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Title: Through a pocket lens
Author: Scherren, Henry
Language: English
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[Illustration:
PTYCHOPTERA PALUDOSA LIMNOBIA REPLICATA
_From enlarged photographs, made at the Yorkshire College,
Leeds, from specimens bred by the Author, and mounted by Messrs.
Watson & Son, High Holborn, London_]
THROUGH
A POCKET LENS
BY
HENRY SCHERREN, F.Z.S.
AUTHOR OF
‘PONDS AND ROCK POOLS,’ ‘A POPULAR HISTORY OF ANIMALS,’ ETC.
_WITH NINETY ILLUSTRATIONS_
THE RELIGIOUS TRACT SOCIETY
56 PATERNOSTER ROW AND 65 ST. PAUL’S CHURCHYARD
1897
Oxford
HORACE HART, PRINTER TO THE UNIVERSITY
CONTENTS
CHAPTER I.
PAGE
THE POCKET LENS, THE DISSECTING MICROSCOPE, AND SOME
SIMPLE APPLIANCES 11
CHAPTER II.
ARTHROPODS AND THEIR CLASSES.--THE MARGINED WATER
BEETLE; THE GREAT WATER BEETLE; THE COCKTAIL
BEETLE 27
CHAPTER III.
COCKROACHES; EARWIGS; THE GREAT GREEN GRASSHOPPER;
THE WATER SCORPION; THE WATER BOATMAN; CORIXA 63
CHAPTER IV.
SPIDERS, MITES, AND MYRIAPODS 96
CHAPTER V.
CRUSTACEANS.--PRAWN, SHRIMP, MYSIS, CRABS; AMPHIPODS;
ISOPODS 128
CHAPTER VI.
AQUATIC INSECT LARVAE 157
INDEX 189
LIST OF ILLUSTRATIONS
FIG. PAGE
_Ptychoptera paludosa. Limnobia replicata_ _Frontispiece_
1. Hand Magnifier and Stand 14
2. Zeiss’s Dissecting Microscope 16
3. Leitz’s Dissecting Microscope 17
4. Two Leitz Lenses in holder (open) 18
5. Two Leitz Lenses in holder (closed) 18
6. Home-made Dissecting Microscope 19
7. Beakers 21
8. Glass Capsule 21
9. Glass Block, with cover 22
10. Glass Box, with cover 22
11. Forceps 23
12. Three forms of Dipping-tube. Method of using it 24
13. Mounted Needles 25
14. Cape Peripatus (natural size) 30
15. Margined Water Beetle (male) 32
16. Shells of Molluscs broken up by Dytiscus 33
17. Outline of Dytiscus 38
18. Male Dytiscus in flight 39
19. To show fold of (right) wing of Dytiscus 40
20. To show fold of (right) wing of Dytiscus 40
21 and 21 A. Head of Dytiscus 42
22. Disposition of mouth parts 43
23. Leg of Cockroach 44
24. Tarsus of Dytiscus (magnified) 45
25. Female Dytiscus swimming 46
26. Upper surface of abdomen of typical Beetle 47
27. Spiracle of Dytiscus (magnified) 48
28. Tracheal tubes of Dytiscus (magnified) 48
29. Great Water Beetle 51
30. Female Hydrophilus constructing a cocoon. (After
Lyonnet) 55
31. Cocktail Beetle 58
32. Cockroaches 66
33. Mouth parts of a Cockroach 69
34. Cockroach, showing Spiracles 71
35. Alimentary Canal of Cockroach 73
36. American Cockroach (male) 75
37. Larva and Pupa of Earwig 77
38. Earwig (male) 78
39. Great Green Grasshopper (female) 81
40. Tibial ear of Great Green Grasshopper 85
41. Land Bug (magnified) 86
42. Water Scorpion 87
43. Organs of Water Scorpion, Egg, and Parasitic Mite.
(After Swammerdam) 90
44. Raptorial leg of Water Scorpion 92
45. Water Boatman 93
46. Water Boatman swimming 94
47. Corixa, with wings expanded 95
48. Scheme of under surface of Wolf Spider (female).
Pedipalp of male (enlarged) 98
49. Garden Spider and Web 99
50. Threads of Spider’s Web 100
51. Anchorage of Web 101
52. Foot of Garden Spider 104
53. Spinnerets of Garden Spider 104
54. Jumping Spider 106
55. Falces of Male Jumping Spider 106
56. Foot of Jumping Spider. Scopula much enlarged 108
57. Diving Spiders 109
58. Cell of Diving Spider 112
59. Red Water Mite 114
60. Larva of Water Mite 117
61. Nymph of Water Mite 117
62. Beetle Mite 119
63. _Lithobius forficatus._ Mouth parts seen from below.
