from the front wall of the mouth
(_epipharynx_), or from the back wall (_hypopharynx_ or _lingua_).[49]
Both epipharynx and hypopharynx project into the mouth, and, in some
Diptera, far beyond it.

[49] Professor Huxley has proposed to call the attached base
_hypopharynx_, and the free tip _lingua_.

The tip of the labium is sometimes produced into a long tongue, called
the _ligula_ (strap).

The mouths of Insects may be classed as:--

BITING.--Orthoptera, Neuroptera, Coleoptera (in some Coleoptera a
licking tongue is developed), most Hymenoptera.

LICKING AND SUCKING.--Some Hymenoptera--_e.g._, Honey Bee.

SUCKING.--(_a_) With lancets--Diptera, Hemiptera. (_b_) Without
lancets--Lepidoptera.

The reference of these to a common plan, and the determination of the
constituent parts, is mainly the work of Savigny. Mouth-parts were made
the basis of the classification of Insects by Fabricius (1745–1808).

[Illustration: Fig. 20.--The Jaws, separated. _Mn_, mandible, seen from
behind (to left) and front (to right); _Mx_' maxilla (first pair);
_Mx_" labium, or second pair of maxillæ. The other letters as before.
× 20.]

The mandibles of the Cockroach are powerful, single-jointed[50] jaws,
each of which is articulated by a convex “condyle” to the lower end
of the epicranial plate, and again by a concave “ginglymus” to the
clypeus. The opposable inner edges are armed with strong tooth-like
processes of dense chitin, which interlock when the mandibles close;
those towards the tip of the mandible are sharp, while others are
blunt, as if for crushing. Each mandible can be moved through an angle
of about 30°. A flexible chitinous flap extends from its inner border
to the labrum. The powerful flexor of the mandible arises within the
epicranial vault; its fibres converge to a chitinous tendon, which
passes outside the central plate of the tentorium, and at a lower
level through a fold on the lower border of the clypeus, being finally
inserted near the ginglymus. A short flexor arises from the crus of the
tentorium. The extensor muscle arises from the side of the head, passes
through the fold formed by the lower end of the gena, and is inserted
close to the outer side of the condyle of the mandible.

[50] Professor J. Wood-Mason points out that in _Machilis_ (one of
the Thysanura) the mandible shows signs of segmentation, while the
apical portion is deeply divided into an inner and an outer half.
Ripe embryos of _Panesthia (Blatta) javanica_ are said to exhibit
folds which indicate the consolidation of the mandible out of
separate joints, while the cutting and crushing portions of the edge
are divided by a “sutural mark,” which may correspond to the line of
junction of the divisions of a biramous appendage (Trans. Ent. Soc.,
1879, pt. 2, p. 145).

The anterior maxillæ lie behind the mandibles, and like them are
unconnected with each other. They retain much more of the primitive
structure of a gnathite than the mandibles, in which parts quite
distinct in the maxillæ are condensed or suppressed. The constituent
pieces are seen in fig. 20. There is a two-jointed basal piece,
consisting of the _cardo_ (_ca_) and the _stipes_ (_st_). The cardo
is a transverse plate bent upon itself, and enclosing muscles; it is
attached to the outward-directed pedicel of the occipital frame, and
carries the vertical stipes. To the side and lower end of the stipes
is attached the five-jointed palp (_pa_), a five-jointed limb used in
feeding and in exploration, while the lacinia (_la_) and galea (_ga_)
are articulated to its extremity. The lacinia is internal and posterior
to the galea; it is broad above, but narrows below to a bifid tooth of
dense chitin; its inner surface is beset with a cluster of strong setæ.
The galea is more flexible, and forms an irregular three-cornered
prism with an obliquely truncated end, upon which are many fine hairs.
A flexible and nearly transparent flap connects the inner edges of the
stipes and cardo, and joins both to the labium. The muscles which move
the bases of the maxillæ spring from the crura, central plate, and keel
of the tentorium.

On the posterior surface of the head, below the occipital foramen,
we find a long vertical flap, the labium, which extends downwards to
the opening of the mouth. It represents a second pair of maxillæ,
fused together in their basal half, but retaining elsewhere sufficient
resemblance to the less modified anterior pair to permit of the
identification of their component parts. The upper edge is applied to
the occipital frame, but is neither continuous with that structure nor
articulated thereto. By stripping off the labium upwards it may be seen
that it is really continuous with the chitinous integument of the neck.
The broad shield-like base is incompletely divided by a transverse
hinge into an upper and larger piece, the _submentum_, and a distal
piece, the _mentum_. To the mentum are appended representatives of the
galeæ (here named _paraglossæ_) and laciniæ, while a three-jointed
palp with an additional basal joint (distinguished as the _palpiger_)
completes the resemblance to the maxillæ of the first pair.[51]
In front of the labium, and lying in the cavity of the mouth is a
chitinous fold of the oral integument, the _lingua_, which lies like
a tongue in the floor of the mouth. The common duct of the salivary
glands enters the lingua, and opens on its hinder surface. The lingua
is supported by the chitinous skeleton represented in the figures of
the salivary glands. (Chap. vii., _infra_.)

[51] The homology of the labium with the first pair of maxillæ is in
no other Insects so distinct as in the Orthoptera.

The epipharynx, which is a prominent part in Coleoptera and Diptera, is
not recognisable in Orthoptera.

_Functions of the Antennæ and Mouth-parts._

We must now shortly consider the functions of the parts just described.
The antennæ have long been regarded as sense-organs, and even the
casual observer can hardly fail to remark that they are habitually
used by the Insect to gain information concerning its immediate
surroundings. Long antennæ, such as those of the Cockroach, are
certainly organs of touch, but it has been much disputed whether they
may not also be the seat of some special sense, and if so, what that
sense may be. Several authors have found reason to suppose that in the
Insect-antenna resides the sense of hearing, but no evidence worth the
name is forthcoming in favour of this view. Much better support can be
found for the belief that the antenna is an olfactory organ,[52] and
some experiments which seem conclusive on this point will be cited in a
later chapter.

[52] Rosenthal, Ueb. d. Geruchsinn der Insekten. Arch. f. Phys. Reil
u. Autenrieth, Bd. X. (1811). Hauser, Zeits. f. wiss. Zool., Bd.
XXXIV. (1880).

In the Cockroach the mandibles and maxillæ are the only important
instruments of mastication. The labium is indirectly concerned
as completing the mouth behind and supporting the lingua, which
is possibly of importance in the ordinary operations of feeding.
Plateau[53] has carefully described the mode of mastication as observed
in a Carabus, and his account seems to hold good of biting Insects in
general. The mandibles and maxillæ act, as he tells us, alternately,
one set closing as the others part. The maxillæ actually push the
morsel into the buccal cavity. When the mandibles separate, the head
is slightly advanced, so that the whole action has some superficial
resemblance to that of a grazing quadruped.

[53] Mém. Acad. Roy. de Belgique, Tom. XLI. (1874). Prof. Plateau’s
writings will often be referred to in these pages. We owe to him the
most important researches into the physiology of Invertebrates which
have appeared for many years.

The palps of the maxillæ and labium have been variously regarded as
sensory and masticatory instruments. Not a few authors believe that
they are useful in both ways. The question has lately been investigated
experimentally by Plateau,[54] who finds that removal of both maxillary
and labial palps does not interfere either with mastication or the
choice of food. He observes that in the various Coleoptera and
Orthoptera submitted to experiment the palps are passive while food is
being passed into the mouth.

[54] Exp. sur le Rôle des Palpes chez les Arthropodes Maxillés. Pt.
I. Bull. Soc. Zool. de France, Tom. X. (1885).

Plateau’s experiments are conclusive as to the subordinate value of the
palps in feeding. The observation of live Cockroaches has satisfied
us that the palps are constantly used when the Insect is active,
whether feeding or not, to explore the surface upon which it moves.
We have seen no ground for attributing to the palps special powers
of perceiving odours or flavours, nor have we observed that they aid
directly in filling the mouth with food.

It is worthy of note that Leydig has described and figured in the
larva of _Hydroporus_ (?), and Hauser in _Dytiscus_, _Carabus_, &c., a
peculiar organ, apparently sensory, which is lodged in the maxillary
and labial palps. It consists of whitish spots, sometimes visible to
the naked eye, characterised by unusual thinness of the chitinous
cuticle and by the aggregation beneath it of a crowd of extremely
minute sensory rods. Of this organ no satisfactory explanation has yet
been given.[55]

[55] Leydig, Taf. z. vergl. Anat., pl. x., fig. 3. Hauser, Zeits.
f. wiss. Zool., Bd. XXXIV., p. 386. Jobert has figured the sensory
organs of the maxillary palps of the Mole-cricket (Ann. Sci. Nat.,
1872), and Forel similar organs in Ants (Bull. Soc. Vaudoise, 1885).

_Comparison of Mouth-parts in different Insects._

The jaws of the Cockroach form an excellent standard of comparison
for those of other Insects, and we shall attempt to illustrate the
chief variations by referring them to this type.[56] Mouth-parts
are so extensively used in the classification of Insects that every
entomologist ought to have a rational as well as a technical knowledge
of their comparative structure. No part of Insect anatomy affords
more striking examples of adaptive modification. In form, size, and
mode of application the jaws vary extremely. It would be hard to find
feeding-organs more unlike, at first sight, than the stylets of a Gnat
and the proboscis of a Moth, yet the study of a few well-selected types
will satisfy the observer that both are capable of derivation from a
common plan. Nor is this common plan at all vague. It is accurately
pictured in the jaws of the Cockroach and other Orthoptera. These
correspond so entirely with the primitive arrangement, inferred by a
process of abstraction from the most dissimilar Insects, as to furnish
a strong argument for the descent of all higher Insects from forms not
unlike Orthoptera in the structure of their mouth-parts.

[56] The reader who desires to follow this subject further is
recommended to study chap. vi. of Graber’s Insekten, which we have
found very useful.

[Illustration: Fig. 21.--Embryo of Aphis. Copied from Mecznikow, Zeits.
f. wiss. Zool., Bd. XVI., taf. xxx., fig. 30. References in text.
× 220.]

Though the jaws of the Cockroach are eminently primitive with respect
to those of most other Insects, they are themselves derived from a far
simpler arrangement, which is demonstrable in all embryonic Insects.
Fig. 21 shows an Aphis within the egg. The rudiments of the antennæ
(_At_), mandibles (_Mn_), and maxillæ (_Mx^1_, _Mx^2_) form simple
blunt projections, similar to each other and to the future thoracic
legs (_L^1_, _L^2_, _L^3_). We see, therefore, that all the appendages
of an Insect are similar in an early stage of growth; and we may add
that a Centipede, a Scorpion, or a Spider would present very nearly
the same appearance in the same stage. A Crustacean in the egg would
not resemble an Insect or its own parent so closely.[57] Aquatic life
favours metamorphosis, and most Crustacea do not begin life with their
full quota of legs, but acquire them as they are wanted.

[57] Freshwater Crustacea, however, are sometimes similar to their
parents at the time of hatching.

Paired appendages of perfectly simple form are therefore the first
stage through which all Insect-jaws must pass. Our second stage is
a little more complex, and not nearly so universal as the first. A
caterpillar (fig. 22) has its own special wants, and these are met
by the unequal development of its jaws. The mandibles are already as
complete as those of the Cockroach, which they closely resemble, but
the maxillæ are stunted cylinders formed mainly of simple rings, and
very like the antennæ. They show, however, the beginnings of three
processes (palp, galea, and lacinia), which are usually conspicuous
in well-developed maxillæ. The second pair of maxillæ (_Lm_) are
coalesced, as usual, and form the spinneret. The mouth-parts of the
Caterpillar do not therefore in all respects represent a universal
stage of development, but show important adaptive modifications. The
mandibles are rapidly pushed forward, and attain their full development
in the larva; the first pair of maxillæ are temporarily arrested
in their growth, and persist for a long time in a condition which
Orthopterous embryos quickly pass through; the maxillæ of the second
pair are not only arrested in their growth, but converted to a special
use, which seems to stop all further progress. The labial palps,
indeed, which are not at all developed in the caterpillar, survive, and
become important parts in the moth; but the greater part of the labium
disappears when the time for spinning the cocoon is over.

[Illustration: Fig. 22.--Head of larva of Goat Moth, seen from behind.
Copied from Lyonnet.]

We come next to the Orthopterous mouth, which is well illustrated
by the Cockroach. This is retained with little modification in all
the biting Insects (Coleoptera and Neuroptera). The mandibles may
become long and pointed, as in _Staphylinus_ and other predatory
forms; in some larvæ of strong carnivorous propensities (Ant-lion,
_Dytiscus_,[58] _Chrysopa_) they are perforate at the tip, and through
them the juices of the prey are sucked into the mouth, which has no
other opening. The labium undergoes marked adaptive change, without
great deviation from the common plan, in the “mask” of the larva of
the Dragon-fly. This well-known implement has a rough likeness, in the
arrangement and use of its parts, to a man’s fore-limb. The submentum
forms the arm, the mentum the fore-arm. Both these are simple, straight
pieces, connected by an elbow-joint. The hand is wider, and carries
a pair of opposable claws, the paraglossæ. In some Coleoptera the
labium is reduced to a stiff spine, while in the Stag-beetle it is
flexible and hairy, and foreshadows the licking tongue of the Bee. The
maxillæ become long and hairy in flower-haunting Beetles, and even
the mandibles are flexible and hairy in the Scarabæus-beetles. Fritz
Müller has found a singular resemblance to the proboscis of a Moth in
a species of _Nemognatha_, where the maxillæ are transformed into two
sharp grooved bristles 12 mm. long, which, when opposed, form a tube,
but are incapable of rolling up.[59]

[58] In _Dytiscus_ the mandibles are perforate at the base, and not
at the tip. See Burgess in Proc. Bost. Soc. Nat. Hist., Vol. XXI.,
p. 223.

[59] Ein Käfer mit Schmetterlingsrüssel, Kosmos, Bd. VI. We take this
reference from Hermann Müller’s Fertilisation of Flowers.

In the Honey Bee (fig. 23) nearly all the mouth-parts of the Cockroach
are to be made out, though some are small and others extremely produced
in length. The mandibles (_Mn_) are not much altered, and are still
used for biting, as well as for kneading wax and other domestic work.
The mandibular teeth have proved inconvenient, and are gone. The
lacinia of the maxilla (_Mx_′) forms a broad and flexible blade, used
for piercing succulent tissues, but the galea has disappeared, and
there is only a vestige of the maxillary palp (_Mxp_). In the second
pair of maxillæ the palp (_Lp_) is prominent; its base forms a blade,
while the tip is still useful as an organ of touch. The paraglossæ
(_Pa_) can be made out, but the laciniæ are fused to form the long,
hairy tongue. This ends in a spoon-shaped lobe (not unlike the “finger”
of an elephant’s trunk), which is used both for licking and for sucking
honey.

The proboscis of the Bee is therefore more like a case of instruments
than a single organ. The mandibles form a strong pair of blunt
scissors. The maxillæ are used for piercing, for stiffening and
protecting the base of the tongue, and when closed they form an
imperfect tube outside the tongue, which, according to Hermann Müller,
is probably suctorial. The labial palps are protective and sensory.
Lastly, the central part, or tongue, is a split tube used for suction;
it is very long, so as to penetrate deep flower-cups, and hairy, so
that pollen may stick to it. When the proboscis is not in use it can
be slid into the mentum (_M_), while it and the mentum together can be
drawn out of the way downwards and backwards.[60]

[60] An interesting account of the structure and mode of action of
the Bee’s tongue is to be found in Hermann Müller’s Fertilisation of
Flowers, where also the evolution of the parts is traced through a
series of graduated types.

[Illustration: Fig. 23.--Mouth-parts of Honey Bee.]

[Illustration: Fig. 23A.--Diagram of Mouth-parts of Honey Bee.]

[Illustration: Fig. 24.--Mouth-parts of Burnet Moth.]

[Illustration: Fig. 24A.--Diagram of Mouth-parts of Moth.]

In the singular suctorial mouth of Moths and Butterflies we observe,
first of all, the great development of the maxillæ. Each forms a
half-tube, which can be accurately applied to its fellow, so as to
form an efficient siphon. In many species the two halves can be held
together by a multitude of minute hooks.[61] At the base of each
maxilla is a rudimentary palp (_Mxp_). The mandibles (_Mn_) are also
rudimentary and perfectly useless. The labium, which was so important
to the larva as a spinneret, has disappeared almost completely, but the
labial palps (_Lp_) are large and evidently important.

[61] See Newport’s figure of Vanessa atalanta (Todd’s Cyc., Art.
Insecta), or Burgess on the Anatomy of the Milk-weed Butterfly, in
Anniversary Mem. of Boston Soc. Nat. Hist., pl. ii., figs. 8–10
(1880).

[Illustration: Fig. 25.--Mouth-parts of Gad-fly (_Tabanus_).]

[Illustration: Fig. 25A.--Diagram of Mouth-parts of Gad-fly.]

In Diptera both piercing and sucking parts are usually present. The
Gad-fly (fig. 25) is typical. Here we recognise the labrum (_Lbr_),
mandible (_Mn_), and maxilla (_Mx_′) of the Cockroach transformed
into stylets. The maxillary palp (_Mxp_) is still sensory. A pointed
process, stiffened by chitinous ribs, is developed from the back of
the labrum. This is the epipharynx (_Ep_), a process undeveloped in
the Cockroach, though conspicuous in some Coleoptera. All these parts
are overtopped by the suctorial labium (_Lm_), which has a two-lobed
expansion at the end. In the more specialised Diptera this becomes a
kind of cupping-glass. The Gad-fly is intermediate between the Gnat,
in which all the mouth-parts are converted into piercing organs of
extraordinary length and sharpness, and such flies as the House-fly and
Blow-fly, where the sucking labium forms an organ of the most elaborate
kind, the piercing organs undergoing a marked reduction. Except where
the labium is short, it is doubly or trebly hinged, so that it can be
readily tucked away under the chin.

In Hemiptera the long four-jointed labium (_Lm_) forms a sheath for
the stylets. When not in use the whole apparatus is drawn up beneath
the head and prothorax. The mandibles (_Mn_) are sharp at the tip, and
close like a pair of forceps, enclosing the maxillæ (_Mx_). These are
of unequal length, only one reaching the end of the mandibular case.
Both have saw teeth on the free edge. Palps are entirely wanting.

[Illustration: Fig. 26.--Mouth-parts of Bug. Copied from Landois,
Zeits. f. wiss. Zool., Bd. XVIII., taf. xi., fig. 3.]

[Illustration: Fig. 26A.--Diagram of Mouth-parts of Bug.]

Comparing the four kinds of suctorial mouths, of which the Bee, the
Moth, the Fly, and the Bug furnish examples, we observe that the
sucking-tube is formed in the Moth out of the two maxillæ, in the
other three out of the labium. Of these last the Bee has the edges of
the labium turned _down_, so that the siphon becomes _ventral_; in
the Bug and Fly the edges are turned _up_, and the siphon becomes
_dorsal_. The more specialised flies have the simple arrangement of the
Bug complicated by a system of branching tubes, which are probably a
special modification of the salivary duct. Similar as the mouth-parts
of the four types may be in regard to their mode of working, they
cannot be reduced to any common plan which differs materially from that
presented by the jaws of the Cockroach.

_Composition of Head._

In all Insects fusion of the primitive elements of the head begins
so early and is carried so far, that it is extremely difficult to
discover the precise way in which they are fitted together. The
following facts have been ascertained respecting the development of
the parts in question. At a very early stage of embryonic life the
body of the Insect becomes divided into a series of segments, which
are at fewest fourteen (in some Diptera), while they are not known to
exceed seventeen.[62] Each segment is normally provided with a pair of
appendages. The foremost segment soon enlarges beyond the rest, and
becomes divided by a median groove into two “procephalic lobes.”[63] Of
the appendages the first eight pairs are usually more prominent than
the rest, and of different form; those of the eighth segment, which may
be altogether inconspicuous, never attain any functional importance.
The first four pairs of appendages are budded off from the future head,
while the next three pairs form the walking legs, and are carried upon
the thoracic segments. All the existing appendages of the fore part of
the body are thus accounted for, but the exact mode of formation of the
head has not yet been made out. The chief part of its walls, including
the clypeus, the compound eyes, and the epicranial plates, arise from
the procephalic lobes, and represent the much altered segment of which
the antennæ are the appendages. The labrum is a secondary outgrowth
from this segment, and, in some cases at least, it originates as a pair
of processes which resemble true appendages, though it is unlikely
that such is their real character. No means at present exist for
identifying the terga and sterna of the head, nor have the gena, the
occipital frame, and the cervical sclerites (described below) been
assigned to their segments.[64] It is worthy of notice that in the
stalk-eyed Crustacea, the head, or what corresponds to the head of
Insecta, consists of either five or six somites, taking into account
a diversity of opinion with respect to the eyestalks, while only four
pairs of appendages can be certainly traced in the head of the Insect.
The mandibles and maxillæ exist to the same number in both groups, and
are homologous organs, so far as is known; the numerical difference
relates therefore to the antennæ, of which the Crustacean possesses two
pairs, the Insect only one. Whether the pair deficient in the Insect is
altogether undeveloped, or represented by the pair of prominences which
give rise to the labrum,[65] is a question of much theoretical interest
and of not a little difficulty.

[62] Balfour, Embryology, Vol. I., p. 337.

[63] Huxley, Med. Times and Gazette, 1856–7; Linn. Trans.,
Vol. XXII., p. 221, and pl. 38 (1858).

[64] “I think it is probable that these cervical sclerites represent
the hindermost of the cephalic somites, while the band with which the
maxillæ are united, and the genæ, are all that is left of the sides
and roof of the first maxillary and the mandibular somites.”--Huxley,
Anat. Invert. Animals, p. 403.

[65] Balfour, Embryology, Vol. I., note to p. 337.

The following table shows the appendages of the head and thorax in
the two classes. The homologies indicated are, however, by no means
established.[66]

CRAYFISH. | COCKROACH.
|
| Antennæ.
---- | ----
Eyestalks. |
Antennules. |
Antennæ. |
Mandibles. | Mandibles.
Maxillæ (1). | Maxillæ (1).
Maxillæ (2). | Maxillæ (2).
---- | ----
Maxillipeds (1). | Thoracic Legs (1).
Maxillipeds (2). | Thoracic Legs (2).
Maxillipeds (3). | Thoracic Legs (3).

[66] J. S. Kingsley in Q. J. Micr. Sci. (1885), has reviewed the
homology of Insect, Arachnid, and Crustacean appendages, and comes
to conclusions very different from those hitherto accepted. He
classifies the appendages as pre-oral (Insect-antennæ) and post-oral,
and makes the following comparisons:--

Pelseneer (Q. J. Micr. Sci., 1885), concludes that both pairs of
antennæ are post-oral in _Apus_, and probably in all other Crustacea.

_Neck._

The neck is a narrow cylindrical tube, with a flexible wall
strengthened by eight plates, the cervical sclerites, two of which
are dorsal, two ventral, and four lateral. The dorsal sclerites lie
immediately behind the head (fig. 14); they are triangular, and closely
approximated to the middle line. The inferior plates (fig. 27) resemble
segments of chitinous hoops set transversely, one behind the other,
rather behind the dorsal sclerites, and close behind the submentum.
There are two lateral sclerites on each side of the neck (fig. 27), a
lower squarish one, which is set diagonally, nearly meeting its fellow
across the ventral surface, and an oblong piece, closely adherent to
the other, which extends forwards and upwards towards the dorsal side.

_Thorax._

The elements of the thoracic exoskeleton are simpler in the Cockroach
than in Insects of powerful flight, where adaptive changes greatly
obscure the primitive arrangement. There are three segments, each
defended by a dorsal plate (_tergum_) and a ventral plate (_sternum_).
The sterna are often divided into lateral halves. Of the three terga
the first (_pronotum_) is the largest; it has a wide free edge on each
side, projects forwards over the neck, and when the head is retracted,
covers this also, its semi-circular fore-edge then forming the apparent
head-end of the animal. The two succeeding terga are of nearly equal
size, and each is much shorter than the pronotum, contrary to the rule
in winged Insects.[67]

[67] Many Orthoptera, which seize their prey with the fore legs, have
a very long pronotum.

[Illustration: Fig. 27.--Ventral Plates of Neck and Thorax of Male
Cockroach. I, prosternum; II, mesosternum; III, metasternum. × 6.]

All the terga are dense and opaque in the female; in the male the
middle one (_mesonotum_) and the hindmost (_metanotum_) are thin and
semi-transparent, being ordinarily overlaid by the wing-covers. While
the thoracic terga diminish backwards, the sterna increase in extent
and firmness, proportionally to the size of the attached legs. The
prosternum is small and coffin-shaped; the mesosternum partly divided
into lateral halves in the male, and completely so in the female.
The metasternum is completely divided in both sexes, while a median
piece, carrying the post-furca, intervenes between its lateral halves
in the male. Behind the sterna, especially in the case of the second
and third, the flexible under-surface of the thorax is inclined,
so as to form a nearly vertical step. In the two hinder of these
steps a chitinous prop is fixed; each is Y-shaped, with long, curved
arms for muscular attachment, and a central notch, which supports
the nerve-cord. The hindmost of these, known as the _post-furca_,
lies immediately behind the metasternum, and its short basal piece
is attached between the lateral halves of that plate. Behind the
mesosternum is a somewhat slighter prop, the _medi-furca_. A third
piece of similar nature (the _ante-furca_), which is well developed in
some Insects--_e.g._, in Ants--is apparently wanting in the Cockroach,
though there is a transverse oval plate behind the prosternum, which
may be a rudimentary furca.

Fig. 27 shows two conical processes which lie in the middle line of
the ventral surface of the thorax, one in front of the metasternum,
the other in front of the mesosternum. These are the thoracic pits,
tubular apodemata, serving for the insertion of muscles. The occurrence
of stink-glands in the thorax of Hemiptera,[68] and of so-called
poison-glands in the thorax of _Solpuga_, led us to look for glands in
connection with these processes, but we have found none.

[68] Also in _Phasmidæ_ (see Scudder, Psyche, Vol. I., p. 137).

_Thoracic Appendages. Legs; Wings._

Three pairs of legs are attached to the thoracic segments; they
regularly increase in size from the first to the third, but hardly
differ except in size; the peculiar modifications which affect the fore
pair in predatory and burrowing Orthoptera (_Mantis_, _Gryllotalpa_),
and the third pair in leaping Orthoptera (Grasshoppers, &c.), being
absent in the cursorial Blattina. Each leg is divided into the five
segments usual in Insects (see fig. 28). The coxa is broad and
flattened. The trochanter is a small piece obliquely and almost
immovably attached to the proximal end of the femur, on its inner side.
The femur is nearly straight and narrowed at both ends; along its inner
border, in the position occupied by the stridulating apparatus of the
hind leg of the Grasshoppers, is a shallow longitudinal groove, fringed
by stiff bristles. The tibia is shorter than the femur in the fore leg,
of nearly the same length in the middle leg, and longer in the hind
leg; it is armed with numerous stiff spines directed towards the free
end of the limb. There are usually reckoned five joints in the tarsus,
which regularly diminish in length, except that the last joint is as
long as the second. All the joints bear numerous fine but stiff hairs
upon the walking surface. The extremity of the fifth joint is segmented
off, and carries a pair of equal and strongly curved claws.[69]

[69] Professor Huxley (Anat. Invert. Animals, p. 404) points out that
the so-called _pulvillus_ ought to be counted as a sixth joint. The
same is true of the foot of Diptera and Hymenoptera, where there are
six tarsal joints, the last carrying the claws. (Tuffen West on the
Foot of the Fly. Linn. Trans., Vol. XXIII.)

[Illustration: Fig. 28.--The three Thoracic Legs of a Female Cockroach.
I, _s_, sternum; _cx_, coxa; _tr_, trochanter; _fe_, femur; _tb_,
tibia; _ta_, tarsus. In IIIA the coxa is abducted, and the joints _a_
(episternum) and _b_ slightly separated. × 4.]

At the base of each leg are several chitinous plates (fig. 28), upon
which no small labour has been bestowed by different anatomists. They
are arranged so as to form two joints intermediate between the coxa and
the sternum, and these two joints admit of a hinge-like movement upon
each other, while their other ends are firmly attached to the coxa and
sternum respectively. (Compare III and IIIA, fig. 28.) These parts in
the Cockroach may be taken for two basal leg-joints which have become
adherent to the thorax. In other cases, however, they plainly belong
to the thorax, and not to the leg. In the Mole-cricket, for instance,
similar plates occur; but here they are firmly united, and form the
lateral wall of the thorax. In the Locust they become vertical, and
lie one in front of the other. Most authors have looked upon them as
regular elements of a typical somite. They regard such a segment as
including two pleural elements--viz., a dorsal plate (_epimeron_), and a
ventral plate (_episternum_). We have already (p. 34) given reasons for
doubting the constancy of the pieces so named. It is not inconvenient,
however, to denote by the term _episternum_ the joint which abuts upon
the sternum; for the joint which is applied to the coxa no convenient
term exists, and its occurrence in Insects is so partial, that the
want need not be supplied at present.[70] Both joints are incompletely
subdivided. In the first thoracic segment of the Cockroach they are
less firmly connected than in the other two.

[70] The nomenclature adopted by Packard (Third Report of U.S.
Entomological Commission) seems to us open to theoretical objections.

Cockroaches of both sexes are provided with wings, which, however,
are only functional in the male. The wing-covers (or anterior pair
of wings) of the male are carried by the second thoracic segment. As
in most _Orthoptera genuina_, they are denser than the hind wings,
and protect them when at rest. They reach to the fifth segment of
the abdomen, and one wing-cover overlaps the other. Branching veins
or nervures form a characteristic pattern upon the surface (figs. 4,
29), and it is mainly by means of this pattern that many of the fossil
species are identified and distinguished. The true or posterior wings
are attached to the metathorax. They are membranous and flexible, but
the fore-edge is stiffened, like that of the wing-covers, by additional
chitinous deposit. When extended, each wing forms an irregular quadrant
of a circle; when at rest, the radiating furrows of the hinder part
close up fan-wise, and the inner half is folded beneath the outer.[71]
The wing reaches back as far as the hinder end of the fourth abdominal
segment. The wing-covers of the female are small, and though movable,
seem never to be voluntarily extended; each covers about one-third of
the width of the mesonotum, and extends backwards to the middle of the
metanotum. A reticulated pattern on the outer fourth of the metanotum
plainly represents the hind wing; it is clearly rather a degeneration
or survival than an anticipation of an organ tending towards useful
completeness.

[71] On wing-plaiting and wing-folding in Blattariæ see Saussure,
Etudes sur l’aile des Orthoptères. Ann. Sci. Nat., Sér. 5^e (Zool.),
Tom. X.

[Illustration: Fig. 29.--Wings and Wing-covers of Male Cockroach. × 4.]

The rudimentary wing of the female Cockroach illustrates the homology
of the wings of Insects with the free edges of thoracic terga, and
this correspondence is enforced by the study of the development of the
more complete wings and wing-covers of the male. The hinder edges of
the terga become produced at the later moults preceding the completely
winged stage, and may even assume something of the shape and pattern
of true wings; it is not, however, true, though more than once stated,
that winged nymphs are common. Adults with imperfectly developed wings
have been mistaken for such.

_Origin of Insect Wings._

The structure of the wing testifies to its origin as a fold of the
chitinous integument. It is a double lamina, which often encloses
a visible space at its base. The nervures, with their vessels and
tracheal tubes, lie between the two layers, which, except at the base,
are in close contact. Oken termed the wings of an Insect “aerial
gills,” and this rather fanciful designation is in some degree
justified by their resemblance to the tracheal gills of such aquatic
larvæ as those of Ephemeridæ, Perlidæ, Phryganidæ, &c. In the larva
of _Chloeon (Ephemera) dipterum_ (fig. 30), for example, the second
thoracic segment carries a pair of large expansions, which ultimately
are replaced by organs of aerial flight. The abdominal segments carry
similarly placed respiratory leaflets, the tracheal gills, which by
their vigorous flapping movements bring a rush of water against their
membranous and tracheated surfaces.

Gegenbaur[72] has argued from the resemblance of these appendages to
wings, that the wing and the tracheal leaflet are homologous parts, and
this view has been accepted as probable by so competent an observer as
Sir John Lubbock.[73]

[72] Grundzüge der Vergl. Anat. (Arthropoden, Athmungsorgane.)

[73] Origin and Metamorphoses of Insects, p. 73.

The leaflets placed most advantageously for propulsion seem to have
become exclusively adapted to that end, while the abdominal gills
have retained their respiratory character. At the time of change
from aquatic to terrestrial life, which takes place in many common
Insects when the adult condition is assumed, and which, according to
Gegenbaur, was a normal event among primitive Insects, the tracheal
gill is supposed to disappear, and in its place, at the next moult, an
opening, the stigma, is formed by the rupture of an air-tube. Gegenbaur
supposes that the primitive Insects were aquatic, and their tracheal
system closed. The tracheal gill he takes to be the common structure
which has yielded organs so unlike as the wing and the stigma.

[Illustration: Fig. 30. Chloeon (Chloeopsis) dipterum. Larva in eighth
stage, with wings and respiratory leaflets. × 14. Copied from Vayssière
(loc. cit.).]

The zoological rank of the Insects (Ephemeridæ, Perlidæ, and
Libellulidæ), in which tracheal gills are most usual, is not
unfavourable to such an explanation. Lubbock has given reasons for
regarding _Campodea_ and the Collembola (of the order Thysanura)
as surviving and not very much altered representatives of the
most primitive Insects, and he has shown that no great amount of
modification would be required to convert the terrestrial _Campodea_
into the aquatic _Chloeon_-nymph.[74] We must not forget, however,
that tracheal gills are by no means restricted to these families of
low grade. Trichoptera, a few Diptera, two Lepidoptera (_Nymphula_ and
_Acentropus_), and two Coleoptera (_Gyrinus_ and _Elmis_),[75] have
tracheal gills, and a closed tracheal system in the larval condition.
We cannot suppose that these larvæ of higher orders represent an
unbroken succession of aquatic forms, but if we refuse to adopt this
alternative, we must admit that the closed tracheal system with
tracheal gills may be an adaptive modification of the open system with
stigmata.

[74] Palmén cites one striking proof of the low position of
Ephemeridæ among Insects. Their reproductive outlets are paired and
separate, as in Worms and Crustacea.