(After Graber) 124
64. The Common Millepede 126
65. Segments of Millepede (magnified) 127
66. Prawn 132
67. First walking leg of Shrimp (enlarged) 134
68. Mysis, or the Opossum Shrimp 135
69. Maxillipedes and Maxilla of Shore Crab. (After
Savigny) 138
70. Stomach of Crab laid open 139
71. Gammarus. (After Sars) 142
72. Maxillipedes of _Gammarus marinus_ (magnified) 146
73. Nest-building Amphipod (from life) 148
74. Water Woodlouse 153
75. Mouth-lock. (After Burgess) 161
76. Dytiscus Larvae 162
77. Pupa of Dytiscus 164
78. Larva of _Limnobia replicata_ 167
79. Forked spine of Limnobia (enlarged) 168
80. Pupa case of Limnobia 169
81. Fore wing of Bee, showing marginal fold (×7) 170
82. Larva of _Paraponyx stratiotata_ (enlarged) 173
83. Diagram of segment of Paraponyx, showing arrangement
of tracheal gills 175
84. Gill of Paraponyx larva. (After De Geer) 176
85. Larva of Sialis (enlarged) 179
86. Diagram of Sialis larva, showing arrangement of gills 181
87. Pupa of Sialis 181
88. Larvae of _Ptychoptera paludosa_ (from life) 184
89. Ptychoptera Larva (enlarged). Tail. (After Lyonnet) 186
90. Pupa of Ptychoptera. (After Lyonnet) 187
THROUGH A POCKET LENS
CHAPTER I
THE POCKET LENS, THE DISSECTING MICROSCOPE,
AND SOME SIMPLE APPLIANCES
The object of this little book is to show how much may be seen
with an ordinary pocket lens, and with a simple microscope; and,
as far as possible, to dispel the idea, far too common, especially
among beginners, that no real work can be done unless one has a
compound microscope, with a large battery of lenses and an array of
‘accessories.’
It would be easy to multiply quotations, from high authorities, in
support of the proposition implied in the foregoing paragraph. Two only
must suffice.
In a recent review of a very good book dealing with Butterflies and
Moths (_Natural Science_, vol. vi. p. 293), the following passage
occurs: ‘The only suggestion we should like to make is that a compound
microscope is unnecessary for any of the details that the author
mentions. A first-rate platyscopic hand lens is much more convenient
and the young naturalist should train himself thoroughly in the use
of it. There is no more common error than the undue use of the higher
powers of a microscope. Except for the intimate details of histology, a
low power or a hand lens is much more easy to use, and its employment
gives a much better idea of the structure.’
The next quotation is of greater interest, as it gives some insight
into the way in which Darwin carried on his investigations. In the
_Life and Letters of Charles Darwin_ (vol. i. pp. 145, 146) we
are told: ‘His natural tendency was to use simple methods and few
instruments. The use of the compound microscope has much increased
since his youth, and this at the expense of the simple one. It strikes
us nowadays as extraordinary that he should have had no compound
microscope when he went his Beagle voyage; but in this he followed
the advice of Robert Brown, who was an authority in such matters. He
always had a great liking for the simple microscope, and maintained
that nowadays it was too much neglected, and that one ought always to
see as much as possible with the simple before taking to the compound
microscope. In one of his letters he speaks on this point, and remarks
that he always suspects the work of a man who never uses the simple
microscope.’
It may be well here to verify the quotations, and also to consult
Darwin’s _Naturalist’s Voyage_, to ascertain what kind of objects
he examined with the simple appliances at his command. In the first
chapter there is an interesting account of a curious limy deposit
on the rocks of the island of St. Paul’s, and of the discoloration
by confervae of the water, which, ‘under a weak lens, seemed as if
covered by chopped bits of hay, with their ends jagged.’ Then we have
an account of the confervae in the Indian Ocean, and of infusoria so
numerous as to tinge the water off the coast of Chile. In the second
chapter we have observations and experiments on planarian worms.
‘Having cut one of them transversely into two nearly equal parts, in
the course of a fortnight both had the shape of perfect animals.’ In
the next chapter he records some observations on the structure of
vitrified tubes formed by lightning striking loose sand. In the fifth
chapter is an elaborate description of a kind of sea-pen; and in the
ninth chapter there are some remarks on the vast number of eggs in the
egg-ribbon of a sea-slug, and on the ‘bird’s-head’ organs in certain
Polyzoa. These remarks were, of course, founded on actual inspection
with the simple microscope.
To this instrument, also, we owe the discovery of the tadpole-like
larvae of Ascidians, or Tunicates, as they are now generally called.
‘At the Falkland Islands I had the satisfaction of seeing, in April,
1833, and therefore some years before any other naturalist, the
locomotive larvae of a compound ascidian.... The tail was about five
times as long as the oblong head, and terminated in a very fine
filament. It was, as sketched by me under a simple microscope, plainly
divided by transverse opaque partitions, which I presume represent the
great cells figured by Kovalevsky. At an early stage of development the
tail was closely coiled round the head of the larva[1].’
* * * * *
We come now to our pocket lens, which may be purchased for a few
shillings of any optician. One can buy a serviceable single lens,
in an ebonite handle, for a shilling; and this cheap instrument is
sufficiently powerful not only to give the worker a good general idea
of the form and structure of objects, but to enable him to do real
work. With it the habits of many of the inmates of his aquaria may
conveniently be watched; he may see their development from stage to
stage of their life-history; and with it, when they are broken up, he
may make out a good deal of their external and internal anatomy.
[Illustration: FIG. 1.--Hand Magnifier and Stand.]
A very good form is shown at Fig. 1, which represents a hand magnifier,
fitted with three lenses of different focus, generally 2 in., 1½
in., and 1 in. Examination of the catalogues of the principal London
opticians shows that such a set of lenses may be bought for about
3_s._ In shape and construction there is sometimes a little
variation; but the form figured is that most generally adopted, and is,
on the whole, fairly convenient. It would, however, be an advantage if
the hole by which the magnifier is mounted on the stand were drilled
in the solid part of the handle. This would not only do away with the
objection that the hole in the case permits dust to penetrate to the
glasses, when carried in the pocket, but would give a longer reach, and
thus obviate the necessity for moving the stand if the observer were
examining a large object. The price of the stand figured is 2_s._
6_d._; and one with a short adjusting arm ought not to cost much
more.
Any one with a mechanical turn may make a stand for himself, though it
may be doubted whether this is quite worth while when these articles
may be bought so cheaply. Nevertheless, there is great pleasure in
making things for oneself; and a home-made stand will enable the
observer to do quite as good work as one that came from the optician’s
shop.
A bill-file weighted at the foot may be bought for a few pence, and
adapted to the purpose. For the slider a large cork cleanly pierced
will answer admirably. This should carry a piece of stout wire, bent
at the end thus __|[image], to serve as a holder for the magnifier,
which should have a hole in the handle, for the reasons stated above.
The only difficulty will be the attachment of the wire to the cork. The
Rev. J. G. Wood advocated winding the wire round the cork in a spiral;
and this is a very good plan. An increase of steadiness is secured, if
a larger cork, or small bung, be used, and the wire inserted in the
side.
There are, of course, more expensive lenses, with which better
definition can be obtained. Zeiss has an excellent magnifier consisting
of two lenses, for use in the dissecting microscope (Fig. 2), and
also as a hand lens, at the price of 6_s._; one of the same
construction, for use in the dissecting microscope alone, may be had
for 4_s._ The Steinheil achromatic lenses are probably the best
of all. These are made in powers ranging from 2 in. to ½ in. focus[2];
and the price varies from 10_s._ up to £1, according to the maker.
Those made by Leitz of Wetzlar cannot be surpassed; and they are sold
in London at 10_s._ each, either mounted in a handle, for use
as hand magnifiers, or with a collar for use in Leitz’s dissecting
microscope (Fig. 3). Mr. Lewis Wright says that ‘the best plan is to
combine both uses, and have two or three powers in collars, with a
spring ring folding into a handle, which will carry any one of them in
that manner. A Steinheil lens at this low price costs little more than
a Coddington, while its performance is infinitely superior[3].’ It is
a difficult thing to get makers to deviate from the beaten track, and
so far as I have been able to learn, Mr. Wright’s wishes have not been
fulfilled.
[Illustration: FIG. 2.--Zeiss’s Dissecting Microscope.]
The lenses and stand (Fig. 1) constitute a simple form of dissecting
microscope. If the worker wishes for something more elaborate, he need
only consult the catalogues of the principal makers to find something
that will meet his requirements. Zeiss’s brass stand, with stage, above
which a lens slides up and down in a holder (Fig. 2), is sold for
9_s._; with blocks for supporting the hands, at 10_s._ It is
a useful instrument for small objects.
My favourite instrument is shown at Fig. 3. Here the focussing of the
lens is effected by rack and pinion work, by means of the screws on
each side the upright pillar. The lens is shown fitted in the collar
which carries it. The stage is of glass--roughly, 2½ in. long by 2
in. wide, and the arm at the top of the pillar can be moved from side
to side, so as to bring a fairly large object within range. The metal
framework of the stage is furnished with nickelled clips (not shown),
which serve to hold an excavated slip. The arm-rests are detachable,
and the uprights are hinged for convenience of packing. The instrument
(with the exception of these rests) packs into a neat, strong mahogany
box, 7½ in. in length, and about 5 in. in height and width. With
two powers--1 in. and ½ in. are very serviceable ones--the cost is
38_s._
[Illustration: FIG. 3.--Leitz’s Dissecting Microscope.]
It is to be wished that the maker would devise some plan by which the
admirable lenses sold with this instrument could be utilized for the
pocket. Mr. C. Curties, of Baker & Co., High Holborn, has kindly done
something in the matter, and has made for me a metal holder. I have
found this convenient, but should be glad to see something further done
in the same direction, so that instrument, lenses, and holder could be
sold for £2. This ought to be within the range of practical optics. The
spring collar advocated by my friend Mr. Wright seems better, and would
certainly be cheaper. The lenses would only need to be dropped in. To
use my pocket holder one must unscrew the metal collar from the lenses
before screwing them into the metal plates which carry them (Fig. 4).
It is, however, something to have made a beginning: it is a step in the
right direction.
[Illustration: FIG. 4.--Two Leitz Lenses in holder
(open).]
[Illustration: FIG. 5.--Two Leitz Lenses in holder
(closed).]
A serviceable dissecting microscope--not a toy, but an instrument with
which real work may be done--can be made at a cost of a few shillings.
Such a one has been made for me by a friend with a positive genius
for such work. The body is fashioned out of a parcel-post box 7 in.
long, 3½ in. in height, and the same in width. From the centre of the
sliding top a piece is cut away, leaving ledges to take a 3 in. by 1
in. excavated slip for small dissections, or a mounted slide of a large
object, such as a whole insect, for examination. A further portion is
cut away on each side to take a small dissecting dish (Fig. 6). To
admit the light, a hole is cut in the side of the box; and the mirror
consists of a piece of silvered glass which was bought of a hawker in
the street. This is placed in the box opposite the square hole, and
sloped at an angle of 45°. The aid of a skilled mechanic was sought
for a small rod carrying a thread, which works in a piece of brass
bent at a right angle. This piece of brass is screwed on the box, just
above the aperture by which light is admitted, and carries a pocket
magnifier, similar to that shown at Fig. 1.
[Illustration: FIG. 6.--Home-made Dissecting Microscope.]