[75] These examples are cited by Palmén.

It is well known[76] that in certain Ephemeridæ (_e.g._, _Tricorythus_
and _Cœnis_) a pair of anterior tracheal gills may become transformed
into large plates, which partly protect the gills behind (fig. 31).
A similar modification of the second and third thoracic gills in
_Prosopistoma_ and _Bætisca_ brings all the functional respiratory
organs under cover, and these enlarged plates resemble stiff and simple
wings very closely.

[76] Eaton, Trans. Ent. Soc., 1868, p. 281; Vayssière, Ann. Sci.
Nat., Zool., 1882, p. 91.

[Illustration: Fig. 31.--Tricorythus. Adult larva, with three
functional leaflets. The next leaflet in front is converted into a
protective plate. × 7.

A, protective plate of Tricorythus larva, seen from the outside. × 26.

B, the same from within, showing the attached respiratory appendage.

C, protective plate of Cœnis larva, without respiratory appendage.

All the figures are copied from Vayssière.]

Palmén[77] has subjected Gegenbaur’s hypothesis to a very searching
examination. He observes that:--

[77] Zur Morphologie des Tracheensystems (1877).

1. In _Campodea_, and presumably in other primitive Insects, the
tracheal system is not closed and adapted for aquatic respiration,
but open. Tracheal gills are not by any means confined to the lowest
Insects. (See above, p. 65.)

2. Tracheal gills are not always homodynamous or morphologically
equivalent. In Ephemeridæ, some are dorsal in position, some ventral
(first abdominal pair in _Oligoneuria_ and _Rhithrogena_); they may be
cephalic, springing from the base of the maxilla, as in _Oligoneuria_
and _Jolia_; _Jolia_ has a branchial tuft at the insertion of each of
the fore legs.[78] In Perlidæ the tracheal gills may have a tergal,
pleural, sternal, or anal insertion. In some Libellulidæ also, anal
leaflets occur.[79]

[78] We take these instances from Eaton, Monograph of Ephemeridæ,
Linn. Trans., 1883, p. 15.

[79] Charles Brongniart has lately described a fossil Insect from the
Coal Measures of Commentry, which he names _Corydaloides Scudderi_,
and refers to the Pseudo-Neuroptera. In this Insect every ring of the
abdomen carries laminæ, upon which the ramified tracheæ can still be
made out by the naked eye. Stigmata co-existed with these tracheal
gills. (Bull. Soc. Sci. Nat. de Rouen, 1885.)

Some Crustacea are furnished with respiratory leaflets, curiously
like those of Tracheates, with which, however, they have no genetic
connection. In Isopod Crustacea the exopodites of the anterior
abdominal segments often form opercula, which protect the succeeding
limbs. In the terrestrial Isopods, _Porcellio_ and _Armadillo_, these
opercula contain ramified air-tubes, which open externally, and
much resemble tracheæ. The anterior abdominal appendages of _Tylus_
are provided with air-chambers, each lodging brush-like bundles of
air-tubes, which open to the outer air. Lamellæ, projecting inwards
from the sides of the abdominal segments, incompletely cover in
the hinder part of the ventral surface of the abdomen, and protect
the modified appendages. (Milne Edwards, Hist. Nat. des Crustacés,
Vol. III.)

3. Tracheal gills never perfectly agree in position and number with the
stigmata throughout the body. Sometimes they occur on different rings,
sometimes on different parts of the same ring. Gegenbaur’s statements
on this point are incorrect.

4. Tracheal gills may co-exist with stigmata. In Perlidæ the tracheal
gills persist in the imago, and may be found, dry and functionless,
beneath the stigmata. In Trichoptera they gradually abort at successive
moults, and in some cases remain after the stigmata have opened.

5. Stigmata do not form by the breaking off of tracheal appendages, but
by the enlargement of rudimentary tracheal branches, which open into
the main longitudinal trunks. In larvæ with aquatic respiration these
branches exist, though they are not functional.

Palmén’s objections must be satisfactorily disposed of before
Gegenbaur’s explanation, interesting as it is, can be fully accepted.
Palmén has proved, what is on other grounds clear enough, that stigmata
are more ancient than tracheal gills, aerial tracheate respiration than
aquatic. But there is nothing as yet to contradict the view that the
first Insect-wings were adapted for propulsion in water, and that they
were respiratory organs before they became motor. It is Gegenbaur’s
explanation of the origin of stigmata, and not his explanation of the
origin of wings, which is refuted by Palmén.

_Abdomen._

In the abdomen of the female Cockroach eight terga (1–7; 10) are
externally visible. Two more (8, 9) are readily displayed by extending
the abdomen; they are ordinarily concealed beneath the seventh tergum.
The tenth tergum is notched in the middle of its posterior margin. A
pair of triangular “podical plates,” which lie on either side of the
anus, and towards the dorsal surface, have been provisionally regarded
by Prof. Huxley as the terga of an eleventh segment. Seven abdominal
sterna (1–7) are externally visible. The first is quite rudimentary,
and consists of a transversely oval plate; the second is irregular
and imperfectly chitinised in front; the seventh is large, and its
hinder part, which is boat-shaped, is divided into lateral halves, for
facilitating the discharge of the large egg-capsule.

In the male Cockroach ten abdominal terga are visible without
dissection (fig. 33, p. 70), though the eighth and ninth are greatly
overlapped by the seventh. The tenth tergum is hardly notched. Nine
abdominal sterna are readily made out, the first being rudimentary, as
in the female. The eighth is narrower than the seventh, the ninth still
narrower, and largely concealed by the eighth; its covered anterior
part is thin and transparent, the exposed part denser. This forms the
extreme end of the body, except that the small sub-anal styles project
beyond it. The podical plates resemble those of the female.

Pleural elements are developed in the form of narrow stigmatic plates,
with the free edge directed backwards. These lie between the terga and
sterna, and defend the spiracle.[80]

[80] Gerstaecker has found in the two first abdominal segments of
_Corydia carunculigera_ (_Blattariæ_) pleural appendages, which
are hollow and capable of protrusion. They have no relation to the
stigmata, which are present in the same segments, and their function
is quite unknown. See Arch. f. Naturg., 1861, p. 107.

The modifications of the hindmost abdominal segments will be more fully
considered in connection with the reproductive organs.

[Illustration: Fig. 32.--Under side of Abdomen of Male and Female
Cockroach. × 4.]

The high number of abdominal segments found in the Cockroach (ten or
eleven) is characteristic of the lower orders of Insecta. It is never
exceeded; though in the more specialised orders, such as Lepidoptera
and Diptera, it may be reduced to nine, eight, or even seven. The
sessile abdomen of the Cockroach is primitive with respect to the
pedunculate abdomen found in such insects as Hymenoptera, where the
constricted and flexible waist stands in obvious relation to the
operations of stinging and boring, or to peculiar modes of oviposition.
The first abdominal segment, which is especially liable to dislocation
and alteration in Insects, occupies its theoretical position in the
Cockroach, though both tergum and sternum are reduced in size. The
sternum is often altogether wanting, while the tergum may unite with
the metathorax.

The externally visible appendages of the abdomen are the cerci and the
styles of the male Cockroach. The cerci are found in both sexes; they
are composed of sixteen rings each, and project beneath the edge of the
tenth tergum. They are capable of erection by special muscles, and are
supplied by large nerves.[81] The sub-anal styles are peculiar in their
insertion, being carried upon the sternum of their segment (the ninth).

[81] Jointed cerci are commonly found in Orthoptera (including
Pseudo-Neuroptera); in the Earwig they become modified and form the
forceps. The “caudal filaments” of _Apus_ are curiously like cerci.

The cerci are concealed in the American _Cryptocercus_, Scudd. (Fam.
_Panesthidæ_).

[Illustration: Fig. 33.--Profile of Male and Female Cockroach. × 4.]

The abdominal segments are never furnished with functional legs in
adult Insects, but representatives of the lost appendages are often met
with in larvæ. According to Bütschli,[82] all the abdominal segments
are provided with appendages in the embryo of the Bee, though they
disappear completely before hatching. Some Hymenopterous larvæ have as
many as eight pairs of abdominal appendages, Lepidopterous larvæ at
most five (3–6; 10).[83]

[82] Entw. der Biene. Zeits. f. wiss. Zool. Bd. XX. Or, see Balfour’s
Embryology, Vol. I., p. 338.

[83] From more recent observations it is probable that abdominal
appendages are usually present in the embryos of Orthoptera,
Coleoptera, Lepidoptera, and possibly Hymenoptera. The subject is
rapidly advancing, and more will be known very shortly.

CHAPTER V.

THE MUSCLES; THE FAT-BODY AND CŒLOM.

_SPECIAL REFERENCES._

VIALLANES. Histologie et Développement des Insectes. Ann. Sci.
Nat., Zool., Tom. XIV. (1882).

KÜHNE in Stricker’s Histology, Vol. I., chap. v.

PLATEAU. Various Memoirs in Bull. Acad. Roy. de Belgique (1865,
1866, 1883, 1884). [Relative and Absolute Muscular Force.]

LEYDIG. Zum feineren Bau der Arthropoden. Müller’s Archiv., 1855.

WEISMANN. Ueber zwei Typen contractilen Gewebes, &c. Zeits. für
ration. Medicin. Bd. XV. (1862).

_Structure of Insect Muscles._

The muscles of the Cockroach, when quite fresh, appear semi-transparent
and colourless. If subjected to pressure or strain they are found to be
extremely tender. Alcohol hardens and contracts them, while it renders
them opaque and brittle.

The minute structure of the voluntary or striped muscular fibres of
Vertebrates is described in common text-books.[84] Each fibre is
invested by a transparent elastic sheath, the sarcolemma, and the space
within the sarcolemma is subdivided by transverse membranes into a
series of compartments. The compartments are nearly filled by as many
contractile discs, broad, doubly refractive plates, which are further
divisible into prismatic columns, the sarcous elements, each being as
long as the contractile disc. Successive sarcous elements, continued
from one compartment to another, form the primitive fibrils of the
muscle. In cross-section the fibrils appear as polygonal areas bounded
by bright lines. Outside the fibres, but within the sarcolemma, are
nuclei, imbedded in the protoplasm, or living and formative element of
the tissue.

[84] See, for example, Klein’s Elements of Histology, chap. ix.

The muscular fibres of Insects present some important differences from
the fibres just described. The nuclei are often found in the centre,
and not on the surface of the fibres in both Insects and Crustacea. In
both classes the fibrils are frequently subdivided into longitudinal
strands, which have not been distinguished in Vertebrate muscles
(Viallanes). The sarcolemma is often undeveloped. Lastly, Insects, like
other Arthropoda, exhibit the remarkable peculiarity that not only
their voluntary muscles, but all, or nearly all, the muscles of the
body, even those of the digestive tube, are striated.[85]

[85] The exceptions relate chiefly to the alary muscles of the
pericardial septum. Lowne (Blow-fly, p. 5, and pl. v.) states that
some of the thoracic muscles of that Insect are not striated.

_General Arrangement of Insect Muscles._

The arrangement of the muscles in an Insect varies greatly according
to situation and mode of action. Some of the abdominal muscles consist
solely of straight parallel bundles, while the muscles of the limbs
usually converge to tendinous insertions. In certain larvæ, where
the segments show hardly any differentiation, the muscles form a
sheet which covers the whole body, and is regularly segmented in
correspondence with the exo-skeleton. As the movements of the body and
limbs become more varied and more energetic, the muscles become grouped
in a more complicated fashion, and the legs and wings of a flying
Insect may be set in motion by a muscular apparatus almost as elaborate
as that of a bird.

_Muscles of the Cockroach._

The following short notes on the muscles of the Cockroach, aided by
reference to the figures, will render the more noteworthy features
intelligible. A very lengthy description, far beyond our space or
the reader’s patience, would be required to explain in detail the
musculature of the head, limbs, and other specialised regions.

STERNAL MUSCLES OF ABDOMEN.--The _longitudinal sternal muscles_
(fig. 34) form a nearly continuous transversely segmented sheet,
covering the ventral surface between the fore-edge of the second
abdominal sternum and the fore-edge of the seventh. These muscles, in
conjunction with the longitudinal tergal muscles, tend to telescope the
segments.

[Illustration: Fig. 34.--Muscles of Ventral Wall, with the Nerve-cord.
× 5.]

[Illustration: Fig. 35.--Muscles of Dorsal Wall, with the Heart and
Pericardial Tendons. × 5.]

The _oblique sternal muscles_ (fig. 34), which are very short, connect
the adjacent edges of the sterna (2–3, 3–4, 4–5, 5–6, 6–7). They
extend inwards nearly to the middle line, but, like the longitudinal
sternal muscles, they are not developed beneath the nerve-cord.
Acting together, the oblique sternal muscles would antagonise the
longitudinal, but it is probable that they are chiefly used to effect
lateral flexion of the abdomen, and that only the muscles of one side
of the abdomen contract at once.

The _tergo-sternal_ (or expiratory) muscles (figs. 35 and 36) form
vertical pairs passing from the outer part of each abdominal sternum
to the corresponding tergum. Their action is to approximate the dorsal
and ventral walls, and thus to reduce the capacity of the abdomen. The
first tergo-sternal muscle has its ventral insertion into the stem
of the postfurca, and takes an oblique course to the first abdominal
tergum.

TERGAL MUSCLES OF ABDOMEN.--The _longitudinal tergal_ muscles extend
from the fore part of each abdominal tergum, including the first,
to the same part of the tergum next behind. They are interrupted by
longitudinal spaces, so that the muscular sheet is less continuous than
on the ventral surface, and has a fenestrated appearance. The direction
of the fibres is slightly oblique.

_Oblique tergal_ muscles, resembling the oblique muscles of the sterna,
are also present.

In the thorax the general arrangement of the muscles is greatly
modified by the altered form of the dorsal and ventral plates, and by
the attachment of powerful limbs.

STERNAL MUSCLES OF THORAX.--Two tubular apodemes, lying one behind the
other, project into the thorax from the ventral surface (p. 59 and
fig. 27). To the foremost of these are attached three paired muscles
and one median muscle. The median muscle passes to the second tubular
apodeme. The anterior pair pass forwards and outwards to the base of
the prothoracic leg; the next pair directly outwards to the base of the
middle leg; while the posterior pair pass outwards and backwards to the
arms of the medifurca. From the second tubular apodeme, in front of the
metasternum, four pairs of muscles spring. Those of the anterior pass
forwards and outwards to the coxa of the fore limb; the second pair
directly outwards to the base of the metathoracic legs; the third pair
backwards and outwards to the arms of the postfurca; the fourth pair
backwards to the second abdominal sternum.

[Illustration: Fig. 36.--Muscles of lateral wall, &c. × 5.]

The muscles attached to the medi- and postfurca (other than those
connecting them with the tubular apodemes) are:--(1) A pair passing
from the posterior edge of the arms of the medifurca to the stem of the
postfurca; (2) a pair which diverge from the stem of the postfurca and
proceed to the fore part of the second abdominal sternum; (3) a pair
passing from the posterior edge of the arms of the postfurca, these are
directed inwards and backwards, and are inserted into the hinder part
of the second abdominal sternum; (4) a pair already mentioned, which
correspond in position and action to the tergo-sternal muscles, and
spring from the stem of the postfurca, passing upwards and outwards to
the sides of the first abdominal tergum.

[Illustration: Fig. 37.--Muscles of left mesothoracic leg, seen
from behind. The muscles are--Adductor and abductor of the coxa;
extensor and flexor of femoral joint; flexor and extensor of tibial
joint; flexor of tarsus; and a retractor tarsi, which swings the
tarsus backwards, so that it points away from the head. It is opposed
by another muscle, which moves the tarsus forwards. Both muscles
parallelise the tarsus to the axis of the body, but in opposite
directions.]

The muscles attached to the arms of each furca pass to other structures
in or near the middle line of the body. The pull of such muscles must
alter the slope of the two steps in the ventral floor of the thorax
(p. 58, and fig. 3, p. 12). When the furca is drawn forwards, the
step is rendered vertical or even inclined forward, the sterna being
approximated; while, on the other hand, a backward pull brings the step
into a horizontal position, and separates the sterna.

TERGAL MUSCLES OF THORAX.--The _longitudinal tergal_ muscles are much
reduced in width when compared with those of the abdomen. Sets of
obliquely placed muscles, which may be called the _lateral thoracic_
muscles, arise from near the middle of each tergum, and converge to
tendinous insertions on the fore edge of each succeeding tergum, close
to the lateral wall of the body.

The principal muscles of the legs are figured and named, and their
action can readily be inferred from the names assigned to them.

_Insect Mechanics._

The mechanics of Insect movements require exposition and illustration
far beyond what is possible in a book like this. Even the elaborate
dissections of Lyonnet and Straus-Dürckheim are not a sufficient basis
for a thorough treatment of the subject, and until we possess many
careful dissections, made by anatomists who are bent upon mastering
the action of the parts, our views must needs be vague and of doubtful
value. Zoologists of great eminence have been led into erroneous
statements when they have attempted to characterise shortly a complex
animal mechanism which they did not think it worth while to analyse
completely.[86]

[86] For example, Prof. Huxley, in his Anatomy of Invertebrated
Animals (p. 254), says that “as the hard skeleton [of Arthropods] is
hollow, and the muscles are inside it, it follows that the body, or
a limb, is bent towards that side of its axis, which is opposite to
that on which a contracting muscle is situated.” The flexor muscles
of the tail of the Crayfish, which, according to the above rule,
should be extensors, the muscles of the mandibles of an Insect, and
the flexors and extensors of Crustacean pincers are among the many
conspicuous exceptions to this rule.

The action of flight and the muscles attached to the wings are best
studied in Insects of powerful flight. The female Cockroach cannot
fly at all, and the male is by no means a good flier. Both sexes are,
however, admirably fitted for running.

In running, two sets, each consisting of three legs, move
simultaneously. A set includes a fore and hind limb of the same side
and the opposite middle leg. Numbering them from before backwards, and
distinguishing the right and left sides by their initial letters, we
can represent the legs which work together as--

┏━━━━━━━━━┷━━━━━━━━━┓
R_{1} L_{2} R_{3}

┏━━━━━━━━━┷━━━━━━━━━┓
L_{1} R_{2} L_{3}

The different legs have different modes of action. The fore-leg may be
compared to a grappling-iron; it is extended, seizes the ground with
its claws, and drags the body towards its point of attachment. The
middle leg is chiefly used to support and steady the body, but has some
pushing power. The hind leg, the largest of the three, is effective in
shoving, and chiefly propels the body.

_Muscular Force of Insects._

The force exerted by Insects has long been remarked with surprise, and
it is a fact familiar, not only to naturalists, but to all observant
persons, that, making allowance for their small size, Insects are the
most powerful of common animals. Popular books of natural history
give striking and sometimes exaggerated accounts of the prodigious
strength put forth by captive Insects in their efforts to escape. Thus
we are told that the flea can draw 70 or 80 times its own weight.[87]
The Cockchafer is said to be six times as strong as a horse, making
allowance for size. A caterpillar of the Goat Moth, imprisoned beneath
a bell-glass, weighing half a pound, which was loaded with a book
weighing four pounds, nevertheless raised the glass and made its escape.

[87] Haller. This and other examples are taken from Rennie’s Insect
Transformations.

This interesting subject has been investigated by Plateau,[88] who
devised the following experiment. The Insect to be tested was confined
within a narrow horizontal channel, which was laid with cloth. A thread
attached to its body was passed over a light pulley, and fastened to a
small pan, into which sand was poured until the Insect could no longer
raise it. Some of the results are given in the following table:--

[88] Bull. Acad. Roy. de Belgique, 2^{e.} Sér., Tom. xx. (1865), and
Tom. xxii. (1866).

_Table of Relative Muscular Force of Insects (Plateau)._

Weight of body Ratio of weight lifted
in grammes. to weight of body.

Carabus auratus 0·703 17·4
Nebria brevicollis 0·046 25·3

Melolontha vulgaris 0·940 14·3
Anomala Frischii 0·153 24·3

Bombus terrestris 0·381 14·9
Apis mellifica 0·090 23·5

One obvious result is that within the class of Insects the relative
muscular force (as commonly understood) is approximately in the inverse
proportion of the weight--that is, the strength of the Insect is (by
this mode of calculation) most conspicuous in the smaller species.

In a later memoir[89] Plateau gives examples from different Vertebrate
and Invertebrate animals, which lead to the same general conclusion.

[89] Loc. cit. 3^{e.} Sér., Tom. vii. (1884). Authorities for the
various estimates are cited in the original memoir.

_Ratio of weight drawn to weight of body (Plateau)._

Horse ·5 to ·83
Man ·86
Crab 5·37
Insects 14·3 to 23·5

The inference commonly drawn from such data is that the muscles
of small animals possess a force which greatly exceeds that of
large quadrupeds or man, allowance being made for size, and that
the explanation of this superior force is to be looked for in some
peculiarity of composition or texture. Gerstaecker,[90] for example,
suggests that the higher muscular force of Arthropoda may be due
to the tender and yielding nature of their muscles. An explanation
so desperate as this may well lead us to inquire whether we have
understood the facts aright. Plateau’s figures give us the ratio of the
weight drawn or raised to the weight of the animal. This we may, with
him, take as a measure of the _relative muscular force_. In reality, it
is a datum of very little physiological value. By general reasoning of
a quite simple kind it can be shown that, for muscles possessing the
same physical properties, the _relative_ muscular force necessarily
increases very rapidly as the size of the animal decreases. For the
contractile force of muscles of the same kind depends simply upon the
number and thickness of the fibres, _i.e._, upon the sectional area of
the muscles. If the size of the animal and of its muscles be increased
according to any uniform scale, the sectional area of a given muscle
will increase as the square of any linear dimension. But the weight
increases in a higher proportion, according to the increase in length,
breadth, and depth jointly, or as the cube of any linear dimension.[91]
The ratio of contractile force to weight must therefore become rapidly
smaller as the size of the animal increases. Plateau’s second table
(_see above_) actually gives a value for the relative muscular force
of the Bee, in comparison with the Horse, which is only one-fourteenth
of what it ought to turn out, supposing that both animals were of
similar construction, and that the muscular fibres in both were equal
in contractile force per unit of sectional area.[92]

[90] Klassen und Ordnungen des Thierreichs, Bd. V., pp. 61–2.

[91] This change in the relation of weight to strength, according to
the size of the structure, has long been familiar to engineers. (See,
for example, “Comparisons of Similar Structures as to Elasticity,
Strength, and Stability,” by Prof. James Thomson, Trans. Inst.
Engineers, &c., Scotland, 1876.) The application to animal structures
has been made by Herbert Spencer (Principles of Biology, Pt. II.,
ch. i.). The principle can be readily explained by models. Place a
cubical block upon a square column. Double all the dimensions in
a second model, which may be done by fitting together eight cubes
like the first, and four columns, also the same as before except in
length. Each column, though no stronger than before, has now to bear
twice the weight.

[92] Contractile force varies as sectional area of muscle. Let _W_
be weight of Horse; _w_, weight of Bee; _R_, a linear dimension of
Horse; _r_, a linear dimension of Bee. Then,

Contr. force of Horse sect. area of muscles (Horse) _R^2_
--------------------- = ----------------------------- = -----.
Contr. force of Bee sect. area of muscles (Bee) _r^2_

_W_ _R^3_ _R^2_ _W_ _r_
But since --- = -----, ----- = --- × ---.
_w_ _r^3_ _r^2_ _w_ _R_

Contr. force of Horse _Wr_
Therefore --------------------- = ----.
Contr. force of Bee _wR_

But, by definition,

Contr. f. of Horse
------------------
Rel. m.f. of Horse _W_ Contr. f. of Horse _w_
------------------ = ╺━━━━━━━━━━━━━╸ = ------------------ × ---
Rel. m.f. of Bee Contr. f. of Bee Contr. f. of Bee _W_
----------------
_w_

_Wr_ _w_ _r_ _r^3_ _w_
= ---- × --- = --- = (---)^{1/3} = (---)^{1/3}.
_wR_ _W_ _R_ _R^3_ _W_

The weight of a horse is about 270,000 grammes, that of a bee
·09 gramme; so that

_w_ ·09
(---)^{1/3} = (-----)^{1/3} = (·000,000,3̅)^{1/3} = ·0015 (nearly) =
_W_ 270,000

Calculated Ratio of Relative Muscular Force of Horse to that of Bee.

·5
The Observed Ratio (Plateau) is ---- = ·02128;
23·5

so that the relative muscular force of the Horse is more than
fourteen times as great in comparison with that of the Bee as it
would be if the muscles of both animals were similar in kind, and the
proportions of the two animals similar in all respects.

A later series of experiments[93] brings out this difference in a
precise form. Plateau has determined by ingenious methods what he
calls the _Absolute Muscular Force_[94] of a number of Invertebrate
animals (Lamellibranch Mollusca, and Crustacea), comparing them with
man and other Vertebrates. His general conclusions may be shortly given
as follows:--The absolute muscular force of the muscles closing the
pincers of Crabs is low in comparison with that of Vertebrate muscles.
The absolute force of the adductor muscles closing a bivalve shell may,
in certain Lamellibranchs, equal that of the most powerful Mammalian
muscles; in others it falls below that of the least powerful muscles of
the frog, which are greatly inferior in contractile force to Mammalian
muscles. We find, therefore, that the low contractile force of Insect
muscles is in harmony, and not in contrast, with common observation of
their physical properties, and that the high _relative_ muscular force,
correctly enough attributed to them, is explicable by considerations
which apply equally well to models or other artificial structures.

[93] Rech. sur la Force Absolue des Muscles des Invertébrés. I^e
Partie. Mollusques Lamellibranches. Bull. Acad. Roy. de Belgique, 3^e
Sér., Tom. VI. (1883).

Do., II^e Partie. Crustacés Décapodes. Ibid., Tom. VII. (1884).

[94] _Statical muscular force_ and _Specific muscular force_ are
synonymous terms in common use. _Contractile force per unit of
sectional area_ gives perhaps the clearest idea of what is meant.

The comparison between the muscular force of Insects and large
animals is sometimes made in another way. For example, in Carpenter’s
Zoology[95] the spring of the Cheese-hopper is described, and we are
told that “the height of the leap is often from twenty to thirty
times the length of the body; exhibiting an energy of motion which is
particularly remarkable in the soft larva of an Insect. A Viper, if
endowed with similar powers, would throw itself nearly a hundred feet
from the ground.” It is here implied that the equation

Height of Insect’s leap Supposed ht. of Viper’s leap (100 ft.)
----------------------- = --------------------------------------
Length of Insect Length of Viper

should hold if the two animals were “endowed with similar powers.”

[95] Vol. II., p. 203. The calculation here quoted is based upon an
observation of Swammerdam, who relates that a Cheese-hopper, 1/4 in.
long, leaped out of a box 6 in. deep.

But it is known that the work done by contraction of muscles of the
same kind is proportional to the volume of the muscles (“Borelli’s
Law”),[96] and in similar animals the muscular volumes are as the
weights. Therefore the equation

Work of Insect Work of Viper
---------------- = ---------------
Weight of Insect Weight of Viper

will more truly represent the imaginary case of equal leaping power.
But the work = weight raised × height, and the weight raised is in both
cases the weight of the animal itself. Therefore

Wt. × Ht. Wt. × Ht.
--------- (Insect) = --------- (Viper),
Wt. Wt.

and Ht. (Insect) = Ht. (Viper). The Viper’s efficiency as a leaping
animal would, therefore, equal that of a Cheese-hopper if it leaped the
same vertical height. Therefore, if the two animals were “endowed with
similar powers,” the heights to which they could leap would be equal,
and not proportional to their lengths, as is assumed in the passage
quoted.

[96] Haughton’s Animal Mechanics, 2nd ed., p. 43.

Straus-Dürckheim observes that a Flea can leap a foot high, which is
200 times its own length, and this has been considered a stupendous
feat. It is really less remarkable than a schoolboy’s leap of two feet,
for it indicates precisely as great efficiency of muscles and other
leaping apparatus as would be implied in a man’s leap to the same
height, viz., one foot.[97]

[97] In any comparison it is necessary to cite not the height cleared
by the man, but the displacement during the leap of his centre of
gravity.

_The Fat-body._

Adhering to the inner face of the abdominal wall is a cellular mass,
which forms an irregular sheet of dense white appearance. This is the
fat-body. Its component cells are polygonal, and crowded together. When
young they exhibit nuclei and vacuolated protoplasm, but as they get
older the nuclei disappear, the cell-boundaries become indistinct, and
a fluid, loaded with minute refractive granules,[98] takes the place of
the living protoplasm. Rhombohedral or hexagonal crystals, containing
uric acid, form in the cells and become plentiful in old tissue. The
salt (probably urate of soda) is formed by the waste of the proteids of
the body. What becomes of it in the end we do not know for certain, but
conjecture that it escapes by the blood which bathes the perivisceral
cavity, that it is taken up again by the Malpighian tubules, and is
finally discharged into the intestine. The old gorged cells probably
burst from time to time, and the infrequency of small cells among
them renders it probable that rejuvenescence takes place, the burst
cells passing through a resting-stage, accompanied by renewal of their
nuclei, and then repeating the cycle of change.

[98] The granules are not shown in the figure, having been removed in
the preparation of the tissue for microscopic examination.

The segmental tubes forming the Wolffian body of Vertebrates have at
first no outlet, and embryologists have hesitated to regard this phase
of development as the permanent condition of any ancestral form.[99] It
is, therefore, of interest to find in the fat-body of the Cockroach an
example of a solid, mesoblastic, excretory organ, functional throughout
life, but without efferent duct.

[99] Balfour, Embryology, Vol. II., p. 603.

[Illustration: Fig. 38.--Fat-body of Cockroach, cleared with
turpentine. _A_, young tissue, with distinct cell-boundaries and
nuclei, a few cells towards the centre with dead contents; _B_, older
ditto, loaded with urates, the cell-walls much broken down, and the
nuclei gone; _tr_, tracheal tubes. × 250.]

The fat-body is eminently a metabolic tissue, the seat of active
chemical change in the materials brought by the blood. Its respiratory
needs are attested by the abundant air-tubes which spread through it in
all directions.

The considerable bulk of the fat-body in the adult Cockroach points to
the unusual duration of the perfect Insect. It is usually copious in
full-fed larvæ, but becomes used up in the pupa-stage.

Extensions of the fat-body surround the nervous chain, the reproductive
organs and other viscera. Sheets of the same substance lie in the
pericardial sinus on each side of the heart.

_The Cœlom._

The fat-body is in reality, as development shows, the irregular
cellular wall of the cœlom, or perivisceral space. Through this space
courses the blood, flowing in no defined vessels, but bathing all the
walls and viscera. In other words, the fat-body is an aggregation of
little-altered mesoblast-cells, excavated by the cœlom, the rest of the
mesoblast having gone to form the muscular layers of the body-wall and
of the digestive tube.

CHAPTER VI.

THE NERVOUS SYSTEM AND SENSE ORGANS.

_SPECIAL REFERENCES._

NEWPORT. Nervous System of Sphinx Ligustri. Phil. Trans. (1832–4).
Todd’s Cyclopædia, Art. “Insecta” (1839).

LEYDIG. Vom Bau des Thierischen Körpers. Bd. I. (1864). Tafeln zur.
vergl. Anat. Hft. I. (1864).

BRANDT (E.) Various memoirs on the Nervous System of Insects in
Horæ Soc. Entom. Ross., Bd. XIV., XV. (1879).

MICHELS. Nervensystem von Oryctes nasicornis im Larven--, Puppen--,
und Käferzustande. Zeits. f. wiss. Zool., Bd. XXXIV. (1881).

DIETL. Organisation des Arthropodengehirns. Zeits. f. wiss. Zool.,
Bd. XXVII. (1876).

FLÖGEL. Bau des Gehirns der verschiedenen Insektenordnungen. Zeits.
f. wiss. Zool., Bd. XXX. Sup. (1878).

NEWTON. On the Brain of the Cockroach. Q. J. Micr. Sci. (1879).
Journ. Quekett Club (1879).

GRENACHER. Sehorgan der Arthropoden. (1879). [Origin, Structure,
and Action of the Compound Eye.]

CARRIERE. Sehorgane der Thiere, vergl.-anat. dargestellt (1885).
[Comparative Structure of various Simple and Compound Eyes.]

_General Anatomy of Nervous Centres._

The nervous system of the Cockroach comprises ganglia and
connectives,[100] which extend throughout the body. We have first, a
supra-œsophageal ganglion, or brain, a sub-œsophageal ganglion, and
connectives which complete the œsophageal ring. All these lie in the
head; behind them, and extending through the thorax and abdomen, is a
gangliated cord, with double connectives. The normal arrangement of the
ganglia in Annulosa, one to each somite, becomes more or less modified
in Insects by coalescence or suppression, and we find only eleven
ganglia in the Cockroach--viz., two cephalic, three thoracic, and six
abdominal.

[100] Yung (“Syst. nerveux des Crustacées Décapodes, Arch. de Zool.
exp. et gén.,” Tom. VII., 1878) proposes to name _connectives_ the
longitudinal bundles of nerve-fibres which unite the ganglia, and
to reserve the term _commissures_ for the transverse communicating
branches.

[Illustration: Fig. 39.--Nervous System of Female Cockroach, × 6. _a_,
optic nerve; _b_, antennary nerve; _c_, _d_, _e_, nerves to first,
second, and third legs; _f_, to wing-cover; _g_, to second thoracic
spiracle; _h_, to wing; _i_, abdominal nerve; _j_, to cerci.]

The nervous centres of the head form a thick, irregular ring,
which swells above and below into ganglionic enlargements, and
leaves only a small central opening, occupied by the œsophagus. The
tentorium separates the brain or supra-œsophageal ganglion from the
sub-œsophageal, while the connectives traverse its central plate. Since
the œsophagus passes above the plate, the investing nervous ring also
lies almost wholly above the tentorium.

[Illustration: Fig. 40.--Side view of Brain of Cockroach, × 25. _op_,
optic nerve; _oe_, œsophagus; _t_, tentorium; _sb_, sub-œsophageal
ganglion; _mn_, _mx_, _mx_′, nerves to mandible and maxillæ. Copied
from E. T. Newton.]

The brain is small in comparison with the whole head; it consists of
two rounded lateral masses or hemispheres, incompletely divided by
a deep and narrow median fissure. Large optic nerves are given off
laterally from the upper part of each hemisphere; lower down, and on
the front of the brain, are the two gently rounded antennary lobes,
from each of which proceeds an antennary nerve; while from the front
and upper part of each hemisphere a small nerve passes to the so-called
“ocellus,” a transparent spot lying internal to the antennary socket
on each side in the suture between the clypeus and the epicranium. The
sub-œsophageal ganglion gives off branches to the mandibles, maxillæ,
and labrum. While, therefore, the supra-œsophageal is largely sensory,
the sub-œsophageal ganglion is the masticatory centre.

The œsophageal ring is double below, being completed by the connectives
and the sub-œsophageal ganglion; also by a smaller transverse
commissure, which unites the connectives, and applies itself closely to
the under-surface of the œsophagus.[101]

[101] This commissure, which has been erroneously regarded as
characteristic of Crustacea, was found by Lyonnet in the larva of
Cossus, by Straus-Dürckheim in Locusta and Buprestis, by Blanchard
in Dytiscus and Otiorhynchus, by Leydig in Glomeris and Telephorus,
by Dietl in Gryllotalpa, and by Liénard in a large number of other
Insects and Myriapods, including Periplaneta. See Liénard, “Const. de
l’anneau œsophagien,” Bull. Acad. Boy. de Belgique, 2^e Sér., Tom.
XLIX., 1880.

Two long connectives issue from the top of the sub-œsophageal
ganglion, and pass between the tentorium and the submentum on their
way to the neck and thorax. The three thoracic ganglia are large (in
correspondence with the important appendages of this part of the
body) and united by double connectives. The six abdominal ganglia
have also double connectives, which are bent in the male, as if to
avoid stretching during forcible elongation of the abdomen. The sixth
abdominal ganglion is larger than the rest, and is no doubt a complex,
representing several coalesced posterior ganglia; it supplies large
branches to the reproductive organs, rectum, and cerci.

_Internal Structure of Ganglia._

Microscopic examination of the internal structure of the nerve-cord
reveals a complex arrangement of cells and fibres. The connectives
consist almost entirely of nerve-fibres, which, as in Invertebrates
generally, are non-medullated. The ganglia include (1) rounded,
often multipolar, nerve-cells; (2) tortuous and extremely delicate
fibres collected into intricate skeins; (3) commissural fibres, and
(4) connectives. The chief fibrous tracts are internal, the cellular
masses outside them. A relatively thick, and very distinct neurilemma,
probably chitinous, encloses the cord. Its cellular matrix, or
chitinogenous layer, is distinguished by the elongate nuclei of its
constituent cells.[102] Tracheal trunks pass to each ganglion, and
break up upon and within it into a multitude of fine branches.

[102] We have not been able to distinguish in the adult Cockroach the
_double_ layer of neurilemmar cells noticed by Leydig and Michels in
various Coleoptera.

[Illustration: Fig. 41.--Transverse section of Third Thoracic Ganglion.
_neu_, neurilemmar cells; _gc_, ganglionic cells; _tr_, tracheal tubes;
_A_, ganglionic cells, highly magnified. × 75.]

[Illustration: Fig. 42.--Longitudinal vertical section of Third
Thoracic Ganglion. _n_, connective. The other references as in fig. 41.
× 75.]

Bundles of commissural fibres pass from the ganglion cells of one
side of the cord to the peripheral nerves of the other. There are
also longitudinal bands which blend to form the connectives, and send
bundles into the peripheral nerves. Of the peripheral fibres, some are
believed to pass direct to their place of distribution, while others
traverse at least one complete segment and the corresponding ganglion
before separating from the cord.

[Illustration: Fig. 43.--Longitudinal horizontal section of Third
Thoracic Ganglion. _n_, peripheral nerves. The other references as
before. × 75.]

Many familiar observations show that the ganglia of an Insect possess
great physiological independence. The limbs of decapitated Insects, and
even isolated segments, provided that they contain uninjured ganglia,
exhibit unmistakable signs of life.

_Median Nerve-Cord._

Lyonnet,[103] Newport,[104] and Leydig[105] have found in large
Insects a system of median nerves, named _respiratory_ (Newport) or
_sympathetic_ (Leydig). These nerves do not form a continuous cord
extending throughout the body, but take fresh origin in each segment
from the right and left longitudinal commissures alternately. The
median nerve lies towards the dorsal side of the principal nerve-cord,
crosses over the ganglion next behind, and receives a small branch
from it. Close behind the ganglion it bifurcates, the branches passing
outwards and blending with the peripheral nerves. Each branch, close
to its origin, swells into a ganglionic enlargement. The median nerve
and its branches differ in appearance and texture from ordinary
peripheral nerves, being more transparent, delicate, and colourless.
They are said to supply the occlusor muscles of the stigmata. In the
Cockroach the median nerves are so slightly developed in the thorax
and abdomen (if they actually exist) that they are hardly discoverable
by ordinary dissection. We have found only obscure and doubtful traces
of them, and these only in one part of the abdominal nerve-cord. The
stomato-gastric nerves next to be described appear to constitute a
peculiar modification of that median nerve-cord which springs from the
circum-œsophageal connectives.

[103] Traité Anat., p. 201, pl. ix., fig. 1.

[104] Phil. Trans., 1834, p. 401, pl. xvi.

[105] Vom Bau des Thierischen Körpers, pp. 203, 262; Taf. z. vergl.
Anat., pl. vi., fig. 3.

_Stomato-gastric Nerves._

In the Cockroach the stomato-gastric nerves found in so many of the
higher Invertebrates are conspicuously developed. From the front of
each œsophageal connective, a nerve passes forwards upon the œsophagus,
outside the chitinous crura of the tentorium. Each nerve sends a branch
downwards to the labrum, and the remaining fibres, collected into two
bundles, join above the œsophagus to form a triangular enlargement,
the frontal ganglion. From this ganglion a recurrent nerve passes
backwards through the œsophageal ring, and ends on the dorsal surface
of the crop (·3 inch from the ring), in a triangular ganglion, from
which a nerve is given off outwards and backwards on either side.
Each nerve bifurcates, and then breaks up into branches which are
distributed to the crop and gizzard.[106] Just behind the œsophageal
ring, the recurrent nerve forms a plexus with a pair of nerves which
proceed from the back of the brain. Each nerve forms two ganglia, one
behind the other, and each ganglion sends a branch inwards to join the
recurrent nerve. Fine branches proceed from the paired nerves of the
œsophageal plexus to the salivary glands.

[106] The stomato-gastric nerves of the Cockroach have been carefully
described by Koestler (Zeits. f. wiss. Zool., Bd. XXXIX., p. 592).

[Illustration: Fig. 44.--Stomato-gastric Nerves of Cockroach. _fr.g._,
frontal ganglion; _at._, antennary nerve; _conn._, connective; _pa.g._,
paired ganglia; _r.n._, recurrent nerve; _v.g._, ventricular ganglion.]

The stomato-gastric nerves differ a good deal in different insects;
Brandt[107] considers that the paired and unpaired nerves are
complementary to each other, the one being more elaborate, according
as the other is less developed. A similar system is found in Mollusca,
Crustacea, and some Vermes (_e.g._, Nemerteans). When highly developed,
it contains unpaired ganglia and nerves, but may be represented only by
an indefinite plexus (earthworm). It always joins the œsophageal ring,
and sends branches to the œsophagus and fore-part of the alimentary
canal. The system has been identified with the sympathetic, and also
with the vagus of Vertebrates, but such correlations are hazardous; the
first, indeed, may be considered as disproved.

[107] “Mem. Acad. Petersb.,” 1835.

_Internal Structure of Brain._

The minute structure of the brain has been investigated by Leydig,
Dietl, Flögel, and others, and exhibits an unexpected complexity. It
is as yet impossible to reduce the many curious details which have
been described to a completely intelligible account. The physiological
significance, and the homologies of many parts are as yet altogether
obscure. The comparative study of new types will, however, in time,
bridge over the wide interval between the Insect-brain and the
more familiar Vertebrate-brain, which is partially illuminated by
physiological experiment. Mr. E. T. Newton has published a clear and
useful description[108] of the internal and external structure of the
brain of the Cockroach, which incorporates what had previously been
ascertained with the results of his own investigations. He has also
described[109] an ingenious method of combining a number of successive
sections into a dissected model of the brain. Having had the advantage
of comparing the model with the original sections, we offer a short
abstract of Mr. Newton’s memoir as the best introduction to the
subject. He describes the central framework of the Cockroach brain as
consisting of two solid and largely fibrous trabeculæ, which lie side
by side along the base of the brain, becoming smaller at their hinder
ends; they meet in the middle line, but apparently without fusion or
exchange of their fibres. Each trabecula is continued upwards by two
fibrous columns, the cauliculus in front, and the peduncle behind; the
latter carries a pair of cellular disks, the calices (the cauliculus,
though closely applied to the calices, is not connected with them);
these disks resemble two soft cakes pressed together above, and bent
one inwards, and the other outwards below. The peduncle divides above,
and each branch joins one of the calices of the same hemisphere.

[108] “Q. J. Micr. Sci.,” 1879, pp. 340–350, pl. xv., xvi.

[109] “Journ. Quekett Micr. Club,” 1879.

[Illustration: Fig. 45.--A, lobes of the brain of the Cockroach, seen
from within; _c_, cauliculus; _p_, peduncle; _t_, trabecula. B, ditto,
from the front; _ocx_, outer calyx; _icx_, inner calyx. C, ditto, from
above. Copied from E. T. Newton.]

This central framework is invested by cortical ganglionic cells,
which possess distinct nuclei and nucleoli. A special cellular mass
forms a cap to each pair of calices, and this consists of smaller
cells without nucleoli. Above the meeting-place of the trabeculæ is
a peculiar laminated mass, the _central body_, which consists of a
network of fibres continuous with the neighbouring ganglionic cells,
and enclosing a granular substance. The antennary lobes consist of a
network of fine fibres enclosing ganglion cells, and surrounded by a
layer of the same. It is remarkable that no fibrous communications
can be made out between the calices and the cauliculi, or between the
trabeculæ and the œsophageal connectives.

[Illustration: Fig. 46.--Model of Cockroach Brain, constructed from
slices of wood representing successive sections.]

[Illustration: Fig. 47.--Right half of Model-brain seen from the
inner side, with the parts dissected away, so as to show the anterior
nervous mass (_cauliculus_), _a_; the median mass (_trabecula_), _m_;
the mushroom-bodies (_calices_), _mb_; and their stems (_peduncles_),
_st_. The cellular cap, _c_, has been raised, so as to display the
parts below: _com_, is a part of the connective uniting the brain
and infra-œsophageal ganglia. [Figs. 45–48 are taken from Mr. E. T.
Newton’s paper in “Journ. Quekett Club,” 1879.]]

[Illustration: Fig. 48.--Diagrammatic outlines of sections of the Brain
of a Cockroach. Only one side of the brain is here represented. The
numbers indicate the position in the series of thirty-four sections
into which this brain was cut. _al_, antennary lobe; _mb_, mushroom
bodies (_calices_), with their cellular covering, _c_, and their stems
(_peduncles_), _st_; _a_, anterior nervous mass (_cauliculus_); _m_,
median nervous mass (_trabecula_). From E. T. Newton.]

[Illustration: Fig. 49.--Frontal section of Brain of Cockroach. _C_,
cellular layer beneath neurilemma; _ICx_, inner calix; _OCx_, outer
calix; _GC_, ganglion-cells; _P_, peduncle; _T_, trabecula; _Op_, optic
nerve; _AnL_, antennary lobe. × 24.]

_Sense Organs. The Eye of Insects._

The sense organs of Insects are very variable, both in position and
structure. Three special senses are indicated by transparent and
refractive parts of the cuticle, by tense membranes with modified
nerve-endings, and by peculiar sensory rods or filaments upon the
antennæ. These are taken to be the organs respectively of sight,
hearing, and smell. Other sense organs, not as yet fully elucidated,
may co-exist with these. The maxillary palps of the Cockroach,
for example, are continually used in exploring movements, and may
assist the animal to select its food; the cerci, where these are
well-developed, and the halteres of Diptera, have been also regarded as
sense organs of some undetermined kind, but this is at present wholly
conjecture.[110]

[110] It is to be remarked that unusually large nerves supply the
cerci of the Cockroach.

[Illustration: Fig. 50.--Plan of Eye of Cockroach, showing the number
of facets along the principal diameters. _as_, antennary socket.]

The compound eyes of the Cockroach occupy a large, irregularly oval
space (see fig. 50) on each side of the head. The total number of
facets may be estimated at about 1,800. The number is very variable in
Insects, and may either greatly exceed that found in the Cockroach, or
be reduced to a very small one indeed. According to Burmeister, the
Coleopterous genus Mordella possesses more than 25,000 facets. Where
the facets are very numerous, the compound eyes may occupy nearly the
whole surface of the head, as in the House-fly Dragon-fly, or Gad-fly.

Together with compound eyes, many Insects are furnished also with
simple eyes, usually three in number, and disposed in a triangle on
the forehead. The white fenestræ, which in the Cockroach lie internal
to the antennary sockets, may represent two simple eyes which have
lost their dioptric apparatus. In many larvæ only simple eyes are
found, and the compound eye is restricted to the adult form; in larval
Cockroaches, however, the compound eye is large and functional.

[Illustration: Fig. 51.--One element of the Compound Eye of the
Cockroach, × 700. _Co. F_, corneal facets; _Cr_, crystalline cones;
_Rm_, nerve-rod (rhabdom); _Rl_, retinula of protoplasmic fibrils.
To the right are transverse sections at various levels. Copied from
Grenacher.]

[Illustration: Fig. 52.--Diagram of Insect Integument, in section.
_bm_, basement-membrane; _hyp_, hypodermis, or chitinogenous layer;
_ct_, _ct_′, chitinous cuticle; _s_, a seta.]

Each facet of the compound eye is the outermost element of a series of
parts, some dioptric and some sensory, which forms one of a mass of
radiating rods or fibres. The facets are transparent, biconvex, and
polygonal, often, but not quite regularly, hexagonal. In many Insects
the deep layer of each facet is separable, and forms a concavo-convex
layer of different texture from the superficial and biconvex lens.
The facets, taken together, are often described as the cornea; they
represent the chitinous cuticle of the integument. The subdivision of
the cornea into two layers of slightly different texture suggests an
achromatic correction, and it is quite possible, though unproved, that
the two sets of prisms have different dispersive powers. Beneath the
cornea we find a layer of crystalline cones, each of which rests by
its base upon the inner surface of a facet, while its apex is directed
inwards towards the brain. The crystalline cones are transparent,
refractive, and coated with dark pigment; in the Cockroach they
are comparatively short and blunt. Behind each cone is a nerve-rod
(rhabdom), which, though outwardly single for the greater part of
its length, is found on cross-section to consist of four components
(rhabdomeres)[111]; these diverge in front, and receive the tip of
a cone, which is wedged in between them; the nerve-rods are densely
pigmented. The rhabdom is invested by a protoplasmic sheath, which
is imperfectly separated into segments (retinulæ), corresponding in
number with the rhabdomeres. Each retinula possesses at least one
nucleus. The retinulæ were found by Leydig to possess a true visual
purple. To the hinder ends of the retinulæ are attached the fibres of
the optic nerve, which at this point emerges through a “fenestrated
membrane.”

[111] The number in Insects varies from eight to four, but seven is
usual; four is the usual number in Crustacea.

[Illustration: Fig. 53.--Section through Eye of Dytiscus-larva, showing
the derivation of the parts from modified hypodermic cells. _L_, lens;
_Cr_, crystalline cones; _R_, nerve-rods; _N. Op._ optic nerve. From
Grenacher.]

In the simple eye the non-faceted cornea and the retinula are readily
made out, but the crystalline cones are not developed as such. The
morphological key to both structures is found in the integument, of
which the whole eye, simple or compound, is a modification. A defined
tract of the chitinous cuticle becomes transparent, and either swells
into a lens (fig. 53), or becomes regularly divided into facets
(fig. 55), which are merely the elaboration of imperfectly separated
polygonal areas, easily recognised in the young cuticle of all parts
of the body. Next, the chitinogenous layer is folded inwards, so as to
form a cup, and this, by the narrowing of the mouth, is transformed
into a flask, and ultimately into a solid two-layered cellular
mass (fig. 53). The deep layer undergoes conversion into a retina,
its chitinogenous cells developing the nerve-rods as interstitial
structures, while the superficial layer, which loses its functional
importance in the simple eye, gives rise by a similar process of
interstitial growth to the crystalline cones of the compound eye
(fig. 55). The basement-membrane, underlying the chitinogenous cells,
is transformed into the fenestrated membrane. The nerve-rods stand upon
it, like organ pipes upon the sound-board, while fibrils of the optic
nerve and fine tracheæ pass through its perforations. The mother-cells
of the crystalline cones and nerve-rods are largely replaced by the
interstitial substances they produce, to which they form a sheath; they
are often loaded with pigment, and the nuclei of the primitive-cells
can only be distinguished after the colouring-matter has been
discharged by acids or alkalis.

[Illustration: Fig. 54.--Section through Simple Eye of Vespa. The
references as above. Simplified from Grenacher.]

Dr. Hickson[112] has lately investigated the minute anatomy of the
optic tract in various Insects. He finds, in the adult of the higher
Insects, three distinct ganglionic swellings, consisting of a network
of fine fibrils, surrounded by a sheath of crowded nerve-cells. Between
the ganglia the fibres usually decussate. In the Cockroach, and some
other of the lower Insects, the outermost ganglion is undeveloped.
The fibres connecting the second ganglion with the eye take a straight
course in the young Cockroach, but partially decussate in the adult.

[112] “Q. J. Micr. Sci.,” 1885.

All the parts between the crystalline cones and the true optic nerve
are considered by Hickson to compose the retina of Insects, which,
instead of ending at the fenestrated-membrane, as has often been
assumed, includes the ganglia and decussating fibres of the optic
tract. The layer of retinulæ and rhabdoms does not form the whole
retina, but merely that part which, in the vertebrate eye, is known as
the layer of rods and cones.

[Illustration: Fig. 55.--Diagrammatic section of Compound Eye. The
references as above.]

As to the way in which the compound eye renders distinct vision
possible, there is still much difference of opinion. A short review
of the discussion which has occupied some of the most eminent
physiologists and histologists for many years past will introduce the
reader to the principal facts which have to be reconciled.

The investigation, like so many other trains of biological inquiry,
begins with Leeuwenhoeck (Ep. ad Soc. Reg. Angl. iii.), who ascertained
that the cornea of a shardborne Beetle, placed in the field of a
microscope, gives images of surrounding objects, and that these
images are inverted. When the cornea is flattened out for microscopic
examination, the images (_e.g._, of a window or candle-flame) are
similar, and it has been too hastily assumed that a multitude of
identical images are perceived by the Insect. The cornea of the
living animal is, however, convex, and the images formed by different
facets cannot be precisely identical. No combined or collective image
is formed by the cornea. When the structure of the compound eye had
been very inadequately studied, as was the case even in Cuvier’s time
(Leçons d’Anat. Comp., xii., 14), it was natural to suppose that all
the fibres internal to the cornea were sensory, that they formed a
kind of retina upon which the images produced by the facets were
received, and that these images were transmitted to the brain, to be
united, either by optical or mental combination, into a single picture.
Müller,[113] in 1826, pointed out that so simple an explanation was
inadmissible. He granted that the simple eye, with its lens and
concave retina, produces a single inverted image, which is able to
affect the nerve-endings in the same manner as in Vertebrates. But the
compound eye is not optically constructed so as to render possible the
formation of continuous images. The refractive and elongate crystalline
cones, with their pointed apices and densely pigmented sides, must
destroy any images formed by the lenses of the cornea. Even if the
dioptric arrangement permitted the formation of images, there is no
screen to receive them.[114] Lastly, if this difficulty were removed,
Müller thought it impossible for the nervous centres to combine a
great number of inverted partial images. How then can Insects and
Crustaceans see with their compound eyes? Müller answered that each
facet transmits a small pencil of rays travelling in the direction
of its axis, but intercepts all others. The refractive lens collects
the rays, and the pigmented as well as refractive crystalline cone
further concentrates the pencil, while it stops out all rays which
diverge appreciably from the axis. Each element of the compound eye
transmits a single impression of greater or less brightness, and the
brain combines these impressions into some kind of picture, a picture
like that which could be produced by stippling. It may be added that
the movements of the insect’s head or body would render the distance
and form of every object in view much readier of appreciation. No
accommodation for distance would be necessary, and the absence of all
means of accommodation ceases to be perplexing. Such is Müller’s theory
of what he termed “mosaic vision.” Many important researches, some
contradictory, some confirmatory of Müller’s doctrine,[115] have since
been placed on record, with the general result that some modification
of Müller’s theory tends to prevail. The most important of the new
facts and considerations which demand attention are these:--

[113] Exner has since determined by measurement and calculation the
optical properties of the eye of Hydrophilus. He finds that the focus
of a corneal lens is about 3mm. away, and altogether behind the eye.

[114] Zur vergl. Phys. des Gesichtsinnes.

[115] A critical history of the whole discussion is to be found in
Grenacher’s “Sehorgan der Arthropoden” (1879), from which we take
many historical and structural details.

Reasons have been given for supposing that images are formed by the
cornea and crystalline cones together. This was first pointed out by
Gottsche (1852), who used the compound eyes of Flies for demonstration.
Grenacher has since ascertained that the crystalline cones of Flies
are so fluid that they can hardly be removed, and he believes that
Gottsche’s images were formed by the corneal facets alone. He finds,
however, that the experiment may be successfully performed with
eyes not liable to this objection, _e.g._, the eyes of nocturnal
Lepidoptera. A bit of a Moth’s eye is cut out, treated with nitric acid
to remove the pigment, and placed on a glass slip in the field of the
microscope. The crystalline cones, still attached to the cornea, are
turned towards the observer, and one is selected whose axis coincides
with that of the microscope. No image is visible when the tip of the
cone is in focus, but as the cornea approaches the focus, a bristle,
moved about between the mirror and the stage, becomes visible. This
experiment is far from decisive. No image is formed where sensory
elements are present to receive and transmit it. Moreover, the image is
that of an object very near to the cornea, whereas all observations of
living Insects show that the compound eye is used for far sight, and
the simple eye for near sight. Lastly, the treatment with acid, though
unavoidable, may conceivably affect the result. It is not certain that
the cones really assist in the production of the image, which may be
due to the corneal facets alone, though modified by the decolorised
cones.

Grenacher has pointed out that the composition of the nerve-rod
furnishes a test of the mosaic theory. According as the percipient
rod is simple or complex, we may infer that its physiological action
will be simple or complex too. The adequate perception of a continuous
picture, though of small extent, will require many retinal rods; on
the other hand, a single rod will suffice for the discrimination of
a bright point. What then are the facts of structure? Grenacher has
ascertained that the retinal rods in each element of the compound eye
rarely exceed seven, and often fall as low as four--further, that the
rods in each group are often more or less completely fused so as to
resemble simple structures, and that this is especially the case with
Insects of keen sight.[116]

[116] Flies, whose eyes are in several respects exceptional, have
almost completely separated rods, notwithstanding their quick sight.

Certain facts described by Schultze tell on the other side. Coming
to the Arthropod eye, fresh from his investigation of the vertebrate
retina, Schultze found in the retinal rods of Insects the same lamellar
structure which he had discovered in Vertebrata. He found also that
in certain Moths, Beetles, and Crustacea, a bundle of extremely fine
fibrils formed the outer extremity of each retinal or nerve-rod. This
led him to reject the mosaic theory of vision, and to conclude that a
partial image was formed behind every crystalline cone, and projected
upon a multitude of fine nerve-endings. Such a retinula of delicate
fibrils has received no physiological explanation, but it is now known
to be of comparatively rare occurrence; it has no pigment to localise
the stimulus of light; and there is no reason to suppose that an image
can be formed within its limits.

The optical possibility of such an eye as that interpreted to us by
Müller has been conceded by physicists and physiologists so eminent
as Helmholz and Du Bois Reymond. Nevertheless, the competence of any
sort of mosaic vision to explain the precise and accurate perception
of Insects comes again and again into question whenever we watch the
movements of a House-fly as it avoids the hand, of a Bee flying from
flower to flower, or of a Dragon-fly in pursuit of its prey. The sight
of such Insects as these must range over several feet at least, and
within this field they must be supposed to distinguish small objects
with rapidity and certainty. How can we suppose that an eye without
retinal screen, or accommodation for distance, is compatible with sight
so keen and discriminating? The answer is neither ready nor complete,
but our own eyesight shows how much may be accomplished by means of
instruments far from optically perfect. According to Aubert, objects,
to be perceived as distinct by the human eye, must have an angular
distance of from 50″ to 70″, corresponding to several retinal rods.
Our vision is therefore mosaic too, and the retinal rods which can be
simultaneously affected comprise only a fraction of those contained
within the not very extensive area of the effective retina. Still
we are not conscious of any break in the continuity of the field of
vision. The incessant and involuntary movements of the eyeball, and
the appreciable duration of the light-stimulus partly explain the
continuity of the image received upon a discontinuous organ. Even more
important is the action of the judgment and imagination, which complete
the blanks in the sensorial picture, and translate the shorthand of the
retina into a full-length description. That much of what we see is seen
by the mind only is attested by the inadequate impression made upon us
by a sudden glimpse of unfamiliar objects. We need time and reflection
to interpret the hints flashed upon our eyes, and without time and
reflection we see nothing in its true relations. The Insect-eye may be
far from optical perfection, and yet, as it ranges over known objects,
the Insect-mind, trained to interpret colour, and varying brightness,
and parallax, may gain minute and accurate information. Grant that the
compound eye is imperfect, and even rude, if regarded as a camera; this
is not its true character. It is intended to receive and interpret
flashing signals; it is an optical telegraph.

Plateau[117] has recently submitted the seeing powers of a number of
different Insects to actual experiment. The two windows of a room five
metres square were darkened. An aperture fitted with ground glass was
then arranged in each window. At a distance of four metres from the
centre of the space between the windows captive Insects were from time
to time liberated. One of the windows was fenced with fine trellis, so
as to prevent the passage of the Insect, or otherwise altered in form,
but the size of the aperture could be increased at pleasure, so as
exactly to make up for any loss of light caused thereby, the brightness
of the two openings being compared by a photometer.

[117] Bull. de l’Acad. Roy. de Belgique, 1885.

It was found that day-flying Insects require a tolerably good light; in
semi-obscurity they cannot find their way, and often refuse to fly at
all. By varnishing one or other set in Insects possessing both simple
and compound eyes, it was found that day-flying Insects provided with
compound eyes do not use their simple eyes to direct their course.
When the light from one window was sensibly greater than that from
the other, the Insect commonly chose the brightest, but the existence
of bars, close enough to prevent or to check its passage, had no
perceptible effect upon the choice of its direction. Alterations in
the shape of one of the panes seemed to be immaterial, provided that
the quantity of light passing through remained the same, or nearly the
same. Plateau concludes that Insects do not distinguish the forms of
objects, or distinguish them very imperfectly.

It is plain, and Plateau makes this remark himself, that such
experiments upon the power of unaided vision in Insects, give a very
inadequate notion of the facility with which an Insect flying at large
can find its way. There the animal is guided by colour, smell, and the
actual or apparent movements of all visible objects. Exner has pointed
out how important are the indications given by movement. Even in man,
the central part of the retina is alone capable of precise perception
of form, but a moving object is observed by the peripheral tract.
Plateau (from whom this quotation is made) adds that most animals are
very slightly impressed by the mere form of their enemies, or of their
prey, but the slightest movement attracts their notice. The sportsman,
the fisherman, and the entomologist cannot fail to learn this fact by
repeated and cogent proofs.

_Sense of Smell in Insects._

The existence of a sense of smell in Insects has probably never been
disputed. Many facts of common observation prove that carrion-feeders,
for example, are powerfully attracted towards putrid animal substances
placed out of sight. The situation of the olfactory organs has only
been ascertained by varied experiments and repeated discussion.
Rosenthal, in 1811, and Lefebvre, in 1838, indicated the antennæ as
the organs of smell, basing their conclusions upon physiological
observations made upon living insects. Many entomologists of that
time were inclined to regard the antennæ as auditory organs.[118]
Observations on the minute structure of the antennæ were made by
many workers, but for want of good histological methods and accurate
information concerning the organs of smell in other animals, these
proved for a long time indecisive. It was by observation of living
insects that the point was actually determined.

[118] References to the literature of the question are given by
Hauser in Zeits. f. wiss. Zool., Bd. XXXIV., and by Plateau in Bull.
Soc. Zool. de France, Tom. X.

Hauser’s experiments, though by no means the first, are the most
instructive which we possess. He found that captive insects, though not
alarmed by a clean glass rod cautiously brought near, became agitated
if the same rod had been first dipped in carbolic acid, turpentine, or
acetic acid. The antennæ performed active movements while the rod was
still distant, and after it was withdrawn the insect was observed to
wipe its antennæ by drawing them through its mouth. After the antennæ
had been extirpated or coated with paraffin, the same insects became
indifferent to strong-smelling substances, though brought quite near.
Extirpation of the antennæ prevented flies from discovering putrid
flesh, and hindered or prevented copulation in insects known to breed
in captivity.

Following up these experiments by histological investigation of many
insects belonging to different orders, Hauser clearly established the
following points, which had been partially made known before:--

The sensory elements of the antennæ are lodged in grooves or pits,
which may be filled with fluid. The nerve-endings are associated with
peculiar rods, representing modified chitinogenous cells. The number
of grooves or pits may be enormous. In the male of the Cockchafer,
Hauser estimates that there are 39,000 in each antenna. He remarks
that in all cases where the female Insect is sluggish and prone to
concealment, the male has the antennæ more largely developed than the
female.

_Sense of Taste in Insects._

F. Will[119] gives an account of many authors who have investigated
with more or less success the sense organs of various Insects. He
relates also the results of his own experiments, and gives anatomical
details of the sensory organs of the mouth in various Hymenoptera.

[119] Zeits. f. wiss. Zool., 1885.

Wasps, flying at liberty, were allowed to visit and taste a packet
of powdered sugar. This was left undisturbed for some hours, and
then replaced by alum of the same appearance. The Wasps attacked the
alum, but soon indicated by droll movements that they perceived the
difference. They put their tongues in and out and cleansed them from
the ill-tasted powder. Two persisted at the alum till they rolled on
the table in agony, but they soon recovered and flew away. In a few
hours the packet was quite deserted. After a day’s interval, during
which the sugar lay in its usual place, powdered, and of course
perfectly tasteless, dolomite was substituted. The wasps licked it
diligently and could not be persuaded for a long time that it could do
nothing for them. Similar experiments were made with other substances,
and Insects whose antennæ and palps had been removed were subjected to
trial. The result clearly proved that a sense of taste existed, and
that its seat is in the mouth.[120] Peculiar nerve-endings, such as
Meinert and Forel had previously found in Ants, were found in abundance
on the labium, the paraglossæ, and the inner side of the maxillæ of
the Wasp. Some lay in pits, through the bases of which single nerves
emerged, and swelled into bulbs, or passed into peculiar conical
sheaths. Interspersed among the gustatory nerve-endings were setæ of
various kinds, some protective, some tactile, and others intended to
act as guiding-hairs for the saliva.

[120] Will confirms, by his own experiments (p. 685), Plateau’s
conclusion (_Supra_, p. 46), that the maxillary and labial palps have
nothing to do with the choice of food.

Will observes that the organs described satisfy the essential
conditions of a sense of taste. The nerve-endings pass free to the
surface, and are thus directly accessible to chemical stimulus.
Further, they are so placed that they and the particles of food which
get access to them are readily bathed by the saliva. Moistened or
dissolved in this fluid, the sapid properties of food are most fully
developed.

The sensory pits and bulbs appropriated to taste are believed to be
unusually abundant in the social Hymenoptera.

_Sense of Hearing in Insects._

The auditory organs of Insects and other Arthropoda are remarkable
for the various parts of the body in which they occur. Thus they have
been found in the first abdominal segment of Locusts, and in the tibia
of the fore-leg of Crickets and Grasshoppers, and more questionable
structures with peculiar nerve-endings have been described as occurring
in the hinder part of the abdomen of various larvæ (_Ptychoptera_,
_Tabanus_, _&c_). The auditory organ of Decapod Crustacea is lodged
in the base of the antennule, that of Stomapods in the tail, while an
auditory organ has been lately discovered on the underside of the head
of the Myriopod _Scutigera_.

Auditory organs are best developed in such Insects as produce sounds as
a call to each other. The Cockroach is dumb, and it is, therefore, not
a matter of surprise that no structure which can be considered auditory
should have ever been detected in this Insect.[121]

[121] For a popular account of auditory organs in Insects, see
Graber’s Insekten, Vol. I., page 287; also J. Müller, Vergl. Phys.
d. Gesichssinn, p. 439; Siebold, Arch. f. Naturg., 1844; Leydig,
Müller’s Arch. 1855 and 1860; Hensen, Zeits. f. wiss. Zool., 1866;
Graber, Denkschr. der Akad. der wiss. Wien, 1875; and Schmidt, Arch.
f. mikr. Anat., 1875.

The sensory hairs of the skin have been already noticed (p. 31).

CHAPTER VII.

THE ALIMENTARY CANAL AND ITS APPENDAGES.

_SPECIAL REFERENCES._

CHOLODKOWSKY. Zur Frage über den Bau und über die Innervation der
Speicheldrüsen der Blattiden. Horæ Soc. Entom. Rossicæ, Tom. XVI.
(1881). [Salivary Glands of Cockroaches.]

SCHINDLER. Beiträge zur Kenntniss der Malpighi’schen Gefässe der
Insekten. Zeits. f. wiss. Zool., Bd. XXX. (1878). [Malpighian
Tubules of Insects.]

CHUN. Ueber den Bau, die Entwickelung, und physiologische Bedeutung
der Rectaldrüsen bei den Insekten. Abh. der Senkenbergischen
Naturforschers Gesellschaft, Bd. X. (1876). [Rectal Glands of
Insects.]

LEYDIG. Lehrbuch der Histologie, &c., and VIALLANES. (_Loc. cit._
_supra_, chap. iv.) [Histology of Alimentary Canal.]

BASCH. Untersuchungen über das Chylopoëtische und Uropoëtische
System der Blatta orientalis. Kais. Akad. der Wissenschaften.
(Math-Nat. Classe.), Bd. XXXIII. (1858). [Digestive and Excretory
Organs of Blatta.]