This modest little instrument generally stands on my work-table, and
has provoked some remark and a little good-natured banter from friends
who have seen it. Nevertheless, I should be sorry to part with it, for
I have found it extremely serviceable in many ways. And more than one
critic has had to confess that better results were obtained than one
would expect from its appearance. The total cost out of pocket was,
3_d._ for the box, 3_s._ for the lens, and 1_d._ for the plate-glass,
while the man who made the pillar and ear-piece would take no more
than 6_d._ for his work. This brings the total to 3_s._ 10_d._ With a
little ingenuity the pillar might be made to carry a collar, and so
take a Steinheil lens. This would swell the total cost to about 11_s._
Other apparatus need not be costly. An incident occurred at the meeting
of the Quekett Microscopical Club on November 22, 1878, which shows
how readily common objects may be utilized for our purpose. The late
Right Hon. T. H. Huxley, who was at that time President, exhibited,
and made some remarks on, the dissecting microscope which now bears
his name. During the discussion which followed, Professor Charles
Stewart exhibited some little saucers, which were admirably adapted
for dissecting purposes. The President said that he should ‘be glad
to know where these convenient little saucers could be obtained.’ The
next paragraph of the minutes is interesting and instructive. ‘Mr.
Stewart said they were to be found at the corners of the streets,
containing three whelks or three mussels for a penny. He bought those
he had brought to the meeting at a shop in the New Cut, where they were
supplied to costermongers[4].’
As very many of the objects with which we are concerned are aquatic,
we shall want vessels of some sort to serve as aquaria. Any glass
vessel will answer our purpose, provided it is clear, to allow of the
examination of our captives; or shallow pie-dishes may be utilized.
The glass pots in which preserves are sold will do admirably, and any
glazier will cut us covers for a few pence. Within reasonable limits,
the smaller the aquaria are the better. The inmates can be seen more
easily, and picked out with less trouble when one wishes to examine
them.
The principles on which aquaria should be kept are now pretty generally
understood. There should always be a small quantity of growing aquatic
vegetation, and a supply of minute life to furnish food for the larger
forms. Excess of light should be avoided, and the temperature should
not be allowed to rise much above 50° F. Carnivorous beetles and their
larvae may be fed with small pieces of meat, small garden worms, or
tadpoles. Most of the smaller larvae treated of will be satisfied with
vegetarian diet, varied with an occasional meal of water-fleas.
[Illustration: FIG. 7.--Beakers.]
[Illustration: FIG. 8.--Glass Capsule.]
If one cannot lay the household stores under contribution for jam-pots,
tumblers, and bottles, beakers (Fig. 7) make capital small aquaria.
They are sold in nests, and may be had either rimmed or lipped--rimmed
for choice. There is no difficulty in obtaining them of any optician or
glass-merchant. Mine have been bought from Messrs. Beck, of Cornhill,
as have the capsules, &c., figured here.
Glass capsules (Fig. 8) are made in different sizes, ranging from 1½
in. to 3 in. in diameter, with a height of 1 in. or 2 in. The largest
size, 3 in. by 2 in., costs 5_d._, and a glass circle to cover it,
1_d._ These capsules will be found useful for small aquaria, and
for isolating aquatic larvae in order to keep them under observation
during their change to perfect insects. It was in a capsule of this
kind that some of my Ptychoptera larvae (p. 184) were kept, and changed
into the pupal condition.
The glass block, with cover (Fig. 9), is convenient for a number of
purposes. In it small creatures may be examined in air or in water, and
it makes an exceedingly convenient little dissecting dish for use with
the mounted hand magnifier (Fig. 1), or with Leitz’s stand (Fig. 3), or
the home-made stand (Fig. 6). The glass box, with cover (Fig. 10), is
extremely good for keeping small creatures under observation.
[Illustration: FIG. 9.--Glass Block, with cover.]
[Illustration: FIG. 10.--Glass Box, with cover.]
Excavated glass slips, 3 in. by 1 in., may be bought from any optician.
They serve for the examination of objects in water, and also for
dissection. The best I have been able to get have been supplied by Mr.
J. Hornell, of the Biological Laboratory, Jersey, and they are very
cheap.
We shall need some forceps to pick up specimens from the vessels
in which they are kept, and the same little instruments will be
found convenient in collecting. Both forms have advantages of their
own; if we are limited to one pair, they should be curved, and of
brass. Forceps with ivory tips are very useful for handling aquatic
vegetation. These articles are not usually sold by opticians, but
are kept by the tradesmen in Clerkenwell who sell jewellers’ and
watchmakers’ tools, and cost from 1_s._ to 1_s._ 6_d._ a
pair.
[Illustration: FIG. 11.--Forceps.]
Dipping-tubes are used to take up small aquatic animals from the
vessels in which they are kept. Very little practice will render the
use of this instrument easy. The tube is held firmly between the thumb
and the third and fourth fingers of either hand, while the index finger
is pressed firmly on the top. Most people naturally prefer the right
hand, but it is well to accustom oneself to use the right or left
indifferently. The open end is then put into the water, just over the
object to be secured, and the index finger lifted. The rush of water
into the tube will carry the object into it, and if the finger be again
applied to the top, the pressure of the atmosphere will prevent the
water from escaping when the tube is lifted out[5].
Small brushes are useful for taking up specimens from the water or from
pickle; common ones will do very well for large objects, but for small
objects and parts it is advisable to have one or two sable brushes, as
these form a better point.
Some needles fixed in handles will also be necessary. These may be
bought, or made by fixing ordinary needles of requisite sizes into the
handles sold for small brushes. The needles must be kept free from
rust, and should always be carefully wiped after use. A good plan to
keep them clean is to stick them in a gallipot in which has been
melted a mixture of lard and paraffin in equal proportions.