SIRODOT. Recherches sur les Sécrétions chez les Insectes. Ann. Sci.
Nat., 4^e Série, Zool., Tom. X. (1859). [Digestive and Excretory
Organs of Oryctes, &c.]

JOUSSET DE BELLESME. Recherches expérimentales sur la digestion des
Insectes et en particulier de la Blatte (1875).

PLATEAU. Recherches sur les Phénomènes de la Digestion chez les
Insectes. Mem. de l’Acad. Roy. de Belgique, Tom. XLI. (1874).
[Now the principal authority on the Digestion of Insects. The
other physiological memoirs cited (Nos. 5, 6, 7) are chiefly of
historical interest.]

PLATEAU. Note additionelle. Bull. Acad. Roy. de Belgique, 2^e Sér.,
Tom. XLIV. (1877). [Contains some corrections of importance.]

_The Alimentary Canal._

The alimentary canal of the Cockroach measures about 2-3/4 inches in
length, and is therefore about 2-3/4 times the length of the body. In
herbivorous Insects the relative length of the alimentary canal may be
much greater than this; it is five times the length of the body in
Hydrophilus. Parts of the canal are specialised for different digestive
offices, and their order and relative size are given in the following
table:--

Œsophagus and crop ·95 in.
Gizzard ·1
Chylific stomach ·5
Small intestine ·1
Colon ·875
Rectum ·25
-----
2·775
=====

[Illustration: Fig. 56.--Alimentary Canal of Cockroach. × 2.]

The principal appendages of the alimentary canal are the salivary
glands, the cæcal diverticula of the stomach, and the Malpighian
tubules.

Considered with respect to its mode of formation, the alimentary canal
of all but the very simplest animals falls into three sections--viz.,
(1) the mesenteron, or primitive digestive cavity, lined by hypoblast;
(2) the stomodæum, or mouth-section, lined by epiblast, continuous with
that of the external surface; and (3) the proctodæum, or anal section,
lined by epiblast folded inwards from the anus, just as the epiblast
of the stomodæum is folded in from the mouth. The mesenteron of the
Cockroach is very short, as in other Arthropoda, and includes only the
chylific stomach with its diverticula. The mouth, œsophagus, and crop
form the stomodæum, while the proctodæum begins with the Malpighian
tubules, and extends thence to the anus. Both stomodæum and proctodæum
have a chitinous lining, which is wanting in the mesenteron. At the
time of moult, or a little after, this lining is broken up and passed
out of the body.

[Illustration: Fig. 57.--Section of Wall of Crop. _Cc_, chitinous
layer; _C_, chitinogenous cells; _Mi_, inner muscular layer; _Mo_,
outer do. × 275.]

The mouth of the Cockroach is enclosed between the labrum in front,
and the labium behind, while it is bounded laterally by the mandibles
and first pair of maxillæ. The chitinous lining is thrown into many
folds, some of which can be obliterated by distension, while others are
permanent and filled with solid tissues. The lingua is such a permanent
fold, lying like a tongue upon the posterior wall of the cavity and
reaching as far as the external opening. The thin chitinous surface of
the lingua is hairy, like other parts of the mouth, and stiffened by
special chitinous rods or bands. The salivary ducts open by a common
orifice on its hinder surface. Above, the mouth leads into a narrow
gullet or œsophagus, with longitudinally folded walls, which traverses
the nervous ring, and then passes through the occipital foramen to the
neck and thorax. Here it gradually dilates into the long and capacious
crop, whose large rounded end occupies the fore-part of the abdomen.
When empty, or half-empty, the wall of the crop contracts, and is
thrown into longitudinal folds, which disappear on distension. Numerous
tracheal tubes ramify upon its outer surface, and appear as fine white
threads upon a greenish-grey ground.

[Illustration: Fig. 58.--Wall of Crop, in successive layers. References
as in fig. 57. × 250.]

Three layers can be distinguished in the wall of the crop--viz., (1)
the muscular, (2) the epithelial, and (3) the chitinous layer.[122]
The muscular layer consists of annular and longitudinal fibres,
crossing at right angles. (See fig. 58.) In most animals the muscles
of organic life, subservient to nutrition and reproduction, are very
largely composed of plain or unstriped fibres. In Arthropoda (with
the exception of the anomalous Peripatus) this is not generally the
case, and the muscular fibres of the alimentary canal belong to the
striped variety. The epithelium rests upon a thin structureless
basement-membrane, which is firmly united in the œsophagus and crop
to the muscular layer and the epithelium. The epithelium consists of
scattered nucleated cells, rounded or oval. These epithelial cells,
homologues of the chitinogenous cells of the integument, secrete
the transparent and structureless chitinous lining. Hairs (setæ) of
elongate, conical form, and often articulated at the base, like the
large setæ of the outer skin, are abundant. In the œsophagus they are
very long, and grouped in bundles along sinuous transverse lines. In
the crop the hairs become shorter, and the sinuous lines run into a
polygonal network. The points of the hairs are directed backwards, and
they no doubt serve to guide the flow of saliva towards the crop.

[122] Here, as generally in the digestive tube of the adult
Cockroach, the peritoneal layer is inconspicuous or wanting. It
occasionally becomes visible--_e.g._, in the outer wall of the
Malpighian tubules, and in the tubular prolongation of the gizzard.

[Illustration: Fig. 59.--Transverse section of Gizzard of Cockroach.
The chitinous folds are represented here as symmetrical. See next
figure. × 30.]

The gizzard has externally the form of a blunt cone, attached by its
base to the hinder end of the crop, and produced at the other end
into a narrow tube (1/4 to 1/3 in. long), which projects into the
chylific stomach. Its muscular wall is thick, and consists of many
layers of annular fibres, while the internal cavity is nearly closed by
radiating folds of the chitinous lining. Six of the principal folds,
the so-called “teeth,” are much stronger than the rest, and project
so far inwards that they nearly meet. They vary in form, but are
generally triangular in cross section and irregularly quadrilateral
in side view. Between each pair are three much less prominent folds,
and between these again are slight risings of the chitinous lining. A
ridge runs along each side of the base of each principal tooth, and
the minor folds, as well as part of the principal teeth, are covered
with fine hairs. The central one of each set of secondary folds is
produced behind into a spoon-shaped process, which extends considerably
beyond the rest, and gradually subsides till it hardly projects from
the internal surface of the gizzard. Behind each large tooth (_i.e._,
towards the chylific stomach) is a rounded cushion set closely with
hairs, and between and beyond these are hairy ridges. (See fig. 61.)
The whole forms an elaborate machine for squeezing and straining
the food, and recalls the gastric mill and pyloric strainer of the
Crayfish. The powerful annular muscles approximate the teeth and folds,
closing the passage, while small longitudinal muscles, which can be
traced from the chitinous teeth to the cushions, appear to retract
these last, and open a passage for the food.[123]

[123] Plateau has expressed a strong opinion that neither in the
stomach of Crustacea nor in the gizzard of Insects have the so-called
teeth any masticatory character. He compares them to the psalterium
of a Ruminant, and considers them strainers and not dividers of the
food. His views, as stated by himself, will be found on p. 131.

[Illustration: Fig. 60.--The Six Primary Folds (teeth) of the Gizzard,
seen in profile.]

[Illustration: Fig. 61.--Part of Gizzard laid open, showing two teeth
(_T_) and the intermediate folds, as well as the hairy pads below.
_A-A_ and _B-B_ are lines of section (see figs. 62 and 63). × 50.]

[Illustration: Fig. 62.--Section through one tooth and two intermediate
spaces (see figure 61, _A-A_). _Cc_, chitinous cuticle; _C_,
chitinogenous layer; _am_, annular muscles; _p_, peritoneal layer.
× 75.]

[Illustration: Fig. 63.--Section through one principal hairy ridge and
two intermediate spaces (see fig. 61, _B-B_); _rm_, radiating muscles;
_tr_, trachea. The other references as before. × 75.]

The gizzard ends below, as we have already mentioned, in a narrow
cylindrical tube which is protruded into the chylific stomach for about
one-third of an inch. Folds project from the wall of this tube, and
reduce its central cavity to an irregular star-like figure. Below it
ends in free processes slightly different from each other in size and
shape. The chitinous lining and the chitinogenous layer beneath pass
to the end of the tube and are then reflected upon its outer wall,
ascending till they meet the lining epithelium of the cæcal tubes.
Between the wall of the gizzard-tube and its external reflected layer,
tracheal tubes, fat-cells, and longitudinal muscles are enclosed.

[Illustration: Fig. 64.--Longitudinal section through Gizzard and
fore-part of Chylific Stomach. _G_, gizzard; _Tu_, cæcal tube; _St_,
stomach; _Ep_, its lining epithelium. _A_ and _B_ are enlarged in the
side figures. × 35.

_A._--The Reflected Chitinogenous Layer of the Tubular Gizzard. _Tr_,
tracheal tube. × 400.

_B._--One of the Tubular Extensions of the same, enclosing muscles and
tracheæ. × 400.]

[Illustration: Fig. 65.--Transverse section of tubular prolongation of
Gizzard, within the Chylific Stomach, part of which is shown at its
proper distance. _R C_, reflected chitinogenous layer; _Tr_, tracheal
tube; _M_, cross section of muscle; _Ep_, epithelium of chylific
stomach. × 100.]

The chylific stomach is a simple cylindrical tube, provided at its
anterior end with eight (sometimes fewer) cæcal tubes, and opening
behind into the intestine. Its muscular coat consists of a loose
layer of longitudinal fibres, enclosing annular fibres. Internal to
these is a basement membrane, which supports an epithelium consisting
of elongate cells which are often clustered into regular eminences,
and separated by deep cavities. The epithelium forms no chitinous
lining in the chylific stomach or cæcal tubes; and this peculiarity,
no doubt, promotes absorption of soluble food in this part of the
alimentary canal. Short processes are given off from the free ends of
the epithelial cells, as in the intestines of many Mammalia and other
animals.

[Illustration: Fig. 66.--Epithelium of Chylific Stomach. In the upper
figure the digestive surface is indented, while in the lower figure
it is flat. Both arrangements are common, and may be seen in a single
section. The epithelial buds are shown below, and again below these the
annular and longitudinal muscles. × 220.]

Between the cells a reticulum is often to be seen, especially where the
cells have burst; it extends between and among all the elements of the
mucous lining, and probably serves, like the very similar structure
met with in Mammalian intestines,[124] to absorb and conduct some of
the products of digestion. Different epithelial cells may be found in
all the stages noticed by Watney--viz., (1) with divided nuclei; (2)
small, newly produced cells at the base of the epithelium; (3) short
and broad cells, overtopped by the older cells around; (4) dome-shaped
masses of young cells, forming “epithelial buds”;[125] (5) full-grown
cells, ranging with those on either side, so as to form an unbroken and
uniform series. The regeneration of the tissue is thus provided for.
The cells come to maturity and burst, when new cells, the product of
the epithelial buds, take their place.

[124] See Watney, Phil. Trans., 1877, Pt. II. The “epithelial buds”
described and figured in this memoir are also closely paralleled in
the chylific stomach of the Cockroach.

[125] These epithelial buds have been described as glands, and we
only saw their significance after comparing them with Dr. Watney’s
account.

The epithelium of the chylific stomach is continued into the eight
cæcal tubes, where it undergoes a slight modification of form.

[Illustration: Fig. 67.--Section of Chylific Stomach, showing the six
bundles of Malpighian tubules. × 70.]

At the hinder end of the chylific stomach is a very short tube about
half the diameter of the stomach, the small intestine. At its junction
with the chylific stomach are attached, in six bundles, 60 or 70 long
and fine tubules, the Malpighian tubules.[126] The small intestine has
the same general structure as the œsophagus and crop; its chitinous
lining is hairy, and thrown into longitudinal folds which become much
more prominent in the lower part of the tube. The junction of the small
intestine with the colon is abrupt, and a strong annular fold assumes
the character of a circular valve (fig. 68).

[126] Development shows that these tubules belong to the proctodæum,
and not to the mesenteron.

[Illustration: Fig. 68.--Junction of Small Intestine with Colon. × 15.]

From the circular valve the colon extends for nearly an inch. Its
diameter is somewhat greater than that of the chylific stomach, and
uniform throughout, except for a lateral diverticulum or cæcum, which
is occasionally but not constantly present towards its rectal end.
The fore part of the colon is thrown into a loose spiral coil. A
constriction divides the colon from the next division of the alimentary
canal, the rectum.

The rectum is about 1/4 inch long, and is dilated in the middle when
distended. Six conspicuous longitudinal folds project into the lumen of
the tube. These folds are characterised by an unusual development of
the epithelium, which is altogether wanting in the intermediate spaces,
where the chitinous lining blends with the basement-membrane, both
being thrown into sharp longitudinal corrugations. Between the six
epithelial bands and the muscular layer are as many triangular spaces,
in which ramify tracheal tubes and fine nerves for the supply of the
epithelium. The chitinous layer is finely setose. The muscular layer
consists of annular fibres strengthened externally by longitudinal
fibres along the interspaces between the six primary folds.[127]

[127] The epithelial bands of the rectum of Insects were first
discovered by Swammerdam in the Bee (Bibl. Nat., p. 455, pl. xviii.,
fig. 1). Dufour called them muscular bands (Rech. sur les
Orthoptères, &c., p. 369, fig. 44).

[Illustration: Fig. 69.--Transverse section of Small Intestine and
Colon, close to their junction. × 50.]

The corrugated and non-epitheliated interspaces may be supposed to
favour distension of the rectal chamber, while the great size of the
cells of the bands of epithelium is perhaps due to their limited
extent. Leydig[128] attributed to these rectal bands a respiratory
function, and compared them to the epithelial folds of the rectum
of Libellulid larvæ, which, as is well known, respire by admitting
fresh supplies of water into this cavity. It is an obvious objection
that Cockroaches and other Insects in which the rectal bands are well
developed do not take water into the intestine at all. Gegenbaur has
therefore modified Leydig’s hypothesis. He suggests (Grundzüge d.
Vergl. Anat.) that the functional rectal folds of Dragon-flies and
the non-functional folds of terrestrial Insects are both survivals of
tracheal gills, which were the only primitive organs of respiration
of Insects. The late appearance of the rectal folds and the much
earlier appearance of spiracles is a serious difficulty in the way
of this view, as Chun has pointed out. It seems more probable that
the respiratory appendages of the rectum of the Dragon-fly larvæ
are special adaptations to aquatic conditions of a structure which
originated in terrestrial Insects, and had primarily nothing to do with
respiration.

[128] “Lehrbuch der Histologie,” p. 337.

[Illustration: Fig. 70.--Transverse section of Rectum. × 50.]

The number of the rectal bands (six) is worthy of remark. We find six
sets of folds in the gizzard and small intestine of the Cockroach,
six bundles of Malpighian tubules, with six intermediate epitheliated
bands. There are also six longitudinal bands in the intestine of the
Lobster and Crayfish. The tendency to produce a six-banded stomodæum
and proctodæum may possibly be related to the six theoretical elements
(two tergal, two pleural, two sternal,) traceable in the Arthropod
exoskeleton, of which the proctodæum and stomodæum are reflected folds.

The anus of the Cockroach opens beneath the tenth tergum, and between
two “podical” plates. Anal glands, such as occur in some Beetles, have
not been discovered in Cockroaches.

_Appendages. The Salivary Glands._

The three principal appendages of the alimentary canal of the Cockroach
are outgrowths of the three primary divisions of the digestive tube;
the salivary glands are diverticula of the stomodæum, the cæcal tubes
of the mesenteron, and the Malpighian tubules of the proctodæum.

[Illustration: Fig. 71.--Salivary Glands and Receptacle, right side.
The arrow marks the opening of the common duct on the back of the
lingua. A, side view of lingua; B, front view of lingua.]

A large salivary gland and reservoir lie on each side of the œsophagus
and crop. The gland is a thin foliaceous mass about 1/3 in. long, and
composed of numerous acini, which are grouped into two principal lobes.
The efferent ducts form a trunk, which receives a branch from a small
accessory lobe, and then unites with its fellow. The common glandular
duct thus formed opens into the much larger common receptacular duct,
formed by the union of paired outlets from the salivary reservoirs. The
common salivary duct opens beneath the lingua. Each salivary reservoir
is an oval sac with transparent walls, and about half as long again
as the gland. The ducts and reservoirs have a chitinous lining, and
the ducts exhibit a transverse marking like that of a tracheal tube.
When examined with high powers the wall of the salivary gland shows
a network of protoplasm with large scattered nuclei, resting upon a
structureless chitinous membrane.

The salivary glands are unusually large in most Orthoptera.[129] In
other orders they are of variable occurrence and of very unequal
development.

[129] Except in Dragon-flies and Ephemeræ.

_The Cæcal Tubes._

There are eight (sometimes fewer) cæcal tubes arranged in a ring
round the fore end of the chylific stomach; they vary in length,
the longer ones, which are about equal to the length of the stomach
itself, usually alternating with shorter ones, though irregularities of
arrangement are common. The tubes are diverticula of the stomach and
lined by a similar epithelium. In the living animal they are sometimes
filled with a whitish granular fluid.

Similar cæcal tubes, sometimes very numerous and densely clustered, are
attached to the stomach in many Crustacea and Arachnida. The researches
of Hoppe Seyler, Krukenberg, Plateau, and others have established the
digestive properties of the fluid secreted in them, which agrees with
the pancreatic juice of Vertebrates.

_The Malpighian Tubules._

The Malpighian tubules mark the beginning of the small intestine, to
which they properly belong. They are very numerous (60–70) in the
Cockroach, as in Locusts, Earwigs, and Dragon-flies; and unbranched,
as in most Insects. They are about ·8 inch in length, and ·002 inch
in transverse diameter, so that they are barely visible to the naked
eye as single threads. In larvæ about one-fifth of an inch long,
Schindler[130] found only eight long tubules, the usual number in
Thysanura, Anoplura, and Termes; but the grouping into six masses, so
plainly seen in the adult, throws some doubt upon this observation. In
the adult Cockroach the long threads wind about the abdominal cavity
and its contained viscera.

[130] Zeitsch. f. wiss. Zool., Bd. XXX.

[Illustration: Fig. 72.--Malpighian Tubules of Cockroach. _A_,
transverse section of young tubule; _p_, its connective-tissue or
“peritoneal” layer; _B_, older tubule, crowded with urates; _tr_,
tracheal tube; _C_, tubule cut open longitudinally, showing three
states of the lining epithelium. × 200.]

In the wall of a Malpighian tubule there may be distinguished (1) a
connective tissue layer, with fine fibres and nuclei; within this, (2)
a basement-membrane, between which and the connective tissue layer runs
a delicate, unbranched tracheal tube; (3) an epithelium of relatively
large, nucleated cells, in a single layer, nearly filling the tube, and
leaving only a narrow, irregular central canal. Transverse sections
show from four to ten of these cells at once. The tubules appear
transparent or yellow-white, according as they are empty or full;
sometimes they are beaded or varicose; in other cases, one half is
coloured and the other clear. The opaque contents consist partly of
crystals, which usually occur singly in the epithelial cells, or heaped
up in the central canal. Occasionally, they form spherical concretions
with a radiate arrangement. They contain uric acid, and probably
consist of urate of soda.[131] In the living Insect the tubules remove
urates from the blood which bathes the viscera; the salts are condensed
and crystallised in the epithelial cells, by whose dehiscence they pass
into the central canals of the tubules, and thence into the intestine.

[131] The contents of the Malpighian tubules may be examined by
crushing the part in a drop of dilute acetic acid, or in dilute
sulphuric acid (10 per cent.). In the first case a cover-slip is
placed on the fluid, and the crystals, which consist of oblique
rhombohedrons, or derived forms, are usually at once apparent. If
sulphuric acid is used, the fluid must be allowed to evaporate. In
this case they are much more elongated, and usually clustered. The
murexide reaction does not give satisfactory indications with the
tubules of the Cockroach.

The Malpighian tubules develop as diverticula from the proctodæum,
which is an invagination of the outer integument and its morphological
equivalent. They are, therefore, similar in origin to urinary organs
opening upon the surface of the body and developed as invaginations
of the integument, like the “shell-glands” of lower Crustacea, and
the “green glands” of Decapod Crustacea. The segmental organs of
_Peripatus_, Annelids, and Vertebrates do not appear to be possible
equivalents of the excretory organs of Arthropods. They arise, not as
involutions, but as solid masses of mesoblastic tissue, or as channels
constricted off from the peritoneal cavity, and their ducts have
only a secondary connection with the outside of the body or with the
alimentary canal.

_Digestion of Insects._

The investigation of the digestive processes in Insects is work of
extreme difficulty, and it is not surprising that much yet remains to
be discovered. Plateau has, however, succeeded in solving some of the
more important questions, which, before his time, had been dealt with
in an incomplete or otherwise unsatisfactory way. The experiments of
Basch, though now superseded by Plateau’s more trustworthy results,
deserve notice as first attempts to investigate the properties of the
digestive fluids of Insects.

Basch set out with a conviction that where a chitinous lining is
present, the epithelium of the alimentary canal secretes chitin only,
and that proper digestive juices are only elaborated in the chylific
stomach, or in the salivary glands. The tests applied by him seemed
to show that the saliva, as well as the contents of the œsophagus
and crop, had an acid reaction, while the contents of the chylific
stomach were neutral at the beginning of the tube and alkaline further
down. From this he concluded that the supposed deep-seated glands of
the chylific stomach secreted an alkaline fluid, which neutralised
the acidity of the saliva. Finding that the epithelial cells of the
stomach were often loaded with oil-drops, he concluded that absorption,
at least of fats, takes place here. The chylific stomach, carefully
emptied of its contents, was found to convert starch into sugar at
ordinary temperatures. The saliva of the Cockroach gave a similar
result, and when a weak solution of hydrochloric acid was added,
Basch thought that the mixture could digest blood-fibrin at ordinary
temperatures.

Plateau’s researches upon _Periplaneta americana_,[132] modified by
subsequent experiments upon _P. orientalis_,[133] and by still more
recent observations, lead him to the following conclusions[134]:--

[132] Bull. Acad. Roy. de Belgique, 1876.

[133] Ib., 1877.

[134] We are indebted to Prof. Plateau for the statement of his views
given in the text.

1.--The saliva of the Cockroach changes starch into glucose; but the
saliva is not acid, it is either neutral (_P. orientalis_) or alkaline
(_P. americana_). Any decided acidity found in the crop is due to the
ingestion of acid food; but a very faint acidity may occur, which
results from the presence in the crop of a fluid secreted by the cæcal
diverticula of the mesenteron.

2.--The glucose thus formed is absorbed in the crop, and no more is
formed in the succeeding parts of the digestive tube.

3.--The function of the gizzard is that of a grating or strainer. It
has no power of trituration. If the animal consumes vegetable food rich
in cellulose, a substance not capable of digestion in the crop, the
fragments are found unaltered as to form and size in the mesenteron. If
it is supplied with plenty of farinaceous food, such as meal or flour,
the saliva is not adequate to the complete solution and transformation
of the starch, and the intestine is found full of uninjured starch
granules, which must have traversed the gizzard without crushing.

4.--The cæcal diverticula secrete a feebly acid fluid. To demonstrate
its acidity an extremely sensitive litmus solution, capable of
indicating one part in twenty thousand of hydrochloric acid, must be
used. The fluid secreted by the cæca emulsifies fats, and converts
albuminoids into peptones.

In all Insects digestion is effected in the following way (which is
particularly easy of demonstration in _Carabus_ and _Dytiscus_). The
crop is filled with food coarsely divided by the mandibles, and the
gizzard being shut to prevent further passage, the fluid secretion of
the cæca ascends to the crop, and there acts upon the food. Digestion
is effected in the crop, and not beyond it. This is clear beyond
doubt. In Decapod Crustacea also it is very easy to prove that the
fluid secreted by the so-called liver ascends into the stomach (which
corresponds to the crop, together with the gizzard of the Insect). To
satisfy ourselves on this point we have only to open a Crayfish during
active digestion.

When digestion in the crop is finished, the gizzard relaxes, and the
contents of the crop, now in a semi-fluid condition, pass into the
mesenteron, which is devoid of chitinous lining, and particularly
fitted for absorption.

5.--There are no absorbent vessels properly so called, and Plateau has
long thought that the products of digestion pass by osmosis directly
through the walls of the digestive tube, to mix with the blood in
the perivisceral space. If we may rely upon what is now known of the
process in Vertebrates, we should be led to modify this explanation.
It is very likely that in Insects, as in Vertebrates, absorption is
effected by the protoplasm of the epithelial cells, which select
and appropriate certain substances formed out of the dissolved
food. Not only do the epithelial cells transmit to the neighbouring
blood-currents the materials which they have previously absorbed, but
they subject certain kinds to further elaboration. The protoplasm of
the epithelial cells of Vertebrates is capable of forming fat. Thus,
a mixture of soap and glycerine, injected into the intestine of a
Vertebrate, is absorbed by the lacteals in the form of oil-drops.
Modern physiologists allow, too, that part of the peptone is similarly
changed into albumen, without transport to a distance, by the activity
of the epithelial lining.

These facts explain why Plateau was unable to isolate the secretion of
the epithelium of the chylific stomach of Insects. The cells are not
secretory, but absorbent; and the secretion vainly sought for does not
actually exist.

CHAPTER VIII.

THE ORGANS OF CIRCULATION AND RESPIRATION.

_SPECIAL REFERENCES._

VERLOREN. Mém. sur la Circulation dans les Insectes. Mém. cour,
par l’Acad. Roy. de Belgique, Tom. XIX. (1847). [Structure of
Circulatory Organs in a number of different Insects.]

GRABER. Ueb. den Propulsatorischen Apparat der Insekten. Arch. f.
mikr. Anat., Bd. IX. (1872). [Heart and Pericardium.]

LEYDIG. Larve von _Corethra plumicornis_. Zeits. f. wiss. Zool.,
Bd. III. (1852). [Valves in Heart.]

LANDOIS, H. Beob. üb. das Blut der Insekten. Zeits. f. wiss. Zool.,
Bd. XIV. (1864). [Blood of Insects.]

JAWOROWSKI. Entw. des Rückengefässes, &c., bei _Chironomus_. Sitzb.
der k. Akad. der Wiss. Wien., Bd. LXXX. (1879). [Minute Structure
and Development of Heart.]

LANDOIS, H., and THELEN. Der Tracheenverschluss bei den Insekten.
Zeits. f. wiss. Zool., Bd. XVII. (1867). [Stigmata.]

PALMEN. Zur Morphologie des Tracheensystems (1877). [Morphology of
Stigmata and Tracheal Gills.]

MACLEOD. La Structure des Trachées et la Circulation
Péritrachéenne. (Brussels, 1880.)

LUBBOCK. Distribution of Tracheæ in Insects. Trans. Linn. Soc.,
Vol. XXIII. (1860).

RATHKE. Untersuch. üb. den Athmungsprozess der Insekten. Schr. d.
Phys. Oek. Gesellsch. zu Königsberg. Jahrg. I. (1861). [Experiments
and Observations on Insect-respiration.]

PLATEAU. Rech. Expérimentales sur les Mouvements Respiratoires
des Insectes. Mém. de l’Acad. Roy. de Belgique, Tom. XLV. (1884).
Preliminary notice in Bull. Acad. Roy. de Belgique, 1882.

LANGENDORFF. Studien üb. die Innervation der Athembewegungen.--Das
Athmungscentrum der Insekten. Arch. f. Anat. u. Phys. (1883).
[Respiratory Centres of Insects.]

_Circulation of Insects._

A very long chapter might be written upon the views advanced by
different writers as to the circulation of Insects. Malpighi first
discovered the heart or dorsal vessel in the young Silkworm. His
account is tolerably full and remarkably free from mistakes. The heart
of the Silkworm, he tells us, extends the whole length of the body,
and its pulsations are externally visible in young larvæ. He supposed
that contraction is effected by muscular fibres, but these he could
not distinctly see. The tube, he says, has no single large chamber, but
is formed of many little hearts (_corcula_) leading one into another.
The number of these he could not certainly make out, but believed
that there was one to each segment of the body. During contraction
each chamber became more rounded, and when contraction was specially
energetic, the sides of the tube appeared to meet at the constrictions.
The flow of blood, he ascertained, was forward, the rhythm not
constant. No arteries were seen to be given off from the heart.[135]
Swammerdam thought that his injections ascertained the existence of
vessels branching out from the heart,[136] but this proved to be a
mistake. Lyonnet added many details of interest to what was previously
known. He came to the conclusion that there was no system of vessels
connected with the heart, and even doubted whether the organ so named
was in effect a heart at all. Marcel de Serres maintained that it was
merely the secreting organ of the fat-body. Cuvier and Dufour doubted
whether any circulation, except of air, existed in Insects. This was
the extreme point of scepticism, and naturalists were drawn back from
it by Herold,[137] who repeated and confirmed the views held by the
seventeenth-century anatomists, and insisted upon the demonstrable fact
that the dorsal vessel of an Insect does actually pulsate and impel a
current of fluid. Carus, in 1826, saw the blood flowing in definite
channels in the wings, antennæ, and legs. Straus-Durckheim followed up
this discovery by demonstrating the contractile and valvular structures
of the dorsal vessel. Blanchard affirmed that a complex system of
vessels accompanied the air tubes throughout the body, occupying
peritracheal spaces supposed to exist between the inner and outer
walls of the tracheæ. This peritracheal circulation has not withstood
critical inquiry,[138] and it might be pronounced wholly imaginary,
except for the fact that air tubes and nerves are found here and there
within the veins of the wings of Insects.

[135] Dissert. de Bombyce, pp. 15, 16 (1669).

[136] Biblia Naturæ, p. 410.

[137] Schrift. d. Marburg. Naturf. Gesellschaft, 1823.

[138] See, for a full account of this discussion, MacLeod sur la
Structure des Trachées, et la Circulation Péritrachéenne (1880). The
peritracheal circulation was refuted by Joly (Ann. Sci. Nat., 1849).

[Illustration: Fig. 73.--Heart, Alary Muscles, and Tracheal Arches,
seen from below; to the left is a side view of the heart. _T^2_, _T^3_,
_A^1_, alary muscles attached to the second thoracic, third thoracic,
and first abdominal terga. × 6. Fig. 35 (p. 74) is not quite correct as
to the details of the heart. The thoracic portion should be chambered,
and additional chambers and alary muscles represented at the end of the
abdomen. These omissions are rectified in the present figure.]

_Heart of the Cockroach._

[Illustration: Fig. 74.--Diagram to show the interventricular valves
and lateral inlets of the Heart. _ML_, median lobe; _V_, valve; _I_,
lateral inlet.]

The heart of the Cockroach is a long, narrow tube, lying immediately
beneath the middle line of the thorax and abdomen. It consists of
thirteen segments (fig. 73), which correspond to three thoracic and
ten abdominal somites. Each segment, as a rule, ends behind in a
conspicuous fold which projects backwards from the dorsal surface;
immediately in front of this are two lateral lobes. The median lobe
passes into the angle between two adjacent terga, and is continuous
with the dorsal wall of the segment next behind, from which it is
separated only by a deep constriction, while the lateral folds conceal
paired lateral inlets,[139] which lead from the pericardial space to
the hinder end of each chamber of the heart. Immediately in front of
each constriction is the interventricular valve, a pear-shaped mass of
nucleated cells, hanging down from the upper wall of the heart, and
inclining forward below. The position of this valve indicates that
during systole it closes upon the constricted boundary between two
chambers, thus shutting off at once the inlets and the passage into the
chambers behind. In this way the progressive and rhythmical contraction
of the chambers impels a steady forward current of blood, allowing an
intermittent stream to enter from the pericardial space, but preventing
regurgitation.

[139] It may be observed that Graber, who has paid close attention to
the heart of Insects, describes the inlets (_e. g._, in _Dytiscus_)
as situated, not at the hinder end, but in the middle of each
segment. We have not been able to discover such an arrangement in the
heart of the Cockroach.

[Illustration: Fig. 75.--Junction of two chambers of the Heart, seen
from above. _ML_, median lobe; _I_, lateral inlet.]

The wall of the heart includes several distinct layers. There are (1) a
transparent, structureless intima, only visible when thrown into folds;
(2) a partial endocardium, of scattered, nucleated cells, which passes
into the interventricular valves; (3) a muscular layer, consisting of
close-set annular, and distant longitudinal fibres. The annular muscles
are slightly interrupted at regular and frequent intervals, and are
imperfectly joined along the middle line above and below, so as to
indicate (what has been independently proved) that the heart arises
as two half-tubes, which afterwards join along the middle. Elongate
nuclei are to be seen here and there among the muscles. The adventitia
(4), or connective tissue layer, is but slightly developed in the adult
Cockroach.

Within the muscular layer is a structure which we have failed to make
out to our own satisfaction. It presents the appearance of regular but
imperfect rings, which do not extend over the upper third of the heart.
They probably meet in a ventral suture, but this and other details are
hard to make out, owing to the transparency of the parts. The rings
stain with difficulty, and we have not observed nuclei belonging to
them. Each extends over more than one bundle of annular muscles.

The difficulty of investigating a structure so minute and delicate
as the heart of an Insect may explain a good deal of the discrepancy
noted on comparing various published descriptions. Perhaps the most
obvious peculiarity which distinguishes the heart of the Cockroach, is
the subdivision of the thoracic portions into three chambers, which,
though less prominent in side-view than the abdominal chambers, are,
nevertheless, perfectly distinct. The number of abdominal chambers is
also unusually high; but it is so easy to overlook the small chambers
at the posterior end of the abdomen, that the number given in some of
the species may have been under-estimated.

_Pericardial Diaphragm and Space._

The heart lies in a pericardial chamber, which is bounded above by
the terga and the longitudinal tergal muscles; below by a fenestrated
membrane, the pericardial diaphragm. The intermediate space, which is
of inconsiderable depth, is nearly filled by a cellular mass laden with
fat, and resembling the fat-body.