Small dissecting-knives are useful, but all the work described here may
be done with an ordinary pocket-knife in good trim.
[Illustration: FIG. 12.--Three forms of Dipping-tube.
Method of using it.]
The best preservative for our purpose is formalin, which is sold in
a forty per cent. solution. This should be treated as absolute, and
a five per cent. solution made. This will really be a two per cent.
solution, and is sufficiently strong for general use.
The most profitable use we can make of specimens is to watch their
habits while living, and to break them up and learn as much as we can
about their structure when they are dead. For us to make a collection
of specimens in tubes would be a waste of material.
[Illustration: FIG. 13.--Mounted Needles.]
Little need be said about collecting. The objects treated of are
so plentiful that no great skill, nor any wealth of appliances, is
needed to secure an ample supply. The following remarks on the methods
employed at the Illinois State Laboratory for the capture of aquatic
insects and larvae are, however, worth quoting:--
‘Insects in vegetation, and on or in the bottom, were taken by means of
a dip-net--a net of about equal depth and width attached to a strong
semicircular ring, firmly fixed to a long handle, the straight side of
the ring being opposite the point of attachment. For the larger and
more active forms, a coarser net was used, and for smaller forms one
made of finer net proved most durable and satisfactory. To collect from
the mud of the bottom, the water immediately over it was violently
stirred and then swept with the net. The surface layer of mud was also
scooped up in the fine dip-net, and then allowed to wash through,
leaving the coarser contents in the net. Insects on the bottom in deep
water were secured by using a dredge, and washing its contents through
net sieves. The aquatic vegetation, when free from mud, was violently
washed in a large pan, many smaller forms being thus dislodged and
coming to the surface. Insects occurring in open water were taken in
drawing an ordinary towing-net[6].’
Here we have, so to speak, the general principles of collecting. It
will be easy to adapt them to particular cases.
In choosing the subjects to be treated of in this little book, some
difficulty has been experienced in deciding what to select from the
multitude that lay ready to hand. It was felt necessary that the
subjects should be connected, since choosing them at random would
lead to purposeless work, and so to waste of time and opportunity.
After some consideration, the author has decided to take all the
examples from the Arthrop´oda--that great sub-kingdom of backboneless
animals which includes the Lobster, the Crab, the Sand-hopper and the
Woodlouse, the Spider and the Mite, the whole world of Insects and the
Centipedes. One cogent reason that influenced this decision was the
fact that these objects are exceedingly common, so that there can be no
difficulty in procuring material on which to work. There is, perhaps,
no other sub-kingdom so full of interest, on account of the many widely
different forms, which may be referred to one common plan.
It may possibly appear to some readers that the powers of the pocket
lens have been exaggerated. As a matter of fact the material for the
book has been gathered by actual observation. The author has seen, with
an ordinary pocket lens, the objects here described. If some are shown
as they would appear under greater magnification than such a lens would
give, this is chiefly for the sake of emphasizing points of interest
which might otherwise be overlooked, but which can readily be made out
with a hand magnifier, when attention has been drawn to them, and the
observer knows what to look for.
CHAPTER II
ARTHROPODS AND THEIR CLASSES.--THE MARGINED
WATER BEETLE; THE GREAT WATER BEETLE;
THE COCKTAIL BEETLE
Having got together our apparatus, which, as we have seen, need be
neither costly nor complicated, the next step will be to acquire some
knowledge of the group from which the examples here treated of will be
taken--the Ar´thropods, or animals with hollow-jointed limbs. These
are the ‘Insects’ of the Linnaean classification, and, for the matter
of that, of popular phraseology; for though few people would now
venture to call a Lobster an ‘insect,’ we still style some of its near
relatives Water ‘Fleas,’ as Swammerdam did two hundred years ago.
The Arthropods form a phylum, or main division of the Animal Kingdom.
Above this phylum comes that of the Molluscs, or soft-bodied animals,
such as the Oyster, the Snail, and the Cuttlefish. Still higher are the
Lancelet, the Sea-squirts, and some few others, that bridge the chasm
between the phyla without, and that phylum with, a backbone. And to
this last Man himself belongs.
Two reasons contributed to the selection of the Arthropods as a subject
for work with the pocket lens: (1) the great interest which surrounds
many of the group; and (2) the ease with which specimens may be
procured and kept under observation.
Every one has pretty clear notions as to the general ‘make’ of a
Vertebrate or backboned animal. An Invertebrate animal has, of course,
no backbone or the semblance of one; the nervecord, where present, lies
on the under surface, and forms a ring round the gullet, and the heart
lies on the upper surface or back. We may verify this by pulling to
pieces a dead insect.
But a phylum, or main division, is much too large to be considered as a
whole. It must, therefore, be broken up into smaller groups, which are
called Classes, generally reckoned as five in number. These, again, may
be grouped into two divisions, according as their members breathe by
means of air-tubes (_tracheae_) or by gills. Our scheme then will
stand thus:--
{ { Peripatus.
{ Breathing by { Centipedes and Millipedes.
{ air-tubes { Insects.
ARTHROPODS { { Spiders and their kin.
{
{ Breathing by { Lobsters, Crabs, Sand-hoppers,
{ gills { and Woodlice.
This scheme looks well on paper; and on the whole is workable. But
among our examples chosen from the Class of Insects, we shall find some
that breathe by gills in their larval stage, and by air-tubes when
adult. And among the Crabs are some, the gills of which have ceased
to perform their normal function, so that these animals cannot live
in water for a single day. And then there are the Sand-hoppers and
Woodlice.
The body of an Arthropod may be represented by a series of similar
rings, thus:
[Illustration]
This similarity is clearly apparent in the Centipede, but is concealed
in the Beetle, the Shrimp, and the Spider. It seems, at first sight,
to be altogether lost in the Crab, and does really vanish in the adult
stage of some parasitic Crustaceans.
It may be plausibly objected that our ideal Arthropod resembles nothing
so much as a worm. In many respects this is true. A primitive Arthropod
was worm-like, as is a Centipede. And Arthropods and Worms were
formerly classed together in one group, as Annulo´sa or ringed animals.
The chief external difference lies in the nature of the appendages
borne by the various rings or segments.
We may represent those of the Worms thus [image], for they are
bristles, or groups, or modifications of bristles. Those of the
Arthropods may be represented thus [image], for the appendages are
really jointed, though, of course, in a fashion different from those of
a backboned animal.
The jointed appendages of Arthropods may be modified to fulfil very
different functions. They may serve as legs for walking, hands for
climbing or seizing prey, jaws for masticating food, feelers or organs
of touch and sense, and, strange as it may seem, in one group, as eyes.
It is well to get some notion of how these joints are formed. To take
the body first: the skin connecting the segments is much thinner than
that of the segments themselves, which is thickened by the deposition
of chitine, and, in some cases, also of carbonate and phosphate of
lime. A portion of the body, then, may be represented thus, [image]
where the heavy lines denote the segments, and the thin ones the spaces
between the segments. It will be seen that this arrangement allows
of considerable play, and also of a telescopic movement by which the
segments can be brought close together.
It is easy to construct a kind of model that shall exemplify these
movements. Make a tube of calico, some six inches long, and having
stuffed it with cotton-wool, paste on it strips of brown paper one inch
in width, leaving an interval between each, as in the last diagram.
Then we shall be able to understand how Arthropods can bend the body or
move it from side to side. And the limb joints are made on a similar
plan.
[Illustration: FIG. 14.--Cape Peripatus (natural size).]
The most archaic Arthropod--Perip´atus--must be mentioned. It is not
found in Britain, nor even in Europe; so that, unless we travel, we
shall only know it from books, or from museum specimens. But it is an
extremely interesting creature, for it is of worm-like aspect, and
breathes by air-tubes, opening all over the body, which has no external
segments. The limbs are imperfectly jointed, and each of them bears
two claws. Most naturalists make this genus a Class by itself, while
some put it with the Centipedes. There are about a dozen species, four
of which are African, two Australian, and the rest are found in South
America and the West Indies. Besides these there are some doubtful
species.
In habit they resemble the Centipedes, and they ensnare the insects on
which they feed by ejecting sticky slime from the small processes near
the mouth. The left process is shown in the illustration, just below
the antenna of that side.
Professor Sedgwick, who described these animals in the _Quarterly
Journal of Microscopical Science_ (1888), and, more popularly,
in the _Cambridge Natural History_, says, that ‘the exquisite
sensitiveness and changing form of the antennae, the well-rounded plump
body, the eyes set like small diamonds on the side of the head, the
delicate feet, and, above all, the rich colouring and velvety texture
of the skin, all combine to give these animals an aspect of quite
exceptional beauty.’
Unfortunately, an illustration in black-and-white can only render
form. We must take the beauty of the colouring for granted. One thing,
however, cannot escape the most cursory examination of the picture--the
resemblance of the creature, in some respects, to a worm, and, in
others, to a caterpillar, which, as everybody knows, is the larval
stage of a butterfly. If this resemblance sets us thinking how it came
about, and what it means, Peripatus will, for the present, have done
its work for us.
* * * * *
With these general notions of Arthropods, we may pass on to put our
pocket lens to some practical use. Our first subject shall be the
Margined Water Beetle (_Dytis´cus margina´lis_), which can be
taken in almost any open pond in the country. Water covered with
duckweed should be avoided in hunting for these beetles, which prefer
ponds with a clear surface, so that they may easily come to the top to
breathe.
Every one has a good general notion of the principal Insect-groups,
technically called Orders--Beetles, Cockroaches and Grasshoppers,
Butterflies, Bees and Wasps, and Flies. Insects may be defined
as animals with hollow-jointed limbs, and divided into three
regions--head, thorax, and abdomen. The head bears a pair of antennae;
the thorax carries three pairs of legs, and (generally) two pairs of
wings; the abdomen is without appendages. Insects when adult breathe
by tubes that open to admit air. In Chapter VI we shall see that many
larvae obtain an air supply in different ways.
[Illustration: FIG. 15.--Margined Water Beetle (male).]
Beetles may be taken as very good types of true Insects. They
constitute the Order Coleop´tera, or Insects with sheathed wings, only
the hinder pair being used for flight (Fig. 18), and at other times
they are folded under the wing-cases, or el´ytra, as in Fig. 15.
We may advantageously compare our Beetle with Peripatus, and note the
points of agreement and of difference.
Now, if our captive Beetles are to yield us the greatest possible
amount of profit, we shall keep them under observation for some time,
so as to watch their habits.
In keeping these Beetles we shall not require a large aquarium. A small
gathering of aquatic weed will be necessary to keep the water in good
condition and the aquarium ready for its tenants.
My interest in these Beetles was quickened by a letter in the
_Field_ (Oct. 28, 1893), in which a correspondent at Weybridge
asked ‘for information as to what animal or bird bisects so neatly the
shells of the Water Snail (_Planorbis_).’ I thought then, and
know now, that the shells were ‘bisected,’ if that is the proper word,
by Water Beetles. From that time I have had, and still have, several
living in small aquaria, but for a long time was unable to get direct
evidence on the subject.