The pericardial diaphragm, or floor of the pericardium, is continuous,
except for small oval openings scattered over its surface. It
consists of loosely interwoven fibres, interspersed with elongate
nuclei (connective-tissue corpuscles) and connected by a transparent
membrane. Into the diaphragm are inserted pairs of muscles, which, from
their shape and supposed continuity with the heart, have been named
_alæ cordis_, or alary muscles.[140] These are bundles of striated
muscle, about ·003 in. wide, which arise from the anterior margin of
each tergum. In the middle of the abdomen every alary muscle passes
inwards for about ·04 in., without breaking-up or widening, and then
spreads out fanwise upon the diaphragm. The fibres unite below the
heart with those of the fellow-muscle, and also join, close to the
heart, those of the muscles in front and behind. The alary muscles
are often said to distend the heart rhythmically by drawing its walls
apart, but this cannot be true. They do not pass into the heart at
all. Even if they did, a pull from opposite sides upon a flexible,
cylindrical tube, would narrow and not expand its cavity. Moreover,
direct observation[141] shows that the heart continues to beat after
all the alary muscles have been divided, and even after it has been cut
in pieces. These facts suggest that the heart of Insects is innervated
by ganglia upon or within it, and indeed transparent larvæ, such as
_Corethra_ or _Chironomus_, exhibit paired cells, very like simple
ganglia, along the sides of the heart.

[140] Lyonnet.

[141] Brandt, Ueb. d. Herz der Insekten u. Muscheln. Mél. Biol. Bull.
Acad. St. Petersb. Tom. VI. (1866).

[Illustration: Fig. 76.--Heart and Pericardial Diaphragm. On the
right, as seen from above; on the left, as seen from below; the bottom
figure represents a transverse section. _Ht_, heart; _PD_, pericardial
diaphragm; _AM_, alary muscle; _Tr_, tracheal tube; _PC_, pericardial
fat-cells; _PC_′, multinucleate fat-cells.]

Scattered over the upper surface of the pericardial diaphragm are
groups of cells, similar to the fat-masses of the perivisceral space.
Over the fan-like expansions of the alary muscles are different
fat-cells, which form branched and multinucleate lobes, and radiate in
the same direction as the underlying muscles.

Tracheal trunks, arising close to the stigmata, ascend upon the tergal
wall towards the heart. They overlie the alary muscles, and end near
the heart by bifurcation, sending one branch forward and another
backward to meet corresponding branches of adjacent trunks. A series of
arches is thus formed by the dorsal tracheæ on each side of the heart.
Occasionally an arch is subdivided into two smaller parallel tubes.
A few branches of distribution are given off to the fat-cells of the
pericardium.

Graber has explained the action of the pericardial diaphragm and
chamber in the following way.[142] When the alary muscles contract,
they depress the diaphragm, which is arched upwards when at rest.
A rush of blood towards the heart is thereby set up, and the blood
streams through the perforated diaphragm into the pericardial chamber.
Here it bathes a spongy or cavernous tissue (the fat-cells), which
is largely supplied with air tubes, and having been thus aerated,
passes immediately forwards to the heart, entering it at the moment of
diastole, which is simultaneous with the sinking of the diaphragm.

[142] Arch. f. mikr. Anat., Bd. IX. (1872); Insekten, ch. x.

In the Cockroach the facts of structure do not altogether justify this
explanation. The fenestræ of the diaphragm are mere openings without
valves. The descent of a perforated non-valvular plate can bring no
pressure to bear upon the blood, for it is not contended that the alary
muscles are powerful enough to change the figure of the abdominal
rings. Moreover, we find comparatively few tracheal tubes in the
pericardial chamber, and can discover no proof that in the Cockroach
the fat-cells adjacent to the heart have any special respiratory
character. The diaphragm appears to give mechanical support to the
heart, resisting pressure from a distended alimentary canal, while the
sheets of fat-cells, in addition to their proper physiological office,
may equalise small local pressures, and prevent displacement. The
movement of the blood towards the heart must (we think) depend, not
upon the alary muscles, but upon the far more powerful muscles of the
abdominal wall, and upon the pumping action of the heart itself.

_Circulation of the Cockroach._

The pulsations of the heart are rhythmical and usually frequent, the
number of beats in a given time varying with the species, the age, and
especially with the degree of activity or excitement of the Insect
observed.[143]

[143] Newport, in Todd’s Cyclopædia of Anatomy and Physiology, Art.
Insecta, pp. 981–2.

Cornelius[144] watched the pulsations in a white Cockroach immediately
after its change of skin, and reckoned them at eighty per minute;
but he remarks that the Insect was restless, and that the beats were
probably accelerated in consequence.

[144] Beitr. zur näheren Kenntniss von Periplaneta orientalis, p. 19.

In the living Insect a wave of contraction passes rapidly along
the heart from behind forwards; and the blood may under favourable
circumstances be seen to flow in a steady, backward stream along the
pericardial sinus, to enter the lateral aperture of the heart. The
peristaltic movement of the dorsal vessel may often be observed to set
in at the hinder end of the tube before the preceding wave has reached
the aorta.

From the heart a slender tube (the aorta) passes forward to the head.
It lies upon the dorsal surface of the œsophagus, which it accompanies
as far as the supra-œsophageal ganglia. In many Insects the thoracic
portion of the dorsal vessel is greatly narrowed and non-valvular,
forming the aorta of most writers on Insect Anatomy. The aorta often
dips downward near its origin, but in the Cockroach the thoracic
portion of the vessel keeps nearly the same level as the abdominal.
It gives off no lateral branches, but suddenly ends immediately in
front of the œsophageal ring in a trumpet-shaped orifice,[145] by
which the blood passes at once into a lacunar system which occupies
the perivisceral space. Here the blood bathes the digestive and
reproductive organs, receives the products of digestion, which are
not transmitted by lacteals, but discharged at once into the blood;
here, too, it gives up its urates to the excretory tubules, and its
superfluous fats to the finely-divided lobules of the fat-body. The
form of the various appendages of the alimentary canal (salivary
glands, cæcal tubes, and Malpighian tubules), as well as of the testes,
ovaries, and fat-body, is immediately connected with the passive
behaviour of the fluid upon which their nutrition depends. Instead
of being compact organs injected at every pulsation by blood under
pressure, they are diffuse, tubular, or branched, so as to expose as
large a surface as possible to the sluggish stream in which they float.

[145] The termination of the aorta has been described by Newport, in
_Sphinx_ (Phil. Trans., 1832, Pt. I., p. 385) _Vanessa, Meloe, Blaps
and Timarcha_. (Todd’s Cycl., Art. “Insecta,” p. 978.)

From the perivisceral space the blood enters the pericardial sinus by
the apertures in its floor, and returns thence by the lateral inlets
into the heart.

No satisfactory injections of the circulatory channels can be made in
Insects, on account of the large lacunæ, or cavities without proper
wall, which are interposed between the heart and the extremities of the
body. In the wings and other transparent organs the blood has been seen
to flow along definite channels, which form a network, and resemble
true blood vessels in their arrangement. Whether they possess a proper
wall has not been ascertained. It is observed that in such cases the
course of the blood is generally forwards along the anterior, and
backwards along the posterior, side of the appendage. The direction of
the current is not, however, quite constant, and the same cross branch
may at different times transmit blood in different directions.[146]

[146] Moseley, Q. J. Micr. Sci. (1871).

_Blood of the Cockroach._

The blood of the Cockroach may be collected for examination by cutting
off one of the legs, and wiping the cut end with a cover-slip. It
abounds in large corpuscles, each of which consists of a rounded
nucleus invested by protoplasm. Amœboid movements may often be
observed, and dividing corpuscles are occasionally seen. Crystals
may be obtained by evaporating a drop of the blood without pressure;
they form radiating clusters of pointed needles. The fresh-drawn
blood is slightly alkaline; it is colourless in the Cockroach, but
milky, greenish, or reddish in some other Insects. The quantity varies
greatly, according to the nutrition of the individual: after a few
days’ starvation, nearly all the blood is absorbed. Larvæ contain much
more blood, in proportion to their weight, than other Insects.

_Respiratory Organs of Insects._

The respiratory organs of Insects consist of ramified tracheal tubes,
which communicate with the external air by stigmata or spiracles. Of
these spiracles the Cockroach has ten pairs--eight in the abdomen and
two in the thorax. The first thoracic spiracle lies in front of the
mesothorax, beneath the edge of the tergum; the second is similarly
placed in front of the metathorax. The eight abdominal spiracles belong
to the first eight somites; each lies in the fore part of its segment,
and hence, apparently, in the interspace between two terga and two
sterna. The first abdominal spiracle is distinctly dorsal in position.

The disposition of the spiracles observed in the Cockroach is common in
Insects, and, of all the recorded arrangements, this approaches nearest
to the plan of the primitive respiratory system of Tracheata, in which
there may be supposed to be as many spiracles as somites.[147] The head
never carries spiracles except in _Smynthurus_, one of the Collembola
(Lubbock). Many larvæ possess only the first of the three possible
thoracic spiracles; in perfect Insects this is rarely or never met with
(_Pulicidæ?_), but either the second, or both the second and third, are
commonly developed. Of the abdominal somites, only the first eight ever
bear spiracles, and these may be reduced in burrowing or aquatic larvæ
to one pair (the eighth), while all disappear in the aquatic larva of
_Ephemera_.

[147] The oldest Tracheate actually known to bear spiracles is the
Silurian Scorpion of Gothland and Scotland (Scudder, in Zittel’s
Palæontologie, p. 738). We need not say that this is very far removed
from the primitive Tracheate which morphological theory requires. The
existing _Peripatus_ makes a nearer approach to the ideal ancestor
of all Tracheates, if we suppose that all Tracheates had a common
ancestor of any kind, which is not as yet beyond doubt.

From the spiracles, short, wide air-tubes pass inwards, and break up
into branches, which supply the walls of the body and all the viscera.
Dorsal branches ascend towards the heart on the upper side of the alary
muscles; each bifurcates above, and its divisions join those of the
preceding and succeeding segments, thus forming loops or arches. The
principal ventral branches take a transverse direction, and are usually
connected by large longitudinal trunks, which pass along the sides
of the body; the Cockroach, in addition to these, possesses smaller
longitudinal vessels, which lie close to the middle line, on either
side of the nerve-cord.[148] The ultimate branches form an intricate
network of extremely delicate tubes, which penetrates or overlies every
tissue.

[148] The longitudinal air-tubes are characteristic of the more
specialised Tracheata. In Araneidæ, many Julidæ, and Peripatus each
spiracle has a separate tracheal system of its own.

[Illustration: Fig. 77.--Tracheal System of Cockroach. Side view of
head seen from without, introducing the chief branches of the left
half. × 15.]

[Illustration: Fig. 78.--Tracheal System of Cockroach. Top and front of
head seen from without. × 15.]

[Illustration: Fig. 79.--Tracheal System of Cockroach. Back of head,
seen from the front, the fore half being removed. × 15. The letters A–J
indicate corresponding branches in figs. 77, 78, and 79.]

[Illustration: Fig. 80.--Tracheal System of Cockroach. The dorsal
integument removed and the viscera in place. × 5.]

[Illustration: Fig. 81.--Tracheal System of Cockroach. The viscera
removed to show the ventral tracheal communications. × 5.]

[Illustration: Fig. 82.--Tracheal System of Cockroach. The ventral
integument and viscera removed to show the dorsal tracheal
communications. × 5.]

_Tracheal Tubes._

The accompanying figures sufficiently explain the chief features of
the tracheal system of the Cockroach, so far as it can be explored by
simple dissection. Leaving them to tell their own tale, we shall pass
on to the minute structure of the air-tubes, the spiracles, and the
physiology of Insect respiration.

The tracheal wall is a folding-in of the integument, and agrees with
it in general structure. Its inner lining, the intima, is chitinous,
and continuous with the outer cuticle. It is secreted by an epithelium
of nucleated, chitinogenous cells, and outside this is a thin and
homogeneous basement membrane. The integument, the tracheal wall, and
the inner layers of nearly the whole alimentary canal are continuous
and equivalent structures. The lining of the larger tracheal tubes at
least is shed at every moult, like that of the stomodæum and proctodæum.

[Illustration: Fig. 83.--Tracheal tube with its epithelium and spiral
thread. Slightly altered from a figure given by Chun (Rectal-drüsen bei
den Insekten, pl. iv., fig. 1).]

_Tracheal Thread._

In the finest tracheal tubes (·0001 in. and under) the intima is to
all appearance homogeneous. In wider tubes it is strengthened by a
spiral thread, which is denser, more refractive, and more flexible
than the intervening membrane. The thread projects slightly into the
lumen of the tube, and is often branched. It is interrupted frequently,
each length making but a few turns round the tube, and ending in a
point. The thread of a branch is never continued into a main trunk.
Both the thread and the intervening membrane become invisible or faint
when the tissue is soaked with a transparent fluid, so as to expel
the air. Both, but especially the thread, absorb colouring matter
with difficulty. The thread, from its greater thickness, offers a
longer resistance to solvents, such as caustic alkalies, and also to
mechanical force; it can therefore be readily unrolled, and often
projects as a loose spiral from the end of a torn tube, while the
membrane breaks up or crumbles away.[149]

[149] Investigators are not yet agreed as to the minute structure
of the tracheal thread. Chun (Abh. d. Senkenberg. Naturf. Gesells.,
Bd. X., 1876) considers it an independent chitinous formation, not a
mere thickening of the intima. He describes the thread as solid. The
intima itself is, he believes, divisible in the larger tubes into
an inner and an outer layer, into both of which the thread is sunk.
Macloskie (Amer. Nat., June, 1884) describes the spiral as a fine
tubule, opening by a fissure along its length. He regards it as a
hollow crenulation of the intima, and continuous therewith. Packard
(Amer. Nat. Mag., May, 1886) endeavours to show that the thread is
not spiral, but consists of parallel thickenings of the intima. He
is unable to find proof of the tubular structure, or of the external
fissure. We have specially examined the trachea of the Cockroach, and
find that the thread can readily be unwound for several turns. It is
truly spiral.

[Illustration: Fig. 84.--Intima (chitinous lining) of a large tracheal
tube. The spiral thread divides here and there. Copied from MacLeod,
loc. cit., fig. 9.]

The large tracheal tubes close to the spiracles are without spiral
thread, and the intima is here subdivided into polygonal areas, each
of which is occupied by a reticulation of very fine threads. This
structure may be traced for a short distance between the turns of the
spiral thread.

The chitinogenous layer of the tracheal tubes is single, and consists
of polygonal, nucleated cells, forming a mosaic pattern, but becoming
irregular and even branched in the finest branches. The cell walls are
hardly to be made out without staining. Externally, the chitinogenous
cells rest upon a delicate basement membrane.

Where a number of branches are given off together, the tracheal tube
may be dilated. Fine branches, such as accompany nerves, are often
sinuous. In the very finest branches the tube loses its thread, the
chitinogenous cells become irregular, and the intima is lost in the
nucleated protoplasmic mass which replaces the regular epithelium of
the wider tubes.[150]

[150] It has been supposed that these irregular cells of the tracheal
endings pass into those of the fat-body, but the latter can always be
distinguished by their larger and more spherical nuclei.

_The Spiracles._

The spiracles of the Cockroach are by no means of complicated
structure, but their small size, and the differences between one
spiracle and another, are difficulties which cost some pains to
overcome.

The first thoracic spiracle (fig. 85) is the largest in the body. It
lies in front of the mesothorax, between the bases of the first and
second legs. It is placed obliquely, the slit being inclined downwards
and backwards, and is closed externally by a large, slightly two-lobed
valve, attached by its lower border. The aperture immediately within
the valve divides into two nearly equal cavities, each of which leads
to a separate tracheal trunk; and between these cavities is a septum,
thickened on its free edge, against which the margin of the valve
appears to close. A special occlusor muscle arises from the integument
below the spiracle, and is inserted into a chitinous process which
projects inwardly from the centre of the valve. A second muscle,
whose connections and mode of action we have not been able to make
out satisfactorily, lies beneath the first, and is inserted into the
thickened edge of the septum.

The second thoracic spiracle (fig. 86) lies in front of the metathorax,
between the bases of the second and third legs. It is much smaller and
simpler than the first. Its valve is nearly semi-circular, and the free
border is strengthened on its deep surface by a chitinous rim, which
terminates beyond the end of the hinge of the valve in a process which
gives insertion to the occlusor muscle.

[Illustration: Fig. 85.--First Thoracic Spiracle (left side), seen from
the outside. × 70. _V_, valve; _I_, setose lining of valve (mouth of
tracheal tube) × 230. The occlusor muscle is shown. The arrow indicates
the direction of air entering the spiracle. In the natural position
this spiracle is set obliquely, the slit being inclined downwards and
backwards. (_P. americana._)]

The abdominal spiracles present quite a different plan of structure.
The external orifice is permanently open, owing to the absence of
valves, but communication with the tracheal trunk may be cut off at
pleasure by an internal occluding apparatus. The external orifice leads
into a shallow oval cup, which communicates with the tracheal trunk
by a narrow slit, or internal aperture of the spiracle. The chitinous
cuticle, surrounding this internal aperture, is richly provided with
setæ, which are turned towards the opening.[151] Fig. 87_C_ represents
a spiracle seen from within, and shows that the slit divides the cup
into two unequal lips, the smaller of which inclines away from the
middle line of the body, is movable, and is strengthened on its deep
surface by a curved chitinous rod, the “bow” of Landois. From the
opposite lip, a pouch is thrown out, which serves for the attachment of
the occlusor muscle. The muscle is inserted into the extremity of the
bow, and when it contracts, the bow is pulled over into the position
shown in fig. 87_D_, and the opening is closed. The antagonist muscle,
which exists in all the abdominal spiracles, is shown in fig. 88; it
arises from the supporting plate of the spiracle, and is inserted
opposite to the occlusor, into the extremity of the bow.

[151] In the first abdominal spiracle the setæ are developed only on
that lip which carries the bow.

[Illustration: Fig. 86.--Second Thoracic Spiracle (left side), seen
from the outside. × 70. _V_, lower (movable) valve. The occlusor
muscle is shown. The arrow indicates the direction of air entering the
spiracle. (_P. americana._)]

[Illustration: Fig. 87.--Four views of the First Abdominal Spiracle
(left side). × 70. The bow is shaded in all the figures. (_P.
americana._)

_A_--The spiracle, seen from the outside; _p_, lateral pouch; _I_,
internal aperture.

_B_-- Do., side view.

_C_-- Do., seen from the inside, the aperture open. The occlusor muscle
is shown.

_D_--The spiracle, seen from the inside, the aperture shut.]

Each of the eight abdominal spiracles is constructed on this plan; the
first merely differs from the others in its larger size and dorsal
position, being carried upon the lateral margin of the first abdominal
tergum, whereas the others are placed on the side of the body, each
occupying an interspace between two terga and two sterna. The bow is of
about the same length in all; hence the apparent disproportion in the
figures of different spiracles. The external aperture of the abdominal
spiracles is oval or elliptical, placed vertically and directed
backwards.

[Illustration: Fig. 88.--Abdominal Spiracle (left side) in side view,
showing the bow: × 70; _p_, lateral pouch of spiracle, seen from
within. The tesselated structure of the spiracle and trachea is shown
at _A_ (× 230), and the margin of the external aperture at _B_ (× 230).
(_P. americana._)]

We have already pointed out that the wall of the air-tube, for a
short distance from the spiracular orifice, has a tesselated instead
of a spiral marking. In the thoracic spiracles the tesselated cells
are grouped round regularly placed setæ (fig. 85 _I_). The chitinous
cuticle within the opening is crowded with fine setæ, which are often
arranged so as to form a fringe on one or both sides of the internal
aperture. (_Supra_, p. 152.)

_Mechanism of Respiration._

In animals with a complete circulation, aërated blood is diffused
throughout the body by means of arteries and capillaries, which deliver
it under pressure at all points. Such animals usually possess a special
aërating chamber (lung or gill), where oxygen is made to combine with
the hæmoglobin of the blood. It is otherwise with Insects. Their blood
escapes into great lacunæ, where it stagnates, or flows and ebbs
sluggishly, and a diffuse form of the internal organs becomes necessary
for their free exposure to the nutritive fluid. The blood is not
injected into the tissues, but they are bathed by it, and the compact
kidney or salivary gland is represented in Insects by tubules, or a
thin sheet of finely divided lobules. By a separate mechanism, air is
carried along ramified passages to all the tissues. Every organ is its
own lung.

We must now consider in more detail how air is made to enter and leave
the body of an Insect. The spiracles and the air-tubes have been
described, but these are not furnished with any means of creating
suction or pressure; and the tubes themselves, though highly elastic,
are non-contractile, and must be distended or emptied by some external
force. Many Insects, especially such as fly rapidly, exhibit rhythmical
movements of the abdomen. There is an alternate contraction and
dilatation, which may be supposed to be as capable of setting up
expirations and inspirations as the rise and fall of the diaphragm of
a Mammal. In many Insects, two sets of muscles serve to contract the
abdomen--viz., muscles which compress or flatten, and muscles which
approximate or telescope the segments.[152] In the Cockroach the
second set is feebly developed, but the first is more powerful, and
causes the terga and sterna alternately to approach and separate with
a slow, rhythmical movement; in a Dragon-fly or Humble-bee the action
is much more conspicuous, and it is easy to see that the abdomen is
bent as well as depressed at each contraction. No special muscles exist
for dilating the abdomen, and this seems to depend entirely upon the
elasticity of the parts. It was long supposed that, when the abdomen
contracted, air was expelled from the body, and the air passages
emptied; that when the abdomen expanded again by its own elasticity,
the air passages were refilled, and that no other mechanism was needed.
Landois pointed out, however, that this was not enough. Air must be
forced into the furthest recesses of the tracheal system, where the
exchange of oxygen and carbonic acid is effected more readily than in
tubes lined by a dense intima. But in these fine and intricate passages
the resistance to the passage of air is considerable, and the renewal
of the air could, to all appearance, hardly be effected at all if the
inlets remained open. Landois accordingly searched for some means
of closing the outlets, and found an elastic ring or spiral, which
surrounds the tracheal tube within the spiracle. By means of a special
muscle, this can be made to compress the tube, like a spring clip upon
a flexible gas pipe. When the muscle contracts, the passage is closed,
and the abdominal muscles can then, it is supposed, bring any needful
pressure to bear upon the tracheal tubes, much in the same way as with
ourselves, when we close the mouth and nostrils, and then, by forcible
contraction of the diaphragm and abdominal walls, distend the cheeks
or pharynx. Landois describes the occluding apparatus of the Cockroach
as completely united with the spiracle. It consists, according to
him, of two curved rods, the “bow” and the “band,” one of which forms
each lip of the orifice. From the middle of the band projects a blunt
process for the attachment of the occlusor muscle, which passes thence
to the extremity of the bow. The concave side of each rod is fringed
with setæ, and turned towards the opening, which lies between the
two. Upon this description of the spiracles of the Cockroach we have
to remark that there is no occluding apparatus at all in the thoracic
spiracles, which are provided with external valves. In the abdominal
spiracles the bow is perfectly distinct, but the “band” of Landois
has no separate existence. Though the actual mechanism in this Insect
does not altogether agree with Landois’ description, it is capable of
performing the physiological office upon which he justly lays so much
stress--viz., the closing of the outlets of the tracheal system, in
order that pressure may be brought upon the contained air.

[152] This subject is treated at greater length in Prof. Plateau’s
contribution on Respiratory Movements of Insects. (_Infra_, p. 159.)

The injection of air by muscular pressure into a system of very fine
tubes may, however, appear to the reader, as it formerly did to
ourselves, extremely difficult or even impossible. Can any pressure
be applied to tubes within the body of an Insect which will force air
along the passages of (say) ·0001 in. diameter? It may well seem that
no pressure would suffice to distend these minute tubules, in which the
actual replacement of carbonic acid by oxygen takes place, but that the
air would either contract to a smaller volume or burst the tissues.

If we question the physical possibility of Landois’ explanation, an
alternative is still open to us. The late Prof. Graham has applied the
principle of Diffusion to the respiration of animals, and has shown
how by a diffusion-process the carbonic acid produced in the remote
cavities would be moved along the smaller tubes, and emptied into wider
tubes, from which it could be expelled by muscular action. The carbonic
acid is not merely exchanged for oxygen, but for a larger volume of
oxygen (O 95 : CO_{2} 81); and there is consequently a tendency to
accumulation within the tubes, which is counteracted by the elasticity
of the air vessels, as well as by special muscular contractions.[153]

[153] Phil. Mag., 1833. Reprinted in “Researches,” p. 44. Graham
expressly applies the law of diffusion of gases to explain the
respiration of Insects. Sir John Lubbock quotes and comments upon the
passage in his paper on the Distribution of the Tracheæ in Insects.
(Linn. Trans. Vol. XXIII.)

Whether diffusion or injection by muscular pressure is the chief means
of effecting the interchange of gases between the outer air and the
inner tissues of the Insect, is a question to be dealt with by physical
enquiry.

If we suppose two reservoirs of different gases at slightly different
pressures to be connected by a capillary tube of moderate dimensions,
such as one of the larger tracheæ of the Cockroach, transference by
the molecular movements of diffusion would be small compared with that
effected by the flow of the gas in mass. But if the single tube were
replaced by a number of others, of the same total area, but of the
fineness (say) of the pores in graphite, the flow of the gas would be
stopped, and the transference would be effected by diffusion only.
We may next consider tubes of intermediate fineness, say a tracheal
tubule of the Cockroach at the point where the spiral thread ceases,
and where the exchange of gases through the wall of the tubule becomes
comparatively unobstructed. Such a tubule is about ·0001 in. diameter.
If we may extend to such tubules the laws which hold good for the flow
of gases in capillary tubes of much greater diameter, the quantity of
air which might be transmitted in a given time by muscular pressure of
known amount can be determined. Suppose the difference of pressure at
the two ends of the tubule to be one-hundredth of an atmosphere, and
further, that the tubule is a quarter of an inch long and ·0001 in.
diameter. The tubule would then be cleared out every four seconds. Such
a flow of air along innumerable tubules might well suffice for the
respiratory needs of the Cockroach. Without laying too much stress upon
this calculation, for which exact data are wanting, we may be satisfied
that an appreciable quantity of air may be made by muscular pressure to
flow along even the finer air passages of an Insect.[154]

[154] For an explanation of the physical principles involved in this
discussion, and for the calculation (based upon our own assumptions),
we are indebted to Mr. A. W. Rücker, F.R.S.

_Respiratory Movements of Insects._

By FÉLIX PLATEAU, Professor in the University of Ghent.

The respiratory movements of large Insects are in general very
apparent, and many observers have said something about what they
have seen in various species. It is only since the publication of
Rathke’s memoir, however, that precise views have been gained as to the
mechanism of these movements. This remarkable work, treating of the
respiratory movements in Insects, the movable skeletal plates, and the
respiratory muscles characteristic of all the principal groups, filled
an important blank in our knowledge. But, notwithstanding the skill
displayed in this research, many questions still remained unanswered,
which required more exact methods than mere observation with the naked
eye or the simple lens.

The writer, who was followed a year later by Langendorff, conceived
the idea of studying, by such graphic methods as are now familiar, the
respiratory movements of perfect Insects. He has made use of two modes
of investigation. The first, or graphic method, in the strict sense of
the term, consisted in recording upon a revolving cylinder of smoked
paper the respiratory movements, transmitted by means of very light
levers of Bristol board, attached to any selected part of the Insect’s
exoskeleton. Unfortunately, this plan is only applicable to insects of
more than average size. A second method, that of projection, consisted
in introducing the Insect, carried upon a small support, into a large
magic lantern fitted with a good petroleum lamp. When the amplification
does not exceed 12 diameters, a sharp profile may be obtained, upon
which the actual displacements may be measured, true to the fraction of
a millimetre. Placing a sheet of white paper upon the lantern screen,
the outlines of the profile are carefully traced in pencil so as to
give two superposed figures, representing the phases of inspiration and
expiration respectively. By altering the position of the Insect, so as
to obtain profiles of transverse section, or of the different parts of
the body, and, further, by gluing very small paper slips to parts whose
movements are hard to observe, the successive positions of the slips
being then drawn, complete information is at last obtained of every
detail of the respiratory movements: nothing is lost.

This method, similar to that employed by the English physiologist,
Hutchinson,[155] is valuable, because it enables us, with a little
practice, to investigate readily the respiratory movements of very
small Arthropods, such as Flies or Lady-birds. It has this advantage
over all others, that it leaves no room for errors of interpretation.

[155] J. Hutchinson, Art. Thorax, Todd’s Cycl. of Anat. and Phys.

Not satisfied with mere observation by such means as these, of the
respiratory movements of Insects, the writer has also studied the
muscles concerned, and, in common with other physiologists (Faivre,
Barlow, Luchsinger, Dönhoff, and Langendorff), has examined the action
of the various nervous centres upon the respiratory organs. The results
at which he has arrived may be summarised as follows:--

1. There is no close relation between the character of the respiratory
movements of an Insect and its position in the zoological system.
Respiratory movements are similar only when the arrangement of the
abdominal segments, and especially when the disposition of the attached
muscles are almost identical. Thus, for example, the respiratory
movements of a Cockroach are different from those of other Orthoptera,
but resemble those of Hemiptera Heteroptera.

2. The respiratory activity of resting Insects is localised in the
abdomen. V. Graber has expressed this fact in a picturesque form, by
saying that in Insects the chest is placed at the hinder end of the
body.

3. In most cases the thoracic segments do not share in the respiratory
movements of an Insect at rest. Among the singular exceptions to this
rule is the Cockroach (_P. orientalis_), in which the terga of the
meso- and meta-thoracic segments perform movements exactly opposite in
direction to those of the abdomen. (See fig. 89, _Ms. th._, _Mt. th._)

[Illustration: Fig. 89.--Profile of Cockroach (_P. orientalis_). The
black surface represents the expiratory contour, while the inspiratory
is indicated by a thin line. The arrows show the direction of the
expiratory movement. _Ms. th._, mesothorax; _Mt. th._, metathorax.
Reduced from a magic-lantern projection.]

4. Leaving out of account all details and all exceptions, the
respiratory movements of Insects may be said to consist of alternate
contraction and recovery of the figure of the abdomen in two
dimensions--viz., vertical and transverse. During expiration the
diameters in question are reduced, while during respiration they revert
to their previous amounts. The transverse expiratory contraction is
often slight, and may be imperceptible. On the other hand, the vertical
expiratory contraction is never absent, and usually marked. In the
Cockroach (_P. orientalis_) it amounts to one-eighth of the depth of
the abdomen (between segments 2 and 3).

5. Three principal types of respiratory mechanism occur in Insects, and
these admit of further subdivision:--

(_a_) Sterna usually stout and very convex, yielding but little.
Terga mobile, rising and sinking appreciably. To this class belong
all Coleoptera, Hemiptera Heteroptera, and Blattina (Orthoptera).

[Illustration: Fig. 90.--Transverse section of Abdomen, Lamellicorn
Beetle. The position of the terga and sterna after an inspiration, is
indicated by the thick line; the dotted line shows their position after
an expiration, and the arrow marks the direction of the expiratory
movement.]

In the Cockroach (_Periplaneta_) the sterna are slightly raised during
expiration. (See figs. 89 and 91.)

[Illustration: Fig. 91.--Transverse section of Abdomen, Cockroach (_P.
orientalis_).]

(_b_) Terga well developed, overlapping the sterna on the sides of
the body, and usually concealing the pleural membrane, which forms
a sunk fold. The terga and sterna approach and recede alternately,
the sterna being almost always the more mobile. To this type belong
Odonata, Diptera, aculeate Hymenoptera, and Acridian Orthoptera.
(Fig. 92.)

(_c_) The pleural membrane, connecting the terga with the sterna,
is well developed and exposed on the sides of the body. The terga
and sterna approach and recede alternately, while the pleural zone
simultaneously becomes depressed or returns to its original figure.
To this type the writer assigns the Locustidæ, the Lepidoptera and
the true Neuroptera (excluding Phryganidæ). (Fig. 93.)

[Illustration: Fig. 92.--Transverse section of Abdomen, Bee (_Bombus_).]

6. Contrary to the opinion once general, changes in length of
the abdomen, involving protrusion of the segments and subsequent
retraction, are rare in the normal respiration of Insects. Such
longitudinal movements extend throughout one entire group only--viz.,
the aculeate Hymenoptera. Isolated examples occur, however, in other
zoological divisions.

[Illustration: Fig. 93.--Transverse section of Abdomen, Hawk Moth
(_Sphingina_).]

7. Among Insects sufficiently powerful to give good graphic tracings,
it can be shown that the inspiratory movement is slower than the
expiratory, and that the latter is often sudden.

8. In most Insects, contrary to what obtains in Mammals, only the
expiratory movement is active; inspiration is passive, and effected by
the elasticity of the body-wall.

9. Most Insects possess expiratory muscles only. Certain Diptera
(_Calliphora vomitoria_ and _Eristalis tenax_) afford the simplest
arrangement of the expiratory muscles. In these types they form a
muscular sheet of vertical fibres, connecting the terga with the
sterna, and underlying the soft elastic membrane which unites the
hard parts of the somites. One of the most frequent complications
arises by the differentiation of this sheet of vertical fibres into
distinct muscles, repeated in every segment, and becoming more and more
separated as the sterna increase in length. (See the tergo-sternal
muscles of the Cockroach, fig. 36, p. 76.) Special inspiratory muscles
occur in Hymenoptera, Acridiidæ, and Phryganidæ.

10. The abdominal respiratory movements of Insects are wholly
reflex. Like other physiologists who have examined this side of the
question, the writer finds that the respiratory movements persist in a
decapitated Insect, as also after destruction of the cerebral ganglia
or œsophageal connectives; further, that in Insects whose nervous
system is not highly concentrated (_e.g._, Acridiidæ and Dragon-flies),
the respiratory movements persist in the completely-detached abdomen;
while all external influences which promote an increased respiratory
activity in the uninjured animal, have precisely the same action upon
Insects in which the anterior nervous centres have been removed, upon
the detached abdomen, and even upon isolated sections of the abdomen.

The view formerly advocated by Faivre, that the metathoracic ganglia
play the part of special respiratory centres, must be entirely
abandoned. All carefully performed experiments on the nervous system
of Arthropoda have shown that each ganglion of the ventral chain is a
motor centre, and in Insects a respiratory centre, for the somite to
which it belongs. This is what Barlow calls the “self-sufficiency” of
the ganglia.

* * * * *

The writer has made similar observations upon the respiration of
Spiders and Scorpions;[156] but to his great surprise he has been
unable either by direct observation, or by the graphic method, or by
projection, to discover the slightest respiratory movement of the
exterior of the body. This can only be explained by supposing that
inspiration and expiration in Pulmonate Arachnida are intra-pulmonary,
and affect only the proper respiratory organs. The fact is less
surprising because of the wide zoological separation between Arachnida
and Insects.