[Illustration: FIG. 16.--Shells of Molluscs broken up by
Dytiscus.
(From a photograph by Cherry Kearton.)]
Many experiments were tried, and at last these proved successful.
Several specimens of Dytiscus[7] were obtained, and put into a small
aquarium in which was no other food for them than some snails and other
molluscs. The Beetles were carefully watched, and were several times
seen trying the snails. In crawling along the inner surface of the
glass, Planorbis and Limnaea both protrude the foot to a considerable
extent, and pieces were ripped out by the strong mandibles of the
Beetles before the shells were actually broken up.
All the shells represented in Fig. 16 were taken from this aquarium,
so that there is good evidence as to what creatures broke them up and
devoured their inmates. In these, as in the specimen kindly sent me by
Mr. Tegetmeier, the Natural History Editor of the _Field_, the
bisection is not complete, though in all cases it is carried far enough
to allow of the extraction of the mollusc. The large Limnaea shell in
the centre has been attacked, but it seems to have been left when the
beetles discovered it was empty. (The empty shell was noted before the
Beetles were put into the tank.) Another Limnaea shell is figured, from
which the snail has been picked out, and that of a fresh-water mollusc.
After these observations had been recorded in the _Field_[8], I
found that I had been anticipated by about forty years. I picked up,
at a bookstall, a copy of G. B. Sowerby’s _Popular History of the
Aquarium_, and there I found that the author had distinctly seen
Dytiscus at this kind of work. He says[9]: ‘I have only once witnessed
him in the act of seizing an unfortunate Planorbis or Flat-coiled Water
Snail. At first, the Dytiscus seemed to be roaming about in quest
of something, first under, then over, the leaves of a water-lily.
At last, in a rather dark corner, he seemed to perceive suddenly a
Planorbis which was browsing upon the stem of a plant just under the
shade of a broad leaf. He darted at this, seized it, and then, putting
his tail out of water, for the purpose of taking in a fresh supply of
air, moved slowly down, bearing the snail with him. He held it by his
fore-feet, turning round the coil until the aperture of the shell was
opposite his mandibles, then he began nibbling away at the animal. In
vain did the poor mollusc try to withdraw within its shelly fortress,
for the beetle picked off the edges of the shell bit by bit, so as to
expose the body as fast as it was withdrawn. All the way down to the
bottom of the tank was this process continued, air-bubbles rising to
the top, and bits of broken shell falling, till the beetle with his
burden reached a stone near the bottom, where I left him still busy at
his work.’
This puts the matter beyond doubt, if any before existed. I at once
wrote to Mr. Tegetmeier to let him know that my experiments had,
unknown to me, been anticipated, long ago, by Mr. Sowerby. Had
he rescued his Planorbis shell, it would have compared very well
with those forwarded to the _Field_ office in 1893. They had
been exhibited at the Malacological Society, and no one was able
to solve the mystery of their mutilation. This shows, to quote the
_Field_[10] on the subject, ‘how easily statements that have been
recorded may subsequently be overlooked and entirely forgotten.’
To return to our Beetle. The male is a handsome creature, from an inch
to an inch and a quarter long, clad in olive-green, bordered with
yellow, and exceedingly active. His mate is smaller, more soberly clad
in brown, without the yellow markings, and the wing-cases are more or
less furrowed.
The first thing to notice is the shape of the body, oval and smooth,
offering no resistance to the water. The hind pair of legs are
flattened and fringed with hairs, so as to make capital paddles. In
swimming the right and left legs are moved together.
Now, though this Beetle lives in the water, it is made, so far as
concerns its breathing apparatus, after the fashion of a Land Beetle,
and consequently is compelled to come to the surface pretty frequently
for a supply of air, which it obtains in this wise. Directly it ceases
paddling it floats to the top of the water; and as the head is heavier
than the tail the latter projects a little above the surface. Then the
wing-cases are raised, and air flows in under them to the breathing
holes on each side. The operation is not a long one, and as soon as it
is over the Beetle is ready for another ramble round his dwelling-house.
But if we do not supply our captive with food that he may take for
himself, it is only right that we should feed him, which may be done at
intervals--say, every other day. ‘Little, and often,’ is an excellent
motto to guide us in our feeding; and though its adoption may entail
some trouble, it will be more than compensated by the success that
will attend our endeavours to keep the inmates of our aquarium in good
condition. And the operation of feeding our Beetle will show us that he
has some capital sense-organs, which are of as much, if not of more,
use to him than his eyes.
He is a flesh-eater. Let us take a small piece of meat or fish in a
pair of forceps, or stuck on a pointed stick, and hold it at a little
distance from his great eyes. The chances are that he will not see it.
Even if we put it in front of him, he is quite likely to disregard it,
for he has nothing corresponding to a nose, with which he may smell.
From his head there spring a pair of long feelers--the antennae--and by
means of these we will let him know that his dinner is ready. That is
effected by drawing the food along the side of one of the antennae. The
creature undergoes a sudden change. Till the antenna was touched with
the food he was resting on his swimming legs. But in a moment down goes
his tail and up goes his head, he stretches out his raptorial legs, and
clutches wildly at the forceps or stick, as the case may be, holding so
tight that he may be dragged round and round the glass vessel. Let go
he will not, of his own accord; and it would be a difficult matter to
shake him off. Similar experiments may be tried with other Beetles, and
the result will be to impress on the mind the fact that the feelers are
capital sense-organs.