[156] De l’absence de mouvements respiratoires perceptibles chez les
Arachnides (Archives de Biologie de Van Beneden et Van Bambeke, 1885.)

_Respiratory Activity of Insects._

The respiratory activity of Insects varies greatly. Warmth, feeding,
and movement are found to increase the frequency of their respirations,
and also the quantity of carbonic acid exhaled. In Liebe’s[157]
experiments a Carabus produced ·24 mgr. of carbonic acid per hour
in September, but only ·09 mgr. per hour in December. A rise of
temperature raised the product temporarily to twice its previous
amount; but when the same insect was kept under experiment for several
days without food, the amount fell in spite of its increased warmth.
Treviranus[158] gives the carbonic acid exhaled by a Humble-bee as
varying from 22 to 174, according as the temperature varied from 56° to
74° F.

[157] Ueb. d. Respiration der Tracheaten. Chemnitz (1872).

[158] See table in Burmeister’s “Manual,” Eng. trans. p. 398.

Larvæ often breathe little, especially such as lie buried in wood,
earth, or the bodies of other animals. The respiration of pupæ is also
sluggish, and not a few are buried beneath the ground or shrouded in
a dense cocoon or pupa-case. Muscular activity originates the chief
demand for oxygen, and accordingly Insects of powerful flight are most
energetic in respiration.

A rise of temperature proportionate to respiratory activity has
been observed in many insects. Newport[159] tells us how the female
Humble-bee places herself on the cells of pupæ ready to emerge, and
accelerates her inspirations to 120 or 130 per minute. During these
observations he found in some instances that the temperature of a
single Bee was more than 20° above that of the outer air.

[159] Art. “Insecta,” Cyc. Anat. and Phys., p. 989.

Some Insects can remain long without breathing. They survive for many
hours when placed in an exhausted receiver, or in certain irrespirable
gases. Cockroaches in carbonic acid speedily become insensible,
but after twelve hours’ exposure to the pure gas they revive, and
appear none the worse. H. Müller[160] says that an Insect, placed in
a small, confined space, absorbs _all_ the oxygen. In Sir Humphry
Davy’s “Consolations in Travel”[161] is a description of the Lago dei
Tartari, near Tivoli, a small lake whose waters are warm and saturated
with carbonic acid. Insects abound on its floating islands; though
water birds, attracted by the abundance of food, are obliged to confine
themselves to the banks, as the carbonic acid disengaged from the
surface would be fatal to them, if they ventured to swim upon it when
tranquil.

[160] Pogg. Ann. 1872, Hft. 3.

[161] Works, Vol. IX., p. 287. This passage has been cited by Rathke.

_Origin of Tracheal Respiration._

Kowalewsky, Bütschli, and Hatschek have described the first stages
of development of the tracheal system. Lateral pouches form in the
integument; these send out anterior and posterior extensions, which
anastomose and form the longitudinal trunks. The tracheal ramifications
are not formed by a process of direct invagination, but by the
separation of chitinogenous cells, which cohere into strings, and then
form irregular tubules. The cells secrete a chitinous lining, and
afterwards lose their distinct contours, fusing to a continuous tissue,
in which the individual cells are indicated only by their nuclei,
though by appropriate re-agents the cell boundaries can be defined.

The ingenious hypothesis propounded by Gegenbaur, that the tracheal
tubes of Insects were originally adapted to aquatic respiration, and
that the stigmata arose as the scars of disused tracheal gills, has
been discussed in chap. iv. Semper has suggested[162] that tracheæ
may be modified segmental organs, but the most probable view of
their origin is that put forth by Moseley,[163] that they arose as
ramified cutaneous glands. In _Peripatus_ the openings are distributed
irregularly over the body; the external orifices lead to pits, from
which simple tubes, with but slight spiral markings, extend into the
deeper tissues.

[162] Arbeiten a. d. Zool. Zoot. Inst. Würzburg. Bd. II., 1874.

[163] Phil. Trans., 1874, p. 757.

CHAPTER IX.

REPRODUCTION.

_SPECIAL REFERENCES._

BRANDT, A. Ueber die Eiröhren der Blatta (Periplaneta) orientalis.
Mem. Acad. St. Petersb. Ser. 7, Vol. XXI. (1874). [Ovarian Tubes of
Cockroach.]

LACAZE-DUTHIERS. Rech. sur l’armure génitale femelle des Insectes
Orthoptères. Ann. Sci. Nat., Zool., 3^e Sér., Tom. XVII. (1852).
[External reproductive organs of female Orthoptera.]

BERLESE. Ricerde sugli organi genitali degli Ortotteri. Atti della
R. Acad. dei Lincei. Ser. 3, Vol. XI. (1882). [Genital Organs of
European Orthoptera.]

KADYI. Beitr. zur Vorgänge beim. Eierlegen der Blatta Orientalis.
Vorläufige Mittheilung. Zool. Anz., 1879, p. 632. [Formation of
egg-capsules of Cockroach.]

BREHM. Comparative structure of the reproductive organs in Blatta
germanica and Periplaneta orientalis. Mem. Soc. Ent. St. Petersb.,
Tom. VIII. (1880). In Russian. [Male organs only.]

RAJEWSKY. Ueber die Geschlechtsorgane von Blatta orientalis, &c.
Nachr. d. kais. Gesellsch. d. Moskauer Universität., Bd. XVI.
(1875). [Testes of Cockroach. The original paper is in Russian; an
abstract is given in Hofmann and Schwalbe’s Jahresbericht, 1875,
p. 425.]

BÜTSCHLI. Bau u. Entwickelung d. Samenfäden bei Insekten u.
Crustaceen. Zeits. f. wiss. Zool., Bd. XXI., pp. 402–414; 526–534.
Pl. xl. xli. (1871). [Spermatozoa and spermatogenesis in the
Cockroach.]

LA VALETTE ST. GEORGE. Spermatologische Beiträge, II.
_Blatta germanica._ Arch. f. mikr. Anat., Bd. XXVII. (1886).
[Spermatogenesis in _B. germanica_.]

MORAVITZ. Quædam ad anat. Blattæ germanicæ pertinentia. Dissertatio
inauguralis. Dorpat. (1853). [An excellent early account of the
anatomy of _B. germanica_, including a description of the male and
female organs. The figures are not trustworthy.]

_Female Reproductive Organs._

The ovaries of the two sides of the body are separated, as in most
Insects, and consist on each side of eight tubes, four dorsal and
four ventral, which open into the inner side of a common oviduct.
The two oviducts unite behind, and form a very short uterus. Tracheæ
and fat-cells tie the ovarian tubes of each side together into a
spindle-shaped bundle. Each tube is about ·4 in. long, and has a
beaded appearance, owing to the eggs which distend its elastic wall.
It gradually tapers in front; then suddenly narrows to a very small
diameter; and lastly, joins with the extremities of the other tubes
to form a slender solid filament, which passes towards the heart, and
becomes lost in the fat-body. The wall of an ovarian tube consists
of a transparent elastic membrane, lined by epithelium, and invested
externally by a peritoneal layer of connective tissue.

[Illustration: Fig. 94.--Female Reproductive Organs. _Od_, oviduct;
_CG_, colleterial gland. × 14.]

The epithelium of an ovarian tube presents some remarkable
peculiarities which disguise its true character. High up in the tube,
the narrow lumen is occupied by a clear protoplasm, in which nuclei,
but no cell walls, can be discerned. Where the tube suddenly widens,
large rounded and nucleated masses of protoplasm appear, interspersed
with nuclei entangled in a network of protoplasm. Passing down the
tube, the large cells, which can now be recognised as eggs, arrange
themselves in a single row, to the number of about twenty. They are
at first polygonal or squarish, but gradually become cylindrical, and
finally oval. Between and around the eggs the nuclei gradually arrange
themselves into one-layered follicles, which are attached, not to the
wall of the tube, but to the eggs, and travel downwards with them. As
the eggs descend, the yolk which they contain increases rapidly, and
the germinal vesicle and spot (nucleus and nucleolus), which were at
first very plain, disappear. A vitelline membrane is secreted by the
inner surface, and a chitinous chorion by the outer surface of the
egg-follicle.

[Illustration: Fig. 95.--Ovarian Tube (acetic acid preparation),
showing scattered nuclei (upper figure), which ultimately form
follicles around the ova (lower figure). Copied from Brandt, _loc.
cit._]

The lowest egg in an ovarian tube is nearly or altogether of the
full size; it is of elongate-oval figure, and slightly curved, the
convexity being turned towards the uterus. It is filled with a clear
albuminous fluid, which mainly consists of yolk. The chorion now forms
a transparent yellowish capsule, which under the microscope appears to
be divided up into very many polygonal areas, defined by rows of fine
dots. These areas probably correspond to as many follicular cells. The
convex surface of the chorion is perforated by numerous micropyles,
fine pores through which it is probable the spermatozoa gain access to
the interior of the egg.

The uterus has a muscular wall and a chitinous lining. Two repeatedly
branched colleterial glands open into its under side. Of these the
left is much the larger, and overlies the other. It consists of many
dichotomous tubes, some of which are a little dilated at their blind
ends. The gland is much entangled with fat-cells, which make it
difficult to unravel. The right gland is probably of no functional
importance; the left gland is filled with a milky substance, containing
many crystals and a coagulable fluid, out of both of which the
egg-capsule is formed.[164]

[164] The crystals have been supposed to consist of oxalate of
lime (Duchamp, Rev. des sci. nat. Montpellier, Tom. VIII.). Hallez
observes that they are prismatic, with rhombic base, the angles
truncated. They are insoluble in water and weak nitric acid, but
dissolve rapidly in strong sulphuric acid without liberation of gas,
and still more rapidly in caustic potash. (Compt. Rend., Aug., 1885.)

At its hinder end the uterus opens by a median vertical slit, which
lies in the 8th sternum, into a genital pouch which represents part
of the external integument, folded back far into the interior of the
abdomen. (See fig. 96.) Upon the dorsal wall of the genital pouch the
orifice of the spermatheca is situated.[165] This is a short tube
dilated at the end, and wound into a spiral of about one turn. From
the tube a cæcal process is given off, which may correspond with
the accessory gland attached to the duct of the spermatheca in many
Insects (_e.g._, Coleoptera, Hymenoptera, and some Lepidoptera). The
spermatheca is filled during copulation, and is always found to contain
spermatozoa in the fertile female.[166] The spermatozoa are no doubt
passed into the genital pouch from time to time, and there fertilise
the eggs descending from the ovarian tubes.

[165] It is usually stated that the spermatheca of the Cockroach
opens into the uterus, as it does in most other Insects, but this is
not true. Locusts and Grasshoppers have the outlet of the spermatheca
placed as in the Cockroach; in other European Orthoptera, it lies
upon the dorsal wall of the uterus. (Berlese, loc. cit., p. 273.)

[166] It is a striking proof of the sagacity of Malpighi, that he
should have observed in the Silkworm the spermatophore of the male
(“in spiram circumvolutum persimile semen”) and the spermatheca of
the female. His reasoning as to the function of the spermatheca
wanted nothing but microscopic evidence of the actual transference
of spermatozoa to establish it in all points. Audouin and Siebold
supplied what was wanting nearly two centuries later, but they
mistook the spirally wound spermatophore for a broken-off penis, and
Stein (Weibl. Geschlechtsorgane der Käfer, p. 85) first arrived at
the complete proof of Malpighi’s explanation.

[Illustration: Fig. 96.--Diagram to show the theoretical (upper figure)
and actual position of the hinder abdominal sterna in the female
Cockroach. _U_, uterus; _s_, spermatheca. The nerve-cord is introduced
into both figures.]

The external reproductive organs of the female Cockroach belong to the
7th, 8th, and 9th somites. The 7th sternum is incompletely divided into
anterior and posterior sections, and the posterior section is split
into lateral halves. These are joined by a flexible membrane, which
admits of the wide separation of the halves, when copulation or the
passage of the large egg-capsule renders it necessary. The vertical
faces of the membrane, which are pressed together when the parts are at
rest, are stiffened by chitinous thickenings.

If the succeeding sterna retained their proper place, as they do in
some Orthoptera (_e.g._, the Mole Cricket), the 8th and 9th sterna
would project beyond the 7th, while the rectum would open beneath the
last tergum, and the uterus between the 8th and 9th sterna. In the
adult female Cockroach, however, the 8th and 9th somites are telescoped
into the 7th, and completely hidden by it. Their terga are reduced to
narrow bands. The 8th sternum forms a semi-transparent plate which
slopes downwards and backwards, and is pierced by a vertical slit, the
outlet of the uterus. The upper edge of this sternum is hinged upon
the projecting basis of the anterior gonapophyses (to be described
immediately), and the parts form a kind of spring joint, ordinarily
closed, but capable of being opened wide upon occasion. The 9th sternum
is a small median crescentic plate, distinct from the 8th; it supports
the spermatheca, whose duct traverses an oval plate which projects from
the fore-edge of the sternum.

[Illustration: Fig. 97.--Hinder end of abdomen of female Cockroach. In
the upper figure the halves of the 7th sternum are closed; in the lower
figure they are open.]

By the telescoping of the 8th and 9th somites the sterna take the
position shown in fig. 96_B_, and a new cavity, the genital pouch, is
formed by invagination. This receives the extremity of the body of
the male during copulation, while it serves as a mould in which the
egg-capsule is cast during oviposition. Its chitinous lining resembles
that of the outer integument. The uterus opens into its anterior end,
which is bounded by the 8th sternum; the spermatheca opens into its
roof, which is supported by the 9th sternum and the gonapophyses; while
its floor is completed by the 7th sternum and the infolded chitinous
membrane.

[Illustration: Fig. 98.--External Reproductive Organs of Female. _T^8_,
&c., terga; _S^7_, &c., sterna; _G_, anterior gonapophysis; _G_′, its
base; _g_, posterior gonapophyses; _Od_, oviduct; _sp_, spermatheca;
_R_, rectum. The upper figure shows the parts in oblique profile;
the left lower figure is an oblique view from before of the outlet
of the uterus, the anterior gonapophyses being cut short; the right
lower figure shows the gonapophyses. Arrows indicate the outlet of the
oviduct and uterus.]

A pair of appendages (anterior gonapophyses) are shown by the
development of the parts to belong to the 8th somite. They are
slender, irregularly bent, and curved inwards at the tips. A small,
forked, chitinous slip connects them with both the 8th and 9th terga,
but their principal attachment is to the upper (properly, posterior)
edge of the 8th sternum. The anterior gonapophyses expand at their
bases into broad horizontal plates, which form part of the roof of the
genital pouch.

Two pairs of appendages, belonging to the 9th somite, form the
posterior gonapophyses. The outer pair are relatively large, soft, and
curved: the inner narrow, hard, and straight.[167]

[167] The descriptions and figures of the reproductive appendages of
female Orthoptera by Lacaze-Duthiers (Ann. Sci. Nat., 1852) are so
often consulted, that it may be useful to explain how we understand
and name the same parts. In pl. xi., fig. 2, 8′ and 9′ are the 8th
and 9th terga; the anterior gonapophyses are seen to be attached to
them below; _a_ (figs. 2 and 4) is the base of the same appendage,
but the twisted ends are incorrect; the 8th sternum is seen at the
back (figs. 2 and 4); _a_′ represents the outer, _f_ the inner pair
of posterior gonapophyses.

The anterior gonapophyses form the lower, and the posterior the upper
jaw of a forceps, which in many Insects can be protruded beyond the
body. Some of the parts are often armed with teeth, and the primary use
of the apparatus is to bore holes in earth or wood for the reception
of the eggs. Hence the apparatus is often called the _ovipositor_. It
forms a prominent appendage of the abdomen in such Insects as Crickets,
Saw-flies, Sirex, and Ichneumons. The sting of the Bee is a peculiar
adaptation of the same organ to a very different purpose. In the
Cockroach the ovipositor is used to grasp the egg-capsule, while it
is being formed, filled with eggs, and hardened; and the notched edge
(fig. 5, p. 23) is the imprint of the inner posterior gonapophyses,
made while the capsule is still soft. The shape of the parts in the
male and female indicates that the ovipositor is passive in copulation,
and is then raised to allow access to the spermatheca.

_Male Reproductive Organs._

The male reproductive organs of Insects, in spite of very great
superficial diversity, are reducible to a common type, which is
exemplified by certain Coleoptera. The essential parts are (1) the
_testes_, which in their simplest form are paired, convoluted tubes;
more commonly they branch into many tubules or vesiculæ, while they
may become consolidated into a single organ; (2) long coiled _vasa
deferentia_, opening into or close to (3) paired _vesiculæ seminales_,
which discharge into (4) the _ejaculatory duct_, a muscular tube, with
chitinous lining, by which the spermatozoa are forcibly expelled.
Opening into the vesiculæ seminales, the ejaculatory duct, or by a
distinct external orifice, may be found (5) _accessory glands_, very
variable in form, size, and number. More than one set may occur in the
same Insect. To these parts, which are rarely deficient, are very often
appended an external armature of hooks or claspers.

The male Cockroach will be found to agree with this description.
It presents, however, two peculiarities which are uncommon, though
not unparalleled. In the first place the testes are functional only
in the young male. They subsequently shrivel, and are functionally
replaced by the vesiculæ seminales and their appendages, where the
later transformations of the sperm-cells are effected. The atrophied
testes are nevertheless sufficiently large in the adult to be easily
made out. Secondly, the accessory glands are numerous, and differ
both in function and insertion. Two sets are attached to the vesiculæ
seminales, and the fore end of the ejaculatory duct (_utriculi majores_
and _breviores_); another large conglobate gland opens separately to
the exterior. We shall now describe the structure of these parts in
more detail.[168]

[168] We propose to notice here the chief differences which we have
found between the figures of Brehm (_loc. cit._), which are the
fullest and best we have seen, and our own dissections.

Figs. 10, 11 (pp. 169–70). The ejaculatory duct and duct of the
conglobate gland are made to end in the penis (_infra_, p. 178).

Figs. 14, 15 (p. 173). These figures seem to us erroneous in many
respects, such as the median position of the penis and titillator.

Fig. 16 (p. 174). The pair of hooks marked _E_ are too small, and
there are additional plates at the base, which are not figured (see
our fig. 102). _F_ (of our fig.) is omitted.

The testes may be found in older larvæ or adults beneath the fifth
and sixth terga of the abdomen. They lie in the fat-body, from which
they are not very readily distinguished. Each testis consists of 30–40
rounded vesicles attached by short tubes to the vas deferens.[169] The
wall of the testis consists of a peritoneal layer and an epithelium,
which is folded inwards along transverse lines. The cells of the
epithelium give rise to spermatocysts,[170] which enclose sperm cells.
By division of the nuclei of the sperm cells spermatozoa are formed,
which have at first nucleated heads and long tails. Subsequently the
enlarged heads disappear. The spermatozoa move actively. In adult males
the testes undergo atrophy, but can with care be discovered in the
enveloping fat-body.

[169] In _Blatta germanica_ the testes are functional throughout
life. They consist of four lobes each. The vasa deferentia are much
shorter than in _P. orientalis_.

[170] The spermatocysts are peculiar to Insects and Amphibia. They
arise by division of the spermatospores, or modified epithelial
cells, and form hollow cysts, within which sperm cells (or
spermatoblasts) are developed by further division. The sperm cells
are usually placed radiately around the wall of the spermatocyst.
They escape by dehiscence, and are transformed into spermatozoa.

[Illustration: Fig. 99.--1. Male Organs, ventral view. _Ts_, testis;
_VD_, vas deferens; _DE_, ductus ejaculatorius; _U_, utriculi majores;
_u_, utriculi breviores. 2. Do., dorsal view, showing termination of
vasa deferentia. 3. Conglobate gland, and its duct. × 8.]

The vasa deferentia are about ·25 inch in length. They pass backwards
from the testes, then turn downwards on each side of the large
intestine, and finally curve upwards and forwards, entering the
vesiculæ seminales on their dorsal side. Each vas deferens divides
once or twice into branches, which immediately reunite; in the last
larval stage the termination of the passage dilates into a rounded,
transparent vesicle.

[Illustration: Fig. 100.--Male Organs, side view. _T^7_, seventh
tergum; _S^7_, seventh sternum; _Ts_, _DE_, as before. _A_, _B_, see
fig. 102. × 8.]

The vesiculæ seminales are simple, rounded lobes in the pupa
(fig. 101), but their appearance is greatly altered in the adult by the
development of two sets of utricles (modified accessory glands). The
longer utricles (_utriculi majores_) open separately into the sides
of the vesiculæ; nearer to the middle line are the shorter and more
numerous _utriculi breviores_, which open into the fore part of the
vesiculæ.

The utricles form the “mushroom-shaped gland” of Huxley, which was long
described as the testis. In the adult male the utricles are usually
distended with spermatozoa, and of a brilliant opaque white.

The ejaculatory duct is about ·15 inch long, and overlies the 6th-9th
sterna. It is wide in front, where it receives the paired outlets of
the vesiculæ seminales. Further back it narrows, and widens again near
to its outlet, which we find to be between the external chitinous
parts, and not into the penis, as described by Brehm. The duct
possesses a muscular wall for the forcible ejection of its contents,
and in accordance with its origin as a folding-in of the outer surface,
it is provided with a chitinous lining. In the adult the fore part of
the duct may be distended with spermatozoa.

[Illustration: Fig. 101.--Vesiculæ Seminales and Ductus Ejaculatorius
of Pupa. _VD_, vas deferens. × 28.]

The ejaculatory duct is originally double (p. 194), and its internal
cavity is still subdivided in the last larval stage or so-called “pupa.”

Upon the ventral surface of the ejaculatory duct lies an accessory
gland of unknown function; it is “composed of dichotomous, monilated
tubes, lined by a columnar epithelium, all bound together by a
common investment into a flattened, elongated mass.”[171] The duct
of this gland does not enter the penis, as described by Brehm, but
opens upon a double hook, which forms part of the external genital
armature (fig. 99, 3). It may be convenient to distinguish this as the
“conglobate gland.”[172]

[171] Huxley, Anat. Invert. Animals, p. 416.

[172] The term “accessory gland,” used by Huxley and others, is
already appropriated to glands which we believe to be represented
by the utricles of the Cockroach, and which have only a general
correspondence with the gland in question.

The external reproductive organs of the male Cockroach are concealed
within the 9th sternum. The so-called penis (fig. 102) is long,
slender, and dilated at the end. It is not perforated, and we do not
understand its use, though it probably conveys the semen.

[Illustration: Fig. 102.--External Male Organs, separated. The
lettering agrees with Brehm’s figures. _A_, titillator; _B_, penis;
_C-F_, hooks and plates. × 8.]

The “titillator” (Brunner von Wattenwyl) is a solid curved hook with
a hollow base. Besides these, are several odd-shaped, unsymmetrical
pieces (fig. 102, _C_, _D_, _E_, _F_), moved by special muscles. A pair
of styles (see figs. 32–3 and 103) project from the hinder edge of the
9th sternum. These paired and unpaired appendages are believed to open
the genital pouch of the female, but we do not understand their action
in detail.[173]

[173] Similar organs, forming a male genital armature, have been
described in various Insects. See Burmeister, Man. of Entomology,
p. 328 (Eng. Transl.); Siebold, Anat. of Invertebrates; Gosse
in Linn. Trans., Ser. 2, Vol. II. (1883); Burgess on Milk-weed
Butterfly, Ann. Mem. Bost. Soc. Nat. Hist.; &c.

Brehm observes that the male reproductive organs of the Cockroach are
most nearly paralleled by those of the Mantidæ. A free penis occurs in
all Orthoptera, except Acridiidæ and Phasmidæ.

The male organs of the House Cricket will be found much easier to
understand than those of the Cockroach. The testes are of irregular,
oval figure, the vasa deferentia very long, tortuous, and enlarged
towards the middle of their length. The vesiculæ seminales bear many
utriculi majores et breviores. The penis is of simple form, and dilated
at the end. The titillator is broad, but produced into a slender prong,
which projects beyond the penis. A pair of subanal styles is found, but
the unpaired hooklets are wanting or very inconspicuous.

[Illustration: Fig. 103.--The Tenth Tergum reflected to show the
external male organs _in situ_. _T^{10}_, tenth tergum; _p_, podical
plates; _A-F_, as in fig. 102; _S_, sub-anal styles. × 8.]

Very little is known about the act of copulation among Cockroaches,
and the opportunities of observation are few. The following account is
given by Cornelius (_loc. cit._, p. 22):--

“The male and female Cockroaches associate in pairs, the females
being generally quiet. The male, on the contrary, bustles about
the female, runs round her, trailing his extended abdomen on the
ground, and now and then raises his wings. If the female moves
away, the male stops the road. At last, when the female has become
perfectly still, the male goes in front of her, brings the end
of his abdomen towards her, then moves backwards, and pushes his
whole length under the female. The operation is so rapid that it is
impossible to give an exact account of the circumstances. Then the
male creeps out from beneath the female, raises high both pairs of
wings, depresses them again, and goes off, while the female usually
remains quiet for some time.”

CHAPTER X.

DEVELOPMENT.

_SPECIAL REFERENCES._

RATHKE. Zur Entwickelungsgesch. der _Blatta germanica_. Meckel’s
Arch. of Anat. u. Phys., Bd. VI. (1832).

BALFOUR. Comparative Embryology, 2 vols. (1880–1).

GRABER. Insekten, Vol. II. (1879).

LUBBOCK. Origin and Metamorphoses of Insects (1874).

KOWALEWSKY. Embryol. Studien an Würmern u. Arthropoden. Mém. Ac.
Petersb. Sér. VII., Vol. XVI. (1871).

WEISMANN. Entw. der Dipteren. Zeits. f. wiss. Zool., Bde. XIII.,
XIV. (1863–4).

METSCHNIKOFF. Embryol. Studien. an Insecten. Ib., Bd. XVI. (1866).

BÜTSCHLI. Entwicklungsgeschichte der Biene. Ib., Bd. XX. (1870).

BOBRETZKY. Bildung d. Blastoderms u. d. Keimblätter bei den
Insecten. Ib., Bd. XXXI. (1878).

NUSBAUM. Rozwój przewodów organów pteiowych u owadów (Polish).
Kosmos. (1884). [Development of Sexual Outlets in Insects.]

---- Struna i struna Leydig’a u owadów (Polish). Kosmos (1886).
[Chorda and Leydig’s chorda in Insects.]

_The Embryonic Development of the Cockroach._[174]

By JOSEPH NUSBAUM, Magister of Zoology, Warsaw.

[174] In the following description it is to be understood that the
observations have been made upon _Blatta germanica_, except where _P.
orientalis_ is expressly named.

The development of the Cockroach is by no means an easy study. It costs
some pains to find an accessible place in which the females regularly
lay their eggs, and the opaque capsule renders it hard to tell in what
stage of growth the contained embryos will be found. Accordingly,
though the development of the Cockroach has lately attracted some
observers, the inexperienced embryologist will find it more profitable
to examine the eggs of Bees, of Aphides, or of such Diptera as lay
their eggs in water.

The Cockroach is developed, like most animals, from fertilised
eggs.[175] The eggs of various animals differ much in size and form,
but always contain a formative plasma or egg-protoplasm, a germinal
vesicle (_nucleus_), and a germinal spot (_nucleolus_). Besides these
essential parts, eggs also always contain a greater or less quantity
of food-yolk, which serves for the supply of the developing embryo.
The quantity of this yolk may be small, and its granules are then
uniformly dispersed through the egg-protoplasm; or very considerable,
in which case the protoplasm and yolk become more or less sharply
defined. Eggs of the first kind are known as _holoblastic_, those of
the second kind as _meroblastic_, names suggested by the complete or
partial segmentation which these kinds of eggs respectively undergo.
When the food-yolk is very abundant it does not at first (and in some
cases does not at any time) exhibit the phenomena of growth, such as
cell-division. If, on the other hand, the yolk is scanty and evenly
dispersed through the egg-protoplasm, the segmentation proceeds
regularly and completely. The eggs of Arthropoda, including those of
the Cockroach, are meroblastic.

[175] Fertilisation consists essentially in the union of an
egg-nucleus (female nucleus) with a sperm-nucleus (male nucleus).
From this union the first segmentation-nucleus is derived.

The eggs of the Cockroach (_P. orientalis_) are enclosed (see p. 23)
sixteen together in stout capsules of horny consistence. They are
adapted to the form of the capsule, laterally compressed, convex on
the outer, and concave on the inner side. The ventral surface of
the embryo lies towards the inner, concave surface of the egg. Each
egg is provided with a very thin brownish shell (_chorion_), whose
surface is ornamented with small six-sided projections. In young eggs,
still enclosed within the ovary, the nucleus (_germinal vesicle_) and
nucleolus (_germinal spot_) can be plainly seen, but by the time they
are ready for deposition within the capsule, so large a quantity of
food-yolk, at first finely--afterwards coarsely--granular, accumulates
within them, that the germinal vesicle and spot cease to be visible.

Since the yolk of the newly-laid egg of the Cockroach is of a
consistence extremely unfavourable to hardening and microscopic
investigation, I have not been able to obtain transverse sections
of the germinal vesicle, nor to study the mode of its division
(segmentation). If, however, we may judge from what other observers
have found in the eggs of Insects more suitable for investigation than
those of the Cockroach, we shall be led to conclude that a germinal
vesicle, with a germinal spot surrounded by a thin layer of protoplasm,
lies within the nutritive yolk of the Cockroach egg. From this
protoplasm all the cells of the embryo are derived.

The germinal vesicle, together with the surrounding protoplasm,
undergoes a process of division or segmentation. Some of the cells thus
formed travel towards the surface of the egg to form a thin layer of
flattened cells investing the yolk, the so-called _blastoderm_, while
others remain scattered through the yolk, and constitute the yolk-cells
(fig. 107).

[Illustration: Fig. 104.--Ventral Plate of _Blatta germanica_, with
developing appendages, seen from below. × 20.]

On the future ventral side of the embryo (and therefore on the concave
surface of the egg) the cells of the blastoderm become columnar, and
here is formed the so-called ventral plate, the first indication of
the embryo. This is a long narrow flattened structure (fig. 104). It
is wider in front where the head segment is situated; further back it
becomes divided by many transverse lines into the primitive segments.
The total number of segments in the ventral plate of Insects is usually
seventeen.[176] Indications of the appendages appear very early. They
give rise to an unpaired labrum, paired antennæ, mandibles, and maxillæ
(two pairs). The first and second pair of maxillæ have originally,
according to Patten,[177] two and three branches respectively. Behind
the mouth-parts are found three rudimentary legs. Upon all the
abdominal segments, according to Patten, rudimentary limbs are formed;
but these soon disappear, except one pair, which persists for a time
in the form of a knobbed stalk; subsequently this, too, completely
disappears. Three or four of the hindmost segments curve under the
ventral surface of the embryo, and apparently (?) give rise to the
modified segments and appendages of the extremity of the abdomen
(fig. 105). The ventral plate lies at first directly beneath the egg
membrane (chorion), but afterwards becomes sunk in the yolk, so that
a portion of the yolk makes its way between the ventral plate and the
chorion. Whilst this portion of the yolk is perfectly homogeneous, the
remainder, placed internally to it, becomes coarsely granular, and
encloses many roundish cavities and yolk-cells. The middle region of
the body is more deeply sunk in the yolk than the two ends, and the
embryo thus assumes a curved position (fig. 105).

[176] Balfour, Embryology, Vol. I., p. 337.

[177] Q. J. Micr. Sci., Vol. XXIV., page 596 (1884).

[Illustration: Fig. 105.--Ventral Plate of _B. germanica_, side view.
× 20.]

[Illustration: Fig. 106.--Diagram to illustrate the formations of
the Embryonic Membranes. _A_, amnion; _S_, serous envelope; _B_,
blastoderm.]

This curvature of the embryo is closely connected with the formation of
the embryonic membranes. On either side of the ventral plate a fold of
the blastoderm arises, and these folds grow towards each other beneath
the chorion. Ultimately they meet along the middle line of the ventral
plate (fig. 106), and thus form a double investment, the outer layer
being the _serous envelope_, the inner the _amnion_. Between the two
the yolk passes in, as has been explained above (fig. 107).

[Illustration: Fig. 107.--Transverse section through young Embryo of
_B. germanica_. _E_, epiblast; _M_, mesoblast; _Y_, yolk-cells.]

At the same time that the embryonic membranes are forming, the
embryonic layers make their appearance. The ventral plate, which
was originally one-layered, forms the _epiblast_ or outer layer of
the embryo, and from this are subsequently derived the middle layer
(_mesoblast_) and the deep layer (_hypoblast_).

[Illustration: Fig. 108.--Diagram to illustrate the formation of the
Germinal Layers. _E_, epiblast; _M_, mesoblast.]

As to the origin of the mesoblast most observers have found[178] that
a long groove (the _germinal groove_) appears in the middle line of
the ventral plate (fig. 108), which bulges into the yolk, gradually
detaches itself from the epiblast, and completes itself into a tube.
The lumen of this tube soon becomes filled with cells, and the solid
cellular mass thus formed divides into two longitudinal tracts, which
lie right and left of the middle line of the ventral plate beneath the
epiblast, and are known as the _mesoblastic bands_. In the Cockroach
I was able to satisfy myself that in this Insect also, the mesoblast,
in all probability, arises by the formation and closure of a similar
groove of the epiblast. _M_ (fig. 108) represents the stage in which
the lumen of the groove has disappeared, and the mesoblast forms a
solid cellular mass.

[178] Kowalewsky in _Hydrophilus_, Graber in _Musca_ and _Lina_,
Patten in _Phryganidæ_, myself in _Meloe_, &c.

The origin of the hypoblast in Insects has not as yet been clearly
determined. Two quite different views on this subject have found
support. Some observers (Bobretsky, Graber, and others) maintain
that the hypoblast originates in the yolk-cells, which form a
superficial layer investing the rest of the yolk. Others (especially
Kowalewsky[179]) believe that the process is altogether different.
According to the latest observations of the eminent embryologist just
named, upon the development of the _Muscidæ_, the germinal groove
gives rise, not only to the two mesoblastic bands, but also, in its
central region, to the hypoblast. This makes its appearance, however,
not as a continuous layer, but as two hourglass-shaped rudiments, one
at the anterior, the other at the posterior end of the ventral plate.
These rudiments have their convex ends directed away from each other,
while their edges are approximated and gradually meet so as to form a
continuous hypoblast beneath the mesoblast. Although I have not been
able completely to satisfy myself as to the mode of formation of the
hypoblast in the Cockroach, I have observed stages of development which
lead me to suppose that it proceeds in this Insect in a manner similar
to that observed by Kowalewsky in _Muscidæ_. The hourglass-shaped
rudiments of the hypoblast become pushed upwards by those foldings-in
of the epiblast which form towards the anterior and posterior ends of
the embryo, and give rise to the stomodæum and proctodæum.[180]

[179] Biolog. Centrablatt. Bd. VI., No. 2 (1886).

[180] These terms are explained on p. 115.

The stage of development in which the germinal groove appears, by
the folding inwards of the epiblast, has been observed in many other
animals, and is known as the Gastræa-stage. In all higher types
(Vertebrates, the higher Worms, Arthropoda, Echinodermata) the
mesoblast and hypoblast are formed in the folded-in part of the Gastræa
in a manner similar to that observed in Insects.

The yolk-cells, which some observers have supposed to form the
hypoblast, are believed by Kowalewsky to have no other function except
that of the disintegration and solution of the yolk. I can, however,
with confidence affirm that in the Cockroach these cells take part in
the formation of permanent tissues (see below).

Each of the two mesoblastic bands which lie right and left of the
germinal groove divides into many successive somites, and each
of these becomes hollow. Every such somite consists of an inner
(dorsal) one-layered and an outer (ventral) many-layered wall, the
latter being in contact with the epiblast. The cavities of all the
somites unite to form a common cavity, the _cœlom_ or perivisceral
space of the Cockroach. The cœlom, like the cavities in which it
originates, is bounded by two layers of mesoblast--an inner, the
so-called _splanchnic_ or visceral layer, which lies on the outer
side of the hypoblast, and an outer _somatic_ or parietal layer,
beneath the epiblast. There are accordingly four layers in the
Cockroach-embryo--viz., (1) _epiblast_, from which the integument and
nervous system are developed; (2) _somatic layer of mesoblast_, mainly
converted into the muscles of the body-wall; (3) _splanchnic layer of
mesoblast_, yielding the muscular coat of the alimentary canal; and (4)
_hypoblast_, yielding the epithelium of the mesenteron.

[Illustration: Fig. 109.--Transverse sections of Embryo of _B.
germanica_, with rudimentary nervous system (Oc. 4, Obj. D.D. Zeiss).
_N_, nervous system; _M_, mesoblastic somites.]

Scattered yolk-cells associate themselves with the mesoblast cells, so
that the constituents of the mesoblast have a two-fold origin. Fig. 109
shows that the yolk-cells are large, finely granular, and provided
with many (3–6) nuclei and nucleoli. They send out many branching
protoplasmic threads, which connect the different cells together,
and thus form a cellular network. Certain cells separate themselves
from the rest, apply themselves to the walls of the somites, and form
a provisional diaphragm (fig. 110, _D_) consisting of a layer of
flattened cells;[181] other cells (fig. 109) pass into and through
the walls of the somites, and reach their central cavity, where they
increase in number and blend with the mesoblast cells. What finally
becomes of them I cannot say; perhaps they form the fat-body.

[181] Cf. Korotneff, Embryol. der Gryllotalpa. Zeits. f. wiss. Zool.
(1885).

[Illustration: Fig. 110.--Transverse section through ventral region
of Embryo of _B. germanica_. The nerve-cord has by this time detached
itself from the epiblast, _E_. _D_ is the temporary diaphragm; _Ch_,
temporary cellular band, from which the neurilemma proceeds; _Ap_,
appendages in section; _M_, mesoblast; _N_, nerve-cord. (Oc. 4. Obj.
BB. Zeiss).]

The ventral plate occupies, as I have explained, the future ventral
surface of the Insect, and here only at first both the embryonic
membranes are to be met with. On the sides and above the yolk is
invested by the serous envelope alone. The ventral plate, however,
gradually extends upwards upon the sides of the egg, in the directions
of the arrows (fig. 107), and finally closes upon the dorsal surface of
the embryo, so as completely to invest the whole yolk. Every segment
of the embryo shows at a certain stage numerous clusters of spherical
granules, which according to Patten (loc. cit.) are composed of urates
(fig. 111, _S_).

We shall now proceed to consider the development of the several organs
of the Cockroach.

[Illustration: Fig. 111.--Transverse section of older Embryo of _B.
germanica_ (abdomen). _E_, Epiblast; _H_, hypoblast; _Ht_, heart; _G_,
reproductive organs; _S_, spherical granules.]

_Nervous System._--Along the middle line of the whole ventral
surface there is formed a somewhat deep groove-like infolding of the
epiblast, bounded on either side by paired solid thickenings, which
detach themselves from the epiblast (fig. 110, _N_) and constitute
the double nervous chain. In many other Insects a median cord (from
which are derived the transverse interganglionic commissures) forms
along the bottom of the nervous fold. This secondary median fold is
very inconspicuous and slightly developed in the Cockroach, so that
the transverse commissures between the developing ganglia are mainly
contributed by the cellular substance of the lateral nervous band.
The brain is formed out of two epiblastic thickenings which occupy
shallow depressions. The so-called _inner neurilemma_, which surrounds
the ventral nerve-cord, is developed as follows:--Along the ventral
nerve-cord, and between its lateral halves, a small solid cellular
band (fig. 110, _Ch_) is developed out of the mesoblastic diaphragm
described above. This grows round the ventral nerve-cord on all sides
(fig. 112, _N_′), passing also inwards between the central fibrillar
tract and the outer cellular layer, and thus forming the thin
membrane which invests the central nervous mass (fig. 112, _N_″). The
above-mentioned solid mesoblastic band, which exists for a very short
time only, may perhaps be homologised with the chorda dorsalis of
Vertebrates, and the chorda of the higher Worms, since in these types
also the chorda forms a solid cellular band of meso-hypoblastic origin,
lying between the nervous system and the hypoblast. The peripheral
nerves arise as direct prolongations of the fibrillar substance of the
nerve-cord.

[Illustration: Fig. 112.--Transverse section of Nerve-cord of Embryo
of _B. germanica_ (Oc. 4, Obj. D.D. Zeiss). _C_, cellular layer; _F_,
fibrillar substance (_punkt-substance_ of Leydig); _Ch_, cellular band;
_N_′ _N_″ inner and outer neurilemma.]

[Illustration: Fig. 113.--Alimentary Canal of Embryo of _B. germanica_.
Copied from Rathke, loc. cit., but differently lettered. _st_,
stomodæum, already divided into œsophagus, crop, and gizzard; _m_,
mesenteron; _pr_, proctodæum, with Malpighian tubules (removed on the
right side). × 12.]

_Alimentary Canal._--The epithelium of the mesenteron is formed out of
the hypoblast, whose cells assume a cubical form and gradually absorb
the yolk. The epithelium of the stomodæum and proctodæum is derived,
however, from two epiblastic involutions at the fore and hind ends of
the embryo. The muscular coat of the alimentary canal is contributed by
the splanchnic layer of the mesoblast. The mesenteron in an early stage
of development appears as an oval sac of greenish colour (fig. 113),
faintly seen through the body-wall. The cæcal tubes are extensions
of the mesenteron, the Malpighian tubules of the proctodæum. The
epiblastic invaginations may be recognised in all stages of growth by
their chitinous lining and layer of chitinogenous cells, continuous
with the similar layers in the external integument.

_Tracheal System._--Tubular infoldings of the epiblast, forming at
regular intervals along the sides of the embryo and projecting into the
somatic mesoblast, give rise to the paired tracheal tubes, which are at
first simple and distinct from one another.[182]

[182] In _Gryllotalpa_ (Dohrn), as in Spiders, some Myriopods
and _Peripatus_ (Moseley, Phil. Trans., 1874), each stigma, with
its branches, constitutes throughout life a separate system. The
salivary glands arise in the same way, not, like the salivary
glands of Vertebrates, as extensions of the alimentary canal, but
as independent pits opening behind the mouth. Both the tracheal and
the salivary passages are believed to be special modifications of
cutaneous glands (Moseley).

_Heart._--The wall of the heart in Insects is of mesoblastic origin,
and develops from paired rudiments derived from that peripheral part of
each mesoblastic band which unites the somatic to the splanchnic layer.
In this layer two lateral semi-cylindrical rudiments appear, which, as
the mesoblastic bands meet on the dorsal surface of the embryo, are
brought into contact and unite to form the heart (fig. 111). The heart
is therefore hollow from the first, its cavity not being constricted
off from the permanent perivisceral space enclosed by the mesoblast,
but being a vestige of the primitive embryonic blastocœl, which is
bounded by the epiblast, as well as by the two other embryonic layers.
Such a mode of the development of the heart was observed by Bütschli in
the Bee, and by Korotneff in the Mole Cricket. I am convinced, from my
own observations, that the heart of the Cockroach originates in this
way, though it is to be observed that, in consequence of Patten’s
results,[183] the question requires further investigation. According
to Patten the mesoblastic layers of the embryo pulsate rhythmically
long before the formation of the heart. Patten also states that the
blood-corpuscles are partially derived from the wall of the heart.

[183] Loc. cit.

[Illustration: Fig. 114.--Young Ovary of _B. germanica_. (Oc. 2, Ob.
DD, Zeiss.)]

[Illustration: Fig. 115.--Young Testis of _B. germanica_. (Oc. 2, Ob.
DD, Zeiss.)]

[Illustration: Figs. 116, 117, 118.--Three stages of development of
tegumentary portion of Male Sexual Organs of _P. orientalis_. (Oc. 1,
Ob. BB, Zeiss.) _VD_, vas deferens; _VS_, vesicula seminalis; _D_,
ductus ejaculatorius; _P_, _p_, penis and its lateral appendages.]

_Reproductive Organs._--In _P. orientalis_ the reproductive organs
are developed as follows:--The reproductive glands have a mesoblastic
origin. The immature ovaries and testes take the form of elongate oval
bodies, which prolong themselves backwards into a long thin thread-like
cord or ligament (figs. 114, 115). These lie in the perivisceral space,
between the somatic and splanchnic layers of the mesoblast, and on the
sides of the abdomen. The glands divide tolerably early into chambers,
which have, however, a communicating passage (figs. 114, 115). From
their backward-directed prolongations arises the epithelium of the vasa
deferentia and oviducts. All other parts of the reproductive ducts are
developed out of tegumentary thickenings of the ventral surface in
the last abdominal segment, and the last but one. These thickenings
are at first paired,[184] but afterwards blend to form single organs
(fig. 118). Within the tegumentary thickenings just described, there
appear in the male Cockroach two anterior closed cavities which unite
to form the single cavity of the permanent mushroom-shaped body
(_vesicula seminalis_). A posterior cavity becomes specialised as
the ductus ejaculatorius, while the hindmost part of the thickening,
which is at first double, afterwards by coalescence single, forms
the penis (figs. 117, 118). The accessory reproductive glands have
also a tegumentary origin. In the female Cockroach the chitinogenous
epithelium of the integument gives rise to the uterus, vagina,[185]
and accessory glands, the muscular and connective tissue layers of the
sexual apparatus being formed out of loose mesoblastic cells.[186]

Joseph Nusbaum.

[184] This arrangement persists only in _Ephemeridæ_ among Insects
(Palmen, Ueb. paarigen Ausführungsgänge der Geschlechtsorgane bei
Insekten, 1884).

[185] Genital pouch of the preceding description.

[186] Indications, which we have not found time to work out, lead us
to think that the development of the specially modified segments and
appendages in the male and female Cockroach needs re-examination. We
hope to treat this subject separately on a future occasion.--L. C. M.
and A. D.

_Post-embryonic Development._

At the time of hatching the Cockroach resembles its parent in all
essentials, the wings being the only organs which are developed
subsequently, not as entirely new parts, but as extensions of the
lateral edges of the thoracic terga. The mode of life of the young
Cockroach is like that of the adult, and development may be said to
be direct, or with only a trifling amount of metamorphosis. In the
Thysanura even this small post-embryonic change ceases to appear,
and the Insect, when it leaves the egg, differs from its parent
only in size. It is probable that development without metamorphosis
was once the rule among Insects. At present such is by no means the
case. Insects furnish the most familiar and striking, though, as will
appear by-and-by, not the most typical examples of development with
metamorphosis. In many text-books the quiescent pupa and the winged
imago are not unnaturally described as normal stages, which are
exceptionally wanting in Orthoptera, Hemiptera, Thysanura, and other
“ametabolous” Insects. It is, however, really the “holometabolous”
Insects undergoing what is called “complete metamorphosis,” which are
exceptional, deviating not only from such little-specialised orders as
Thysanura and Orthoptera, but from nearly all animals which exhibit a
marked degree of metamorphosis. We shall endeavour to make good this
statement, and to show that the Cockroach is normal in its absence of
conspicuous post-embryonic change, while the Butterfly, Bee, Beetle,
and Gnat are peculiar even among metamorphic animals.

_Animal Metamorphoses._

To investigate the causes of metamorphosis, let us select from the
same sub-kingdom two animals as unlike as possible with respect to the
amount of post-embryonic change to which they are subject. We can find
no better examples than Amphioxus and the Chick.

The newly-hatched Amphioxus is a small, two-layered, hollow sac, which
moves through the sea by the play of cilia which project everywhere
from its outer surface. It is a Gastræa, a little simpler than the
Hydra, and far simpler than a Jelly-fish. As yet it possesses no
nervous system, heart, respiratory organs, or skeleton. The most
expert zoologist, ignorant of its life-history, could not determine
its zoological position. He would most likely guess that it would turn
either into a polyp or a worm.

The Chick, on the other hand, at the tenth day of incubation, is
already a Bird, with feathers, wings, and beak. When it chips the shell
it is a young fowl. It has the skull, the skeleton, the toes, and the
bill characteristic of its kind, and no child would hesitate to call it
a young Bird.

Amphioxus is, therefore, a Vertebrate (if for shortness we may so name
a creature without vertebræ, brain, or skull), which develops with
metamorphosis, being at first altogether unlike its parent. The Chick
is a Vertebrate which develops directly, without metamorphosis. Let us
now ask what other peculiarities go with this difference in mode of
development.

Amphioxus produces many small eggs (1/10 mm. in diameter) without
distinct yolk, and consequently segmenting regularly. The adult is of
small size (2 to 3 in. long), far beneath the Chick in zoological rank,
and of marine habitat.

The Fowl lays one egg at once, which is of enormous size and provided
with abundant yolk, hence undergoing partial segmentation. The Fowl is
much bigger than Amphioxus, much higher in the animal scale, and of
terrestrial habitat.

Which of the peculiarities thus associated governs the rest? Is it the
number or size of the eggs? Or the size, zoological rank, or habitat of
the adult? The question cannot be answered without a wider collection
of examples. Let us run over the great divisions of the Animal
Kingdom, and collect all the facts which seem to be significant. We may
omit the Protozoa, which never develop multicellular tissues, and in
which segmentation and all subsequent development are therefore absent.

PORIFERA (Sponges).--Nearly all marine and undergoing metamorphosis,
the larva being wholly or partially ciliated.

CŒLENTERATES undergo metamorphosis, the immediate product of the
ovum being nearly always a _planula_, or two-layered hollow sac,
usually devoid of a mouth, and moving about by external cilia. In
many Cœlenterates the complicated process of development known as
Alternation of Generations occurs. The sedentary Anemones pass
through a planula stage, but within the body of the parent. Among the
few Cœlenterates which have no free planula stage is the one truly
fluviatile genus--Hydra.

WORMS are remarkable for the difference between closely allied
forms with respect to the presence or absence of metamorphosis. The
non-parasitic freshwater and terrestrial Worms, however (_e.g._,
Earthworms, Leeches, all freshwater Dendrocœla, and Rhabdocœla), do not
undergo metamorphosis. In the parasitic forms complicated metamorphosis
is common, and may be explained by the extraordinary difficulties often
encountered in gaining access to the body of a new host.

All POLYZOA are aquatic (fluviatile or marine), and all produce
ciliated embryos, unlike the parent.

BRACHIOPODA are all marine, and produce ciliated embryos.

ECHINODERMS usually undergo striking metamorphosis, but certain
viviparous or marsupial forms develop directly. There are no fluviatile
or terrestrial Echinoderms.

LAMELLIBRANCHIATE MOLLUSCA have peculiar locomotive larvæ, provided
with a ring of cilia, and usually with a long vibratile lash. These
temporary organs are reduced or suppressed in the freshwater forms.
There are no terrestrial Lamellibranchs.

SNAILS have also a temporary ciliated band, but in the freshwater
species it is slightly developed (_Limnæus_), and it is totally wanting
in the terrestrial _Helicidæ_.

CEPHALOPODA, which are all marine, have no ciliated band, and the
post-embryonic changes do not amount to metamorphosis. There is usually
a much larger yolk-sac than in other Mollusca.

CRUSTACEA usually pass through well-marked phases. _Peneus_ presents
five stages of growth (including the adult), the earlier being common
to many lower Crustacea. The Crab passes through three, beginning with
the third of _Peneus_; the Lobster through two; while the freshwater
Crayfish, when hatched, is already in the fifth and last.

FISHES seldom undergo any post-embryonic change amounting to
metamorphosis. _Amphioxus_ (if _Amphioxus_ be indeed a fish) is the
only well-marked case.

AMPHIBIA develop without conspicuous metamorphosis, except in the case
of the Frogs and Toads (Anura), which begin life as aquatic, tailed,
gill-bearing, and footless tadpoles.

REPTILES, BIRDS, and MAMMALS do not undergo transformation.

This survey, hasty as it necessarily is, shows that habitat is
a material circumstance. Larval stages are apt to be suppressed
in fluviatile and terrestrial forms. Further, it would seem that
zoological rank is not without influence. Metamorphosis is absent in
Cephalopoda, the highest class of Mollusca, and in all but the lowest
Vertebrates, while it is almost universal in Cœlenterates, Echinoderms,
and Lamellibranchs.

It has often been remarked that the quantity of food-yolk indicates
the course of development. If a large store of food has been laid up
for the young animal, it can continue its growth without any effort
of its own, and it leaves the egg well equipped for the battle of
life. Where there is little or no yolk, the embryo is turned out in an
ill-furnished condition to seek its own food. This early liberation
implies metamorphosis, for the small and feeble larva must make use of
temporary organs. Some very simple locomotive appendages are almost
universally needed, to enable it to get away from the place of its
birth, which is usually stocked with as much life as it can support.

Some animals, therefore, are like well-to-do people, who can provide
their children with food, clothes, schooling, and pocket-money. Their
fortunate offspring grow at ease, and are not driven to premature
exercise of their limbs or wits. Others are like starving families,
which are forced to send their children to sell matches or newspapers
in the streets. It is a question of the amount of capital or
accumulated food which is at command.

The connection between zoological rank and the absence of metamorphosis
is also explained by what we see among men. High zoological position
ordinarily implies strength or intelligence, and the strong and knowing
can do better for their offspring than the puny and sluggish. It does
not cost a Shark or a quadruped too much to hatch its young in its
own body, while Spiders and Earwigs,[187] which are among the highest
Invertebrates, defend their progeny, as do Mammals and Birds, the
highest Vertebrates.

[187] It may be useful to point out the following examples of
parental care among animals in which, as a rule, the eggs are left
to take care of themselves. It will be found that in general this
instinct is associated with high zoological rank (best exemplified
by Mammals and Birds), land or freshwater habitat, reduced number of
eggs, and direct development.

AMPHIBIA.--The eggs are sometimes hatched by the male (_Alytes
obstetricans_, _Rhinoderma Darwinii_), or placed by the male in
pouches on the back of the female (_Pipa dorsigera_, _Notodelphis
ovifera_, _Nototrema marsupiatum_), or carried during hatching by the
female (_Polypedates reticulatus_).

FISHES.--The Stickleback and others build nests. Of eleven genera of
nest-building Fishes, eight are freshwater. The number of eggs is
unusually small. Many Siluroids have the eggs hatched in the mouth
of the males, a few under the belly of the female. The species are
both marine and freshwater, the eggs few and large. Lophobranchiate
fishes usually have the eggs hatched by the male. They are marine;
the eggs few and large. Many sharks hatch their eggs, which are very
few, within the body. _Mustelus lævis_ has a placenta formed out of
the yolk-sac.

INSECTS.--De Geer has described the incubation of the Earwig, and the
care of the brood by the female. The cases of the social Hymenoptera,
&c., are universally known.

SPIDERS.--The care of the female spider for her eggs is well known.

CRUSTACEANS.--The Crayfish hatches and subsequently protects her
young. _Mysis_, _Diastylis_ (_Cuma_), and some Isopods hatch their
eggs. _Gammarus locusta_ is followed about by her brood, which
shelter beneath her when alarmed. _Podocerus capillatus_ builds a
nest among corallines. Several of the _Caprellidæ_ hatch or otherwise
protect their young. All these, except the Crayfish, are marine; the
eggs commonly fewer than usual.

ECHINODERMS.--Many cases of “marsupial development” have been
recorded in the species of the Southern seas. Here development,
contrary to the rule in Echinodermata, is direct.

But what has all this to do with habitat? Are fluviatile and
terrestrial animals, as a rule, better off than marine animals?
Possibly they are. In the confined and isolated fresh waters at
least, the struggle for existence is undoubtedly less severe than in
the waters of the sea. This is shown by the slow rate of change in
freshwater types. Many of our genera of land and freshwater shells
date back at least as far as to Purbeck and Wealden times, while our
common pond-mussel is represented in the Coal Measures. The comparative
security of fresh waters is probably the reason why so many marine
fishes enter rivers to spawn.

More important, and less open to question, is the direct action of the
sphere of life. The cheap method of turning the embryo out to shift
for itself can seldom be practised with success on land. But in water
floating is easy, and swimming not difficult. A very slightly-built
larva can move about by means of cilia, and a whole brood can disperse
far and wide in search of food, while still in a mere planula
condition--hollow sacs, without mouth, nerves, or sense-organs.
Afterwards the little locomotive larva settles down, opens a mouth, and
begins to feed. Nearly the whole of its development is carried on at
its own charge.

The extra risks to which marine animals are exposed also tell in favour
of transformation, for they are met by an increase in the number of
ova. Marine species commonly lay more eggs than freshwater animals of
like habits. The Cod is said to produce nine million eggs; the Salmon
from twenty to thirty thousand; the Stickleback only about one hundred,
which are guarded during hatching by the male. The Siluroid fish,
_Arius_, lays a very few eggs, as big as small cherries, which the male
carries about in his mouth.

Without laying stress upon such figures as these, which cannot be
impartially selected, we can safely affirm that marine forms are
commonly far more prolific than their freshwater allies. But high
numbers increase the difficulty of providing yolk for each, and
thus tend to early exclusion, and subsequent transformation. We may
rationally connect marine habitat with small eggs, poorly supplied with
yolk, segmenting regularly, and producing larva which develop with
metamorphosis.

In fresh waters dispersal can seldom be very effective. The area is
usually small, and communicates with other freshwater basins only
through the sea. Migration to a considerable distance is usually
impossible, and migration to a trifling distance useless. Moreover,
competition is not too severe to prevent some accumulation of food by
the parent on behalf of the family.

On land the conditions are still less favourable to larval
transformation. Very early migration is altogether impossible. Any kind
of locomotion by land implies muscles of complicated arrangement, and,
as a rule, there must be some sort of skeleton to support the weight
of the body. The larva, if turned out in a Gastræa condition would
simply perish without a struggle.[188] Nor is great precocity needful.
The terrestrial animal is commonly of complicated structure, active,
and well furnished with means of information. It can lay-by for its
offspring, and nourish them within its own body, or at least by food
stored up in the egg.

[188] The minute and early larvæ of _Tœnia_ and _Distomum_ may appear
to contradict this statement. They really inhabit the film of water
which spreads over wet grass, though they are capable of enduring dry
conditions for a short time, like Rotifers and many Infusoria.

The influence of habitat upon development may be recapitulated as
follows:--

MARINE HABITAT.--Eggs many. Yolk small. Segmentation often regular.
Young hatched early. Development with metamorphosis. [The most
conspicuous exceptions are Cephalopoda and marine Vertebrata.]

FLUVIATILE HABITAT.--Eggs fewer. Yolk larger. Segmentation often
unequal. Young hatched later. Development direct, or with late
metamorphosis only. [The most obvious exceptions are Frogs and Toads,
which develop with metamorphosis.]

TERRESTRIAL HABITAT.--Eggs few. Yolk large [except where the young are
supplied by maternal blood]. Segmentation often partial. Young hatched
late. Development without metamorphosis. [An exception is found in
Insects, which usually exhibit conspicuous metamorphosis, though the
yolk is large, and the type of segmentation partial or unequal.]

Let us now take up the exceptions, and see whether these are capable of
satisfactory explanation.

1.--Cephalopoda and marine Vertebrates, unlike other inhabitants of
the sea, develop without metamorphosis. But these are large animals of
relatively high intelligence, well able to feed and protect their young
until development is completely accomplished.

2.--Frogs and Toads, unlike other fluviatile animals, develop with
metamorphosis. The last and most conspicuous change, however, from
the gill-bearing and tailed tadpole to the air-breathing and tailless
frog, hardly belongs to the ordinary period of embryonic development.
When the tadpole has four limbs and a long tail it has already
reached the point at which the more primitive Amphibia (_Menopoma_,
_Proteus_, &c.) become sexually mature. The loss of the tail, the
lengthening of the hind limbs, and the complete adaptation to pulmonary
respiration, relate to the mode of dispersal of the adult. Cut off
from early dispersal by the isolation of their breeding-places and
the difficulty of land migration, Frogs migrate from pool to pool as
full-grown animals. The eggs are thus laid in new sites, and very small
basins--ditches and pools which dry up in summer--can be used for
spawning. To this peculiar facility in finding new spawning grounds
the Anura no doubt owe their success in life, of which the vast number
of nearly-allied species furnishes an incontrovertible proof. But the
adaptation to terrestrial locomotion necessarily comes late in life,
after the normal and primitive adult Amphibian condition has been
attained. It is by a _secondary adult metamorphosis_ that the aquatic
tadpole turns into the land-traversing frog. The change is not fairly
comparable to any process of development by which other animals gain
the adult structure characteristic of their class and order, but (in
respect of the time of its occurrence) resembles the late assumption of
secondary sexual characters, such as the antlers of the stag, or the
train of the peacock.

3.--Lastly, we come to the exceptional case of Insects which, unlike
other terrestrial animals, develop with metamorphosis. The Anurous
Amphibia have prepared us to recognise this too as a case of secondary
adult (post-embryonic) metamorphosis. Thysanuran or Orthopterous larvæ
cannot differ very widely from the adult form of primitive Insects.
From wingless, hexapod Insects, like Cockroach larvæ in all essentials
of external form, have been derived, on the one hand, the winged imago,
adapted in the more specialised orders to a brief pairing season
exclusively spent in migration and propagation; on the other hand, the
footless maggot or quiescent pupa.

Insects, like Frogs, disperse as adults, because of the difficulty
of the medium, aerial locomotion being even more difficult than
locomotion by land, and implying the highest muscular and respiratory
efficiency. The flying state is attained by a late metamorphosis, which
has not yet become universal in the class, while it is not found in
other Tracheates at all. _Peripatus_, Scorpions, and Myriopods become
sexually mature when they reach the stage which corresponds to the
ordinary less-modified Insect-nymph, with segmented body, walking legs,
and mouth-parts resembling those of the parent.[189]

[189] It is possible that the curious cases of agamogenetic
reproduction of the larvæ of _Aphis_, _Cecidomyia_, and _Chironomus_
are vestiges of the original fertility of Insect larvæ.

The Caterpillar is not, as Harvey[190] maintained, a kind of walking
egg; it is rather the primitive adult Tracheate modified in accordance
with its own special needs. It may be sexually immature, imperfect,
destined to attain more elaborate development in a following stage, but
it nevertheless marks the stage in which the remote Tracheate ancestor
attained complete maturity. Where it differs from the primitive form,
hatched with all the characters of the adult, the changes are adaptive
and secondary.[191]

[190] “Alia vero semen adhuc imperfectum et immaturatum recludunt,
incrementum et perfectionem, sive maturitatem, soris acquisiturum;
ut plurima genera piscium, ranæ, item mollia, crustata, testacea, et
cochleæ: quorum ova primum exposita sunt, veluti origines duntaxat,
inceptiones et vitelli; qui postea albumina sibi ipsis circum
circa induunt; tandemque alimentum sibi attrahentes, concoquentes
et apponentes, in perfectum semen atque ovum evadunt. Talia sunt
insectorum semina (vermes ab Aristotele dicta) quæ initio imperfecte
edita sibi victum quærunt indeque nutriuntur et augentur, de eruca
in aureliam; de ovo imperfecto in perfectum ovum et semen.”--_De
generatione_, Exc. II., p. 183 (1666). Viallanes justifies this view
by applying it to the histolysis and regeneration of the tissues in
Diptera. But these remarkable changes are surely secondary, adaptive,
and peculiar, like the footless maggot itself, whose conversion into
a swift-flying imago renders necessary so complete a reconstruction.

[191] The reader is recommended to refer to Fritz Müller’s Facts and
Arguments for Darwin, especially chap. xi.; to Balfour’s Embryology,
Vol. II., chap. xiii., sect. ii.; and to Lubbock’s Origin and
Metamorphoses of Insects.

_The Genealogy of Insects._

To construct from embryological and other data a chart of the descent
of Insects, and of the different orders within the class, is an
attempt too hazardous for a student’s text-book.[192] A review of
the facts of Arthropod development led Balfour[193] to conclude that
the whole of the Arthropoda cannot be united in a common phylum. The
Tracheata are probably “descended from a terrestrial Annelidan type
related to _Peripatus_.... The Crustacea, on the other hand, are
clearly descended from a Phyllopod-like ancestor, which can be in no
way related to _Peripatus_.” The resemblances between the Arthropoda
appear therefore to be traceable to no nearer common ancestors than
some unknown Annelid, probably marine, and furnished with a chitinous
cuticle, an œsophageal nervous ring, and perhaps with jointed
appendages. Zoological convenience must give place to the results of
embryological and historical research, and we shall probably have
to reassign the classes hitherto grouped under the easily defined
sub-kingdom of Arthropoda.

[192] Those who care to see a bold experiment of this kind may refer
to Haeckel’s Schöpfungsgeschichte.

[193] Comp. Embryology, Vol. I., p. 451.

Sir John Lubbock has explained, in his very interesting treatise on
the Origin and Metamorphoses of Insects, the reasons which lead him
to conclude “that Insects generally are descended from ancestors
resembling the existing genus _Campodea_ [sub-order Collembola],
and that these again have arisen from others belonging to a type
represented more or less closely by the existing genus _Lindia_” [a
non-ciliated Rotifer].

Present knowledge does not, therefore, justify a more definite
statement of the genealogy of Insects than this, that in common with
Crustacea they had Annelid ancestors, and that _Lindia_, _Peripatus_,
and _Campodea_ approximately represent three successive stages of the
descent. When we reflect that Cockroaches themselves reach back to
the immeasurably distant palæozoic epoch, we get some misty notion of
the antiquity and duration of those still remoter ages during which
Tracheates, and afterwards Insects, slowly established themselves as
new and distinct groups of animals.

CHAPTER XI.

THE COCKROACH OF THE PAST.

By S. H. SCUDDER, of the U.S. Geological Survey.

_SPECIAL REFERENCES._

BERENDT, G. C. Mémoire pour servir à l’histoire des Blattes
antédiluviennes (Ann. Soc. Entom., France, V.). Paris, 1836. 8vo.

BRODIE, P. B. A History of the Fossil Insects in the Secondary
Rocks of England. London, 1845. 8vo.

GEINITZ, F. E. Die Blattinen aus der unteren Dyas von Weissig (Nova
Acta. Acad. Leop.-Carol., XLI.). Halle, 1880. 4to.

GERMAR, E. F., und BERENDT, G. C. Die im Bernstein befindlichen
Hemipteren und Orthopteren der Vorwelt. Berlin, 1856. Fol.

GOLDENBERG, F. Zur Kenntniss der Fossilen Insekten in der
Steinkohlenformation (Neues Jahrb. Miner). Stuttgart, 1869. 8vo.

---- Fauna Saræpontana Fossilis. Heft 1–2, Saarbrücken, 1873, 1877.
4to.

HEER, O. Ueber die fossilen Kakerlaken (Viertelj. Naturf. Ges.,
Zürich, IX.). Zürich, 1864. 8vo.

KLIVER, M. Ueber einige Blattarien ... aus der Saarbrücker
Steinkohlenformation (Palæontogr. XXIX.). Cassel, 1883. 4to.

KUSTA, J. Ueber enige neue Böhmische Blattinen (Sitzungsb. böhm.
Ges. Wissensch, 1883). Prag. 8vo.

SCUDDER, S. H. Palæozoic Cockroaches (Mem. Bost. Soc. Nat. Hist.,
III.). Boston, 1879. 4to.

---- The Species of Mylacris (_Ibid_). Boston, 1884. 4to.

---- A Review of Mesozoic Cockroaches (_Ibid_). Boston, 1886. 4to.

---- Triassic Insects from the Rocky Mountains (Amer. Journ. Sc.
Arts[3], XXVIII.). New Haven, 1884. 8vo.

---- Systematische Uebersicht der fossilen Myriopoden, Arachnoideen
und Insekten (Zittel, Handb. Palæont. I. Abth., Bd. II.). München,
1885. 8vo.

WESTWOOD, J. O. Contributions to Fossil Entomology (Quart. Journ.
Geol. Soc., Lond., X.). London, 1854. 8vo.

Like all useful scavengers, the Cockroach is looked upon nowadays as
an unmitigated pest. It has, however, a certain right to our regard,
for it comes of a venerable antiquity. Indeed, palæontologically
considered, no Insect is so interesting as the Cockroach. Of no other
type of Insects can it be said that it occurs at every horizon where
Insects have been found in any numbers; in no group whatever can the
changes wrought by time be so carefully and completely studied as
here; none other has furnished more important evidence concerning
the phylogeny of Insects. Even the oldest known air-breathing animal
has been claimed (though I think erroneously) as a Cockroach; yet,
however that may be, it is certain that in the most ancient deposits
which have yielded any abundance of Insect remains, the Coal Measures,
they so far outnumber all other types of Insects, that this period,
as far as its hexapodal fauna is concerned, may fairly be called
the _Age of Cockroaches_. And though the subsequent periods show an
ever-diminishing percentage of this family when compared with the
total synchronous Insect fauna, yet the existing species are counted
by hundreds, and the fecundity of some, attested by every housewife,
may be looked upon as a sufficient explanation of the persistence of
this antique type. The Cockroach is, therefore, a very aristocrat among
Insects.

Our knowledge of its past is derived almost entirely from its wings;
perhaps because these organs are the farthest removed from the
nourishing fluids of the body, which on death become one of the agents,
or at least the media, of putrefaction and consequent obliteration. At
all events, whatever the cause, these chitinous membranes, with their
network of supporting rods, and even not infrequently with the minutest
reticulation of the membrane itself, are preserved with extraordinary
fidelity, and in such abundance that, by comparison with similar parts
in existing forms, we may reach some general conclusions concerning the
life of the past of no little interest.

The first thing that would strike an observer, looking at the ancient
Cockroaches, would be their _general_ resemblance to the living.
Excepting for their usually larger size,[194] were we to have the
oldest known Cockroaches in our kitchens to-day, the householder would
take no special note of them--unless, indeed, the transparency of their
wings (shortly to be mentioned) were to give them a somewhat peculiar
aspect. There would be the same rounded pronotal shield, the same
overlapping wings, coursed by branching veins, the same smooth curves
and oval flattened form of the whole creature, and doubtless also
the same scurrying movements. Indeed, some accurate observers--so, I
suppose, we must call them--have failed to take note of some important
and very general distinctions between the living and the dead. Thus
Gerstaecker, in a work begun twenty years ago, and not yet finished,
said, near its beginning,[195] “Not a single species of Insect has
yet been found in the Carboniferous rocks which does not fall, on
closer examination (_mit voller Evidenz_), not only in an existing
order, but even almost completely in the same family as some living
form, and only presents striking distinctions when compared with the
species themselves.” He further specifies the Cockroaches described
from the Coal Measures, by Germar and Goldenberg, as agreeing in every
distinguishing family characteristic with those of the present day.

[194] Yet none were so large as our largest living forms; their
average size was very nearly that of _Periplaneta americana_.

[195] Die Klassen und Ordnungen der Arthropoden. Leipzig, 8vo, p. 292.

In one sense, indeed, this is true. We separate the living Cockroaches
from other kinds of Orthoptera as a “family” group, and “Cockroaches”
have existed since the Coal Measures at least; yet the structure of
every one of the older types is really so peculiar that none of them
can be brought within the limits of the family as it now exists. We
recognise ours, indeed, as the direct descendants of the ancient forms,
but so changed in structure as to form a distinct group. A parallel
case is found in the Walking-sticks, and is even more obvious. The
recent researches of M. Charles Brongniart have brought to view a
whole series of forms in Carboniferous times, which are manifestly
the progenitors of living Walking-sticks, with their remarkably long
and slender stick-like body, attenuated legs, and peculiar appendages
at the tip of the abdomen. Existing forms are either wingless or else
have opaque elytron-like front wings, and very ample, gauzy, fan-like
hind wings; while the Carboniferous species are furnished with four
membranous wings, almost precisely alike, and so utterly different from
those of existing types that, before the discovery of the bodies, these
wings were universally classed as the wings of Neuropterous Insects
(sensu Linneano). Thus Gerstaecker, in the very place already quoted,
says of these same wings, known under the generic name _Dictyoneura_,
that they show at least a very close relationship to the _Ephemeridæ_
of to-day.

One principal difference here alluded to--the exact resemblance,
except in minor details, of the front and hind wings, and, as
consequent therewith, _equal diaphaneity in both_--is found indeed
in all palæozoic insects, with exceedingly few exceptions;[196] it
is one of their most characteristic and pervading peculiarities.
It marks one phase of the movement in all life from homogeneity to
heterogeneity--from the uniform to the diverse. In the Cockroaches of
to-day a few are found in which the tegmina are nearly as diaphanous
as the hind wings; but in the great mass the texture of the tegmina,
as in Orthoptera generally (excepting most Gryllides), is decidedly
coriaceous; and in some, _e.g._, _Phoraspis_, the veins are nearly
obliterated in the thickness and opacity of the membrane, so as to
resemble many Coleopterous elytra.

[196] A few elytra of Coleoptera are recently announced from the
Silesian “culm.”

Three principal differences have been noticed between the ancient and
modern forms of Cockroaches. Doubtless others could be found were we
able to compare the structure of all parts of the body; and perhaps
future research and more happy discovery may yet bring them to light;
at present, however, we are compelled to restrict our comparisons to
the wings alone.

First, we have to remark the similarity of the front and hind wings in
the ancient types: a similarity which extends to their general form
(the extended anal area of the hind wings in modern types being as yet
only slightly differentiated); their nearly equal size (a corollary, to
a certain extent, of the last); the general course of their neuration
(true, in a limited sense only, of modern types); and the complete
transparency of the front as well as of the hind wing.

Second, the same number of principal veins is developed in the front
and hind wings of ancient Cockroaches; while in the front wings of
modern types two or more of the veins are blended, so as to reduce the
number of the principal stems below the normal, the hind wing at the
same time retaining its original simplicity. These principal veins
are six, counting the marginal vein, which here merely thickens the
anterior border, as one; to use the terminology of Heer, and starting
from the anterior margin, they are the _marginal_, _mediastinal_,
_scapular_, _externomedian_, _internomedian_, and _anal_. The general
disposition of these veins is as follows:--The mediastinal and scapular
veins, with their branches, which are superior (_i.e._, part from the
main vein on the upper or anterior side), terminate upon the anterior
margin. The internomedian and anal take the opposite course, and their
branches are inferior, or, at least, directed toward the inner margin;
while the externomedian, interposed between these two sets, terminates
at the tip of the wing, and branches indifferently on either side.

[Illustration: Fig. 119.--Schematic view of Wing of Palæozoic
Cockroach, showing the veins and areas.]

Now these veins are all present in both front and hind wings of
palæozoic Cockroaches, and also in the hind wings of existing species;
but in the front wings or tegmina of the latter the number is never
complete, the externomedian vein being always amalgamated either with
the scapular, or with the internomedian, and the mediastinal frequently
blended with the scapular vein.

The hind wings are thus shown to be conservative elements of structure,
since they have preserved from the highest antiquity both their
transparency and their normal number of veins. They have retained the
use to which they were first put, and the changes that have come about,
such as the wider expansion of the anal area, have been in fuller
development of the same purpose; while the front wings, in virtue of
their position in repose, have become more and more protectors of the
hind wings, and have gradually lost, in part, if not entirely, their
original use. The hind wings of existing Insects, thus protected,
have given less play to selective action, and have become to some
degree interpreters for us of the more complicated structure, the more
modernised anatomy, the more varied organisation of the front wing.

A third distinction between palæozoic and modern Cockroaches is found
in the veinlets of the anal area. These, unlike the branches of the
other veins, do not part from the main anal vein at various points
along its course, but form a series of semi-independent veinlets, and
in palæozoic Cockroaches take the same general course as the main anal
vein, or “anal furrow” (the curved, deeply sunken vein that marks
off the anal area from the rest of the front wing, both in ancient
and modern Cockroaches), and terminate at sub-equidistant intervals
upon the inner margin; while in modern Cockroaches these veins either
run sub-parallel to the inner margin and terminate on the descending
portion of the anal furrow, or they form a fusiform bundle and
terminate in proximity to one another and to the tip of the anal furrow.

These differences, which were mentioned by Germar and Goldenberg,
and their universality pointed out in my memoir on Palæozoic
Cockroaches,[197] seem to warrant our separating the older forms from
the modern as a family group, under the name of _Palæoblattariæ_; this
family has been thus characterised:--

[197] Memoirs Bost. Soc. Nat. Hist., III., 23 seq. (1880).

Fore wings diaphanous, generally reticulated, and nearly symmetrical on
either side of a median line. Externomedian vein completely developed,
forking in the outer half of the wing, its branches generally occupying
the apical margin; internomedian area broad at base (beyond the anal
area), rapidly tapering apically, and filled with oblique mostly
parallel veins, having nearly the same direction as the anal veinlets,
which, like them, strike the inner margin.

About eighty palæozoic species have been published up to the present
time, and have been grouped in two sub-families and thirteen genera.
Besides these, Brongniart has not yet given any hint of how many have
been found at Commentry, a French locality which may be expected to
increase the number largely, and about twenty undescribed species are
known to me from the American Carboniferous rocks.

[Illustration: Fig. 120.--_Etoblattina mazona_ Scudd. × 3. (The outline
of natural size.) Carboniferous, Illinois.]

The two tribes or sub-families differ in the structure of the
mediastinal vein; in one type (_Blattinariæ_) the branches part from
the main stem as in the other veins, at varying distances along its
course (see the figure of _Etoblattina_); in the other (_Mylacridæ_)
they spread like unequal rays of a fan from the very base of the wing
(see the figure of _Mylacris_). What is curious is that the latter
type has been found only in the New World, while the former is common
to Europe and America. The latter appears to be the more archaic type,
since it is probable that the primeval Insect wing was broad at the
base, as is the general rule in palæozoic wings, and had the veins
somewhat symmetrically disposed on either side of a middle line; in
this case the mediastinal and anal areas would be somewhat similar and
more or less triangular in form, and the space they occupied would be
most readily filled by radiating veins; such a condition of things,
which we find in the _Mylacridæ_, would naturally precede one in which
the mediastinal vein, to strengthen the part of the wing most liable
to strain, should, as in the _Blattinariæ_, follow the basal curve of
the costal margin, and throw its branches off at intervals toward the
border, much after the fashion of the mediastinal vein.

[Illustration: Fig. 121.--_Mylacris anthracophilum_ Scudd. × 2.
Carboniferous, Illinois.]

This view of the relative antiquity of the two tribes of
_Palæoblattariæ_ is supported by the fact that while in both of them
the internomedian branches show a tendency to repeat the general course
of the anal nervules, as in the corresponding veins of the costal
region, this tendency is lost in modern types; and among those ancient
_Blattinariæ_, which are esteemed highest in the series, there is a
marked tendency toward a loss of this repetition of the style of
branching of the mediastinal and anal offshoots by the scapular and
internomedian respectively.

A certain amount of geological evidence may also be claimed in support
of this view. A survey of the species of the two groups found up to
the present time in America, published and unpublished, shows that all
the _Mylacridæ_ are found below the Upper Carboniferous, while more
than half the _Blattinariæ_ are found in or above it. This results
largely from a recent and as yet unpublished discovery of _Blattinariæ_
in the Upper Coal Measures of Ohio and West Virginia, which in their
general features are much nearer than previously discovered American
Cockroaches to the European _Blattinariæ_, the latter of which come
generally from Upper Carboniferous beds. The _Mylacridæ_ have therefore
been found in America in strata generally regarded as older than those
which in Europe have yielded Cockroaches, and this gives a sufficient
explanation why no _Mylacridæ_ have yet been found in the Old World.
In America one is mostly dealing with absolutely older forms, and they
naturally give that continent a more old-fashioned look, when we regard
the Carboniferous fauna as a whole. As already stated, a wing from the
French Silurian (_Palæoblattina Douvillei_ Brongn.) has been claimed as
a Cockroach, but without good reason, and to see a real old Cockroach
one must look to America.

Up to this point we have contrasted the palæozoic Cockroaches with the
existing forms only, and finding such important distinctions between
them, we naturally turn with some curiosity to the intermediate
mesozoic and tertiary formations.

Now, not only are the mesozoic species as numerous (actually, but not
relatively) as the palæozoic, but a recent discovery of a Triassic
fauna of considerable extent, in the elevated parks of Colorado,
presents us with a series of intermediate forms between those peculiar
to the Coal Measures and those characteristic of the later mesozoic
rocks. Excluding, however, for a moment this Triassic fauna, we may
say of the later mesozoic species that they are _Neoblattariæ_, not
_Palæoblattariæ_, though they still show some lingering characteristics
of their ancestry. Thus the front wings are in general of a less
dense texture than in modern times, but without the perfect
diaphaneity of the palæozoic species; in some the anal veins fall in
true palæoblattarian fashion on the inner margin, while in others
which cannot be dissociated generically from them, the anal veins
are disposed as in modern types. But in all there is a loss of one
of the principal veins, or rather an amalgamation of two or more--a
characteristic of more fundamental character. As a general rule,
moreover, to which we shall again advert, the mass of the species are
of small size, in very striking contrast to the older types.

[Illustration: Fig. 122.--_Neorthroblattina Lakesii_ Scudd. × 5. Trias,
Colorado.]

To return now to the Triassic deposits of Colorado, we recognize here
an assemblage of forms of a strictly intermediate character. Here
are _Palæoblattariæ_ and _Neoblattariæ_, side by side. The larger
proportion are _Palæoblattariæ_, but all of them are specifically,
and most of them generically, distinct from palæozoic species, and
all rank high among _Blattinariæ_; still further, the species are
all of moderate size, their general average being but little above
that of mesozoic Cockroaches, and only a little more than half that
of palæozoic types. The _Neoblattariæ_ of this Triassic deposit are
still smaller, being actually smaller than the average mesozoic
Cockroach, and one or two of them, of the genus _Neorthroblattina_
(see figure of _N. Lakesii_), have marked affinity to one of the
genera of Palæoblattariæ (_Poroblattina_) peculiar to the same beds,
differing mainly in the union or separation of the mediastinal
and scapular veins; while others, as _Scutinoblattina_, have a
_Phoraspis_-like aspect and density of membrane. This novel assemblage
of species bridges over the distinctions between the _Palæoblattariæ_
and _Neoblattariæ_. We find, first, forms in which the front wings
are diaphanous, with distinct mediastinal and scapular veins, and
the anal veinlets run to the border of the wing (_Spiloblattina_,
_Poroblattina_); next, those having a little opacity of the front
wings, with blended mediastinal and scapular, and the anal veins as
before (some species of _Neorthroblattina_); then those with still
greater opacity, with the same structural features (other species
of _Neorthroblattina_); next, those having a coriaceous or leathery
structure, blended mediastinal and scapular, and anal veins falling on
the inner margin (some species of _Scutinoblattina_); and, finally,
similarly thickened wings with blended mediastinal and scapular,
and anal veins impinging on the anal furrow (other species of
_Scutinoblattina_).

It is not alone, however, by the union of the mediastinal and scapular
stems that the reduction of the veins in the wings of later Cockroaches
has come about; for in many mesozoic types the externomedian vein is
blended with one of its neighbours, while in others not only are the
mediastinal and scapular united, but at the same time the externomedian
and internomedian.

As regards the other structural distinction between the _Palæoblattariæ
and Neoblattariæ_--the course of the anal nervules--there is much
diversity, and very imperfect knowledge, since this very portion of
the wing is not infrequently lost, a fracture most readily occurring
at the anal furrow. In most of the mesozoic genera, the anal nervules,
as far as known, strike the margin; but the larger portion of these
show a decided tendency to trend toward the tip of the anal furrow,
as in many modern forms. This feature can hardly be considered
as firmly established in mesozoic times, and the same genus, as
_Scutinoblattina_, may contain species which differ in this respect.

A further peculiarity of mesozoic Cockroaches, already alluded to, is
their generally small size. The average length of the front wing of
palæozoic Cockroaches has been estimated to be 26 mm., that of the
Triassic _Palæoblattariæ_ is about 16 mm., while that of the mesozoic
_Neoblattariæ_ is 12·5 mm. One exception to this small size must be
noted in the species from the Jura of Solenhofen, all of which were
large and some gigantic, one wing reaching the length of 60 mm., or
about the size of our largest tropical _Blaberæ_. If we omit these
exceptional forms, the average length of the wing of the mesozoic
Cockroach would be scarcely more than 11 mm. Now an average of the
243 species of which the measurements are given in Brunner’s Système
des Blattaires (1865), gives the length of the front wing of living
Cockroaches as a little over 18 mm.; so that the mesozoic Cockroaches
were as a rule considerably smaller, the palæozoic Cockroaches much
larger, than the living.

[Illustration: Fig. 123.--_Mesoblattina Brodiei_ Scudd. × 4. Purbecks,
England.]

Nearly eighty species of mesozoic _Neoblattariæ_ are known, and they
are divided into thirteen genera,[198] one of which, _Mesoblattina_
(see figure of _M. Brodiei_), contains upwards of twenty species,
mainly from the Lias and Oolites of England. The Upper Oolite has
proved the most prolific, considerably more than half the species
having been found in the English Purbecks, while nearly a fourth occur
in the Lias of England, Switzerland, and Germany. Many of the English
species have been figured in Brodie’s Fossil Insects of the Secondary
Rocks of England, in Westwood’s paper on Fossil Insects in the tenth
volume of the Quarterly Journal of the Geological Society, and in
the memoir alluded to above. No species has yet been found in rocks
of different geological horizons, and the genera of the Trias are
peculiar to it. So, too, are some of the genera of the Oolite, but all
of the Liassic genera occur also in the Oolite.

[198] See a paper on mesozoic Cockroaches now printing in the Memoirs
Bost. Soc. Nat. Hist., Vol. III., p. 439 seq.

Among these mesozoic Cockroaches are some of very peculiar aspect; one,
_Blattidium_ (see figure of _B. Simyrus_), found only in the lower
Purbecks, has ribbon-shaped wings with parallel sides, longitudinal
neuration, and anal nervures with a course at right angles to their
usual direction; another, _Pterinoblattina_ (see figure of _P.
intermixta_), geologically widespread, is very broad, more or less
triangular, and has an exceedingly fine and delicate neuration, so
arranged as to resemble the barbs of a feather.

[Illustration: Fig. 124.--_Blattidium Simyrus_ Westw. × 3. Lower
Purbecks, England.]

A comparison of the neuration of the tegmina of mesozoic and recent
Cockroaches, to determine as far as possible the immediate relations
of the former to existing types, gives as yet little satisfaction.
The prolific genera, _Mesoblattina_ and _Rithma_, may be said to bear
considerable resemblance to the _Phyllodromidæ_, and the peculiar
neuration of _Elisama_ is in part repeated in the _Panchloridæ_, as
well as in some _Phyllodromidæ_ and _Epilampridæ_. _Scutinoblattina_
also reminds one in certain features of some _Epilampridæ_, like
_Phoraspis_. The other genera appear to have no special relations to
any existing type. As a whole, it would appear as if the _Blattariæ
spinosæ_ approached closer to the mesozoic forms than do the _Blattariæ
muticæ_.

[Illustration: Fig. 125.--_Pterinoblattina intermixta_ Scudd. × 4.
Upper Lias, England.]

As to the tertiary Cockroaches we know very little, exceedingly few
having been preserved, even in amber--that wonderful treasury of fossil
Insects. Here first we come across apterous forms, _Polyzosteria_
having been recognised in Prussian amber,[199] together with winged
species, which seem to be _Phyllodromidæ_; these are the only
_Blattariæ spinosæ_ known from the Tertiaries. Of the other group,
we have _Zetobora_, one of the _Panchloridæ_, and _Paralatindia_,
one of the _Corydidæ_, from American rocks, and _Heterogamia_ and
_Homœogamia_, one from Parschlug in Steiermark, the other from
Florissant in Colorado, belonging to the sub-family _Heterogamidæ_.
Others are mentioned, generally under the wide generic term _Blatta_,
from Oeningen, Eisleben, Rott, and even from Spitzbergen and Greenland;
but little more than their names are known to us. _Paralatindia_, from
the Green River beds of Wyoming, U.S., is the only tertiary Cockroach
yet referred to an extinct genus; but close attention has not yet been
paid even to the few tertiary Cockroaches which we know. There is no
reason to suppose that they will be found to differ more from the
existing types than is generally the case with other Insects. The more
we learn of cænozoic Insects, the more truly do we find that the early
Tertiary period was in truth the dawn of the present, the distinction
between the faunas of these remotely separated times (though not to
be compared in character) being scarcely greater than is found to-day
between the Insects of the temperate and torrid zones.

[199] The wingless creature from the Carboniferous deposits of
Saarbrücken, described by Goldenberg as a Cockroach, under the name
of _Polyzosterites granosus_, appears to be a Crustacean.

We began this review with the statement that no Insect was so important
palæontologically as the Cockroach. This would more clearly appear had
we space to pass in review the geological history of all the Insect
tribes; for then it could be shown that it was only in the passage from
palæozoic to mesozoic times that the great ordinal groups of Insects
were differentiated, and that the Triassic period therefore becomes the
expectant ground of the student of fossil Insects. Up to the present
time we do not know half a dozen Insects besides Cockroaches from
these rocks. Yet, notwithstanding this advantage on the part of the
Cockroaches, how meagre is the history, how striking the “imperfection
of the geological record” concerning them, the following tabulation of
the fossil species by their genera will show.

It here appears that there are about 80 species known from the
palæozoic rocks, two or three more than that from the mesozoic, and
only nine from the cænozoic! When we call to mind that half the
palæozoic Insects were Cockroaches, and that seven or eight hundred
species exist to-day, what shall we say of the paltry dozen[200] from
the rich tertiaries? Shall we claim that these figures represent
their true numerical proportion to their numbers in the more distant
past? Then, indeed, must the palæozoic period have been the Age of
Cockroaches; for all research into the past shows that a type once
losing ground continues to lose it, and does not again regain its
strength. The Cockroaches of to-day are no longer, as once, a dominant
group; they are but a fragment of the world’s Insect-hosts; yet even
now the species are numbered by hundreds. If this be a waning type,
what must its numbers have been in the far-off time, when the warm
moisture which they still love was the prevailing climatic feature of
the world; and how few of that vast horde have been preserved to us!
The housekeeper will thank God and take courage.

[200] This includes all possible forms; our table shows but nine.

GEOLOGICAL DISTRIBUTION OF FOSSIL COCKROACHES.

Figures in _italics_ represent the number of American species; in
roman, of European.

+---------------------+-------------------+----+----+----+----+----+----+
| |Carboniferous.| | | | U | O | M | |
| | L | M | U | P | T | L | p | l | i | T |
| | o | i | p | e | r | i | p | i | o | O |
| | w | d | p | r | i | a | e | g | c | T |
| | e | d | e | m | a | s. | r | o | e | A |
| | r. | l | r. | i | s. | | J | c | n | L |
| | | e. | | a | | | u | e | e. | S. |
| | | | | n. | | | r | n | | |
| | | | | | | | a. | e. | | |
+---------------------+----+----+----+----+----+----+----+----+----+----+
|PALÆOBLATTARIÆ. | | | | | | | | | | |
| _Mylacridæ_-- | | | | | | | | | | |
| Mylacris |_10_| .. | .. | .. | .. | .. | .. | .. | .. | 10 |
| Promylacris | _1_| .. | .. | .. | .. | .. | .. | .. | .. | 1 |
| Paromylacris | _1_| .. | .. | .. | .. | .. | .. | .. | .. | 1 |
| Lithomylacris | _2_| _2_| .. | .. | .. | .. | .. | .. | .. | 4 |
| Necymylacris | _2_| .. | .. | .. | .. | .. | .. | .. | .. | 2 |
| _Blattinariæ_-- | | | | | | | | | | |
| Etoblattina | _1_| 1 |15+ | 3+ | | .. | .. | .. | .. |} |
| | .. | .. | _6_| _1_| _1_| .. | .. | .. | .. |}28 |
| Spiloblattina | .. | .. | .. | .. | _4_| .. | .. | .. | .. | 4 |
| Archimylacris | _3_| .. | .. | .. | .. | .. | .. | .. | .. | 3 |
| Anthracoblattina | .. | 2 | 6 | 4 | _1_| .. | .. | .. | .. | 13 |
| Gerablattina | _1_| 1 | 10 | .. | .. | .. | .. | .. | .. | 12 |
| Hermatoblattina | .. | .. | 1 | 1 | .. | .. | .. | .. | .. | 2 |
| Progonoblattina | .. | .. | 2 | .. | .. | .. | .. | .. | .. | 2 |
| Oryctoblattina | _1_| .. | 1 | 1 | .. | .. | .. | .. | .. | 3 |
| Petrablattina | _1_| .. | .. | 1 | _2_| .. | .. | .. | .. | 4 |
| Poroblattina | .. | .. | .. | .. | _2_| .. | .. | .. | .. | 2 |
| |(23)| (6)|(41)|(11)|(10)| | | | |(91)|
|NEOBLATTARIÆ. | | | | | | | | | | |
|N t | | | | | | | | | | |
|o o {Ctenoblattina | .. | .. | .. | .. | .. | 1 | 2 | .. | .. | 3 |
|t {Neorthroblattina| .. | .. | .. | .. | _4_| .. | .. | .. | .. | 4 |
| s {Rithma | .. | .. | .. | .. | .. | 2 | 10 | .. | .. | 12 |
|y u {Mesoblattina | .. | .. | .. | .. | .. | 7 | 15 | .. | .. | 22 |
|e b {Elisama | .. | .. | .. | .. | .. | 1 | 5 | .. | .. | 6 |
|t - {Pterinoblattina | .. | .. | .. | .. | .. | 3 | 6 | .. | .. | 9 |
| f {Blattidium | .. | .. | .. | .. | .. | .. | 2 | .. | .. | 2 |
|r a {Nannoblattina | .. | .. | .. | .. | .. | .. | 3 | .. | .. | 3 |
|e m {Dipluroblattina | .. | .. | .. | .. | .. | .. | 1 | .. | .. | 1 |
|f i {Diechoblattina | .. | .. | .. | .. | .. | .. | 2 | .. | .. | 2 |
|e l {Scutinoblattina | .. | .. | .. | .. | _3_| .. | .. | .. | .. | 3 |
|r i {Legnophora | .. | .. | .. | .. | 1 | .. | .. | .. | .. | 1 |
|r e {Aporoblattina | .. | .. | .. | .. | .. | 3 | 6 | .. | .. | 9 |
|e s | | | | | (8)|(17)|(52)| | |(77)|
|d . | | | | | | | | | | |
| | | | | | | | | | | |
| _Phyllodromidæ_-- | | | | | | | | | | |
| “Blatta” | .. | .. | .. | .. | .. | .. | .. | 3 | .. | 3 |
| _Periplanetidæ_-- | | | | | | | | | | |
| Polyzosteria | .. | .. | .. | .. | .. | .. | .. | 2 | .. | 2 |
| _Panchloridæ_-- | | | | | | | | | | |
| Zetobora | .. | .. | .. | .. | .. | .. | .. | _1_| .. | 1 |
| _Corydidæ_-- | | | | | | | | | | |
| Paralatindia | .. | .. | .. | .. | .. | .. | .. | _1_| .. | 1 |
| _Heterogamidæ_-- | | | | | | | | | | |
| Homœogamia | .. | .. | .. | .. | .. | .. | .. | _1_| .. | 1 |
| Heterogamia | .. | .. | .. | .. | .. | .. | .. | .. | 1 | 1 |
| | | | | | | | | (8)| (1)| (9)|
+---------------------+----+----+----+----+----+----+----+----+----+----+
| GRAND TOTALS | 23 | 6 | 41 | 11 | 18 | 17 | 52 | 8 | 1 |177 |
+---------------------+----+----+----+----+----+----+----+----+----+----+

Samuel H. Scudder.

APPENDIX.

PARASITES OF THE COCKROACH.

_Spirillum, sp._ [Vibrio]. SCHIZOMYCETES.

Rectum.

_Ref._--Bütschli, Zeits. f. wiss. Zool., Bd. XXI., p. 254 (1871).

_Hygrocrocis intestinalis_, Val. CYANOPHYCEÆ.

Filaments of a very minute Alga abound in the rectum of the
Cockroach, where this species is said by Valentin to occur. The
intestine of the Crayfish is given as another habitat. Leidy
observes that the filaments which he found in the rectum of the
Cockroach are inarticulate, and do not agree with Valentin’s
description of the species.

_Ref._--Valentin, Repert. f. Anat. u. Phys., Bd. I., p. 110 (1836);
Robin, Végét. qui croissent sur l’Homme, p. 82 (1847); Leidy,
Smithsonian Contr., Vol. V., p. 41 (1853); Bütschli, Zeits. f.
wiss. Zool., Bd. XXI., p. 254 (1871).

_Endamœba (Amœba) Blattæ_, Bütschli. RHIZOPODA.

Rectum.

_Ref._--Siebold, Naturg. wirbelloser Thiere (1839) _fide_ Stein;
Stein, Organismus d. Infusions-thiere, Bd. II., p. 345 (1867);
Bütschli, Zeits. f. wiss. Zool., Bd. XXX., p. 273, pl. xv. (1878);
Leidy, Proc. Acad. N. S. Phil., Oct. 7th, 1879, and Freshwater
Rhizopods of N. America, p. 300 (1879).

_Gregarina (Clepsidrina) Blattarum_, Sieb. GREGARINIDA.

Encysted in chylific stomach and gizzard; free in large intestine.

_Ref._--Siebold, Naturg. wirbelloser Thiere, pp. 56, 71 (1839);
Stein, Müll. Arch., 1848, p. 182, pl. ix., figs. 38, 39; Leidy,
Trans. Amer. Phil. Soc., Vol. X., p. 239 (1852); Bütschli, Zeits.
f. wiss. Zool., Bd. XXI., p. 254 (1871), and Bd. XXXV., p. 384
(1881); Schneider, Grégarines des Invertébrés, p. 92, pl. xvii.,
figs. 11, 12 (1876).

_Nyctotherus ovalis_, Leidy. INFUSORIA.

Small and large intestines.

_Ref._--Leidy, Trans. Amer. Phil. Soc., Vol. X., p. 244, pl. xi.
(1852).

_Plagiotoma (Bursaria) blattarum_, Stein. INFUSORIA.

Rectum.

_Ref._--Stein, Sitzb. d. königl. Böhm. Ges., 1860, pp. 49, 50.

_Lophomonas Blattarum_, Stein. INFUSORIA.

Rectum.

_Ref._--Stein (loc. cit.); Bütschli, Zeits. f. wiss. Zool., Bd.
XXX., p. 258, plates xiii., xv. (1878).

_L. striata_, Bütschli. INFUSORIA.

Rectum.

_Ref._--Bütschli, Zeits. f. wiss. Zool., Bd. XXX., p. 261, plates
xiii., xv. (1878).

_Gordius_, sp. NEMATELMINTHA.

Specimens in the Museum at Hamburg, from Venezuela. Obtained from
some species of Cockroach.

_Oxyuris Diesingi_, Ham. NEMATELMINTHA.

Rectum, frequent.

_Ref._--Hammerschmidt, Isis, 1838; Bütschli, Zeits. f. wiss. Zool.,
Bd. XXI., p. 252, pl. xxi. (1871).

_O. Blattæ orientalis_, Ham. NEMATELMINTHA.

Rectum (much rarer than _O. Diesingi_).

_Ref._--Hammerschmidt (_loc. cit._); Bütschli, Zeits. f. wiss.
Zool., Bd. XXI., p. 252, pl. xxii. (1871).

Other species of _Oxyuris_ are said to occur in the same situation,

_e.g._, _O. gracilis_ and _O. appendiculata_ (Leidy, Proc. Acad.
N. S. Phil., Oct. 7th, 1879), and _O. macroura_ (Radkewisch, quoted
by Van Beneden in Animal Parasites, Engl. trans., p. 248).

_Filaria rhytipleurites._ NEMATELMINTHA.

Encysted in the fat-body of the Cockroach; sexual state in the
alimentary canal of the Rat. _Spiroptera obtusa_ is similarly
shared by the Meal-worm (larva of _Tenebrio molitor_) and the Mouse.

_Ref._--Galeb, Compt. Rend., July 8th, 1878.

_Acarus_, sp. ARACHNIDA.

Found by Cornelius upon the sexual organs of a male Cockroach.

_Ref._--Cornelius, Beitr. zur nähern Kenntniss von _Periplaneta
orientalis_, p. 35, fig. 23 (1853).

_Evania appendigaster_, L. INSECTA (_Hymenoptera_).

A genus of Ichneumons, parasitic upon _Periplaneta_ and _Blatta_.

_Ref._--Westwood, Trans. Ent. Soc., Vol. III., p. 237; Ib., Ser.
II., Vol. I., p. 213.

_Symbius Blattarum_, Sund. INSECTA (_Coleoptera_).

The apterous female is parasitic upon _P. americana_ and _B.
germanica_.

_Ref._--Sundevall, Isis, 1831.

* * * * *

SENSE OF SMELL IN INSECTS.

Since the printing of the sheets which describe the organs of special
sense, we have become acquainted with two experimental researches of
recent date, instituted for the purpose of determining whether other
organs, besides the antennæ, may be specially concerned with the
perception of odours by Insects.

Prof. Graber (Biol. Centralblatt, Bd. V., 1885) has described extensive
and elaborate experiments upon various Insects, tending to the
conclusion that the palps and the cerci may be sensitive to odours,
and that in special cases the palps may be even more sensitive in this
respect than the antennæ. Cockroaches, decapitated, but kept alive for
some days, were found to perceive odours by means of their cerci. His
general conclusion is that Insects have no special sense of smell,
but that various parts of the surface of the body are furnished with
nerve-endings capable of perceiving strong odours. Prof. Graber’s
results are known to us only through the abstract given by Prof.
Plateau in the paper next to be mentioned.

Prof. Plateau (Compt. rend. de la Soc. Entom. de Belgique, 1886)
relates experiments upon the powers of scent resident in different
organs of the Cockroach. Two Cockroaches had their palps (maxillary and
labial) removed; two others had the antennæ removed. An evaporating
dish, 8 inches in diameter, was then partly filled with fine sand.
In the centre of the dish was set a circular box of card, without
bottom, 2 inches in diameter, and 1-4/5 inches high. In this box bread
moistened with beer, a bait very attractive to Cockroaches, was placed,
and renewed daily. The four Cockroaches were allowed to run about in
the dish outside the box, and to feed upon the bread at pleasure by
climbing over the enclosure. Observations were made late at night
for a month, when it was found that, except on the first night, when
the Insects ran all over the dish, none of the Cockroaches without
antennæ made their way to the food; while twenty-three times one of the
Cockroaches without palps, but with antennæ intact, was found to be
feeding; in one instance, both were so found.

Plateau observes that a special sense of smell can only be claimed for
organs which are able to detect faint and distant odours, and that
experiments made with powerful odours close to the body of the Insect
may lead to fallacious results. The perception of faint odours cannot
be effected by the palps or cerci of the Cockroach, but only by the
antennæ.

THE END.

Printed by McCORQUODALE & CO, Limited, Leeds.

*** End of this LibraryBlog Digital Book "The Structure and Life-history of the Cockroach - Studies in Comparative Anatomy—III" ***

Home