If we are to turn our Beetle to the best account, we shall need to
handle him. It may be inconvenient to wait till he dies, so we will
kill him quickly and painlessly by plunging him into boiling water, and
he may be preserved by putting him into a tube containing about equal
parts of water and spirit, or a five per cent. solution of formalin.
Dissections should properly be made under water. The Beetle should be
fastened, back upwards, to a piece of cork weighted with lead, and
placed in a deep saucer, or dissecting dish, and covered with water.
But a good deal of rough dissection, as is ours, may be done in air,
and the Beetle may be fastened to any convenient piece of board, or
even held in the palm of the left hand. Very little practice is needed
to run over the external parts of a large Beetle in this manner.
[Illustration:
FIG. 17.--Outline of Dytiscus (male). _a_,
antenna; _b_, maxillary palp; _c_, eye; _d_,
fore-leg; _e_, thorax; _f_, middle leg; _g_,
elytron; _h_, suture; _i_, hind leg; _j_, claw;
_k_, tarsus or foot; _l_, tibia or shank; _m_,
femur or thigh; _n_, first three joints of foot, widened
into a plate with suckers beneath.]
First, let us look over our Beetle, and get some general notions of
its make. As it lies, back upwards, it is clear that it consists of
three parts or regions------------[image], the first of which is the
head, the second the thorax, and the third the abdomen. Not only in our
Beetle, but in Insects generally, these parts correspond to the words
that denote them, in that the thorax is longer than the head, and the
abdomen longer than the thorax, as shown by the three dashes, a few
lines above.
These divisions are well shown in Fig. 17, where other parts are also
marked. It will pay to go over our own specimen with this figure before
us, and so make acquaintance with the several parts, to some of which
we shall return in greater detail.
[Illustration: FIG. 18.--Male Dytiscus in flight.]
At this point, if we have not done so before, it will be convenient
to fasten our Beetle, in the position figured, by a stout pin driven
between the thorax and the abdomen, just above the suture (_h_).
We want to raise one of the wing-cases.
If a needle be taken in each hand, between the thumb and first two
fingers, and that in the left hand be used to steady the creature, the
wing-case on the right may be raised with the needle in the right hand,
and then cut off. The small filmy membrane, of somewhat triangular
shape, which comes off with the wing-case, is the winglet. There is
one on each side; and their vibration causes the humming noise made by
these insects in flight. When the water dries up in one pond, or food
becomes scarce, they will leave and fly off to another.
The wing lies folded upon the abdomen. A good deal of very interesting
matter has been written on the way in which Insects fold their wings,
but we can see for ourselves how this Beetle folds them. All we have to
do is to take the wing, and draw it gently away from us, and so unfold
it. We may use finger and thumb, or a small pair of forceps. When let
go, it will spring back to its old position. Reference to the expanded
wing in Fig. 18, and to the diagrams Figs. 19 and 20, will show how the
wing is folded.
[Illustration: FIG. 19.--To show fold of (right) wing of
Dytiscus.]
[Illustration: FIG. 20.--To show fold of (right) wing of
Dytiscus.]
The cross-mark in the diagram represents a joint in the chitinous rod
that forms the wings. This lies just above the cell (which is left
white in Fig. 18). The shorter part of the rod is bent down, forming an
acute angle (Fig. 20); of course, carrying with it the membranous part
of the wing.
This may seem a little difficult. But if it be tried on a specimen, no
real difficulty will be experienced. When the wing has been unfolded,
it will, if let go, spring back to its old position, the shorter part
lying underneath, and the chitinous rod fitting into a groove formed by
the projecting sides of the segments of the abdomen.
To this point the sum of our knowledge about Dytiscus amounts
to this: It is aquatic in habits; its body is divided into three
regions; and it has a pair of membranous wings, covered by chitinous
wing-cases, or sheaths, technically called _el´ytra_ (each being
an _el´ytron_). Wing-cases of this kind are the distinguishing
mark of the Beetles, or _Coleop´tera_, though they are not always
so well developed as in the specimen with which we are dealing. This we
can discover for ourselves by examining all the Land Beetles met with
in a country ramble or in a stroll round the garden.
Now let us unpin our Beetle, turn it on its back, and examine it from
the under side. Head, thorax, and abdomen may be made out more clearly
than before, and we can see that the last two regions are divided into
segments.
Let us deal with the head first. This may be easily separated from the
thorax with a dissecting needle, or with a pocket-knife--an exceedingly
handy tool. The huge goggle-eyes cannot escape observation; and, even
without a magnifier, they may be seen to be compound--that is, made up
of a number of facets, which show like a fine network.
Just in front of the eyes are the antennae, which serve as organs of
touch and perhaps also of other senses.
Kirby has recorded facts which seem to show that the antennae (in
some cases) are also organs of hearing. Other authorities, after many
observations, have come to the same conclusion. The matter, however,
is beset with difficulty. It is certain that some Insects have their
ears in their legs; and for the present, at any rate, we may be
satisfied to know that the antennae are sense-organs, certainly of
touch, probably of smell, and, in some cases, of hearing. An excellent
authority on the subject is Sir John Lubbock’s book, _The Senses of
Animals_[11], which contains references to very many original papers.
[Illustration: FIG. 21.
FIG. 21.--Upper surface of head of Dytiscus. _a_,
labrum, or upper lip; _b_, clypeus or shield; _c_,
mandible dissected out, and (_d_) reversed; _e_, eye;
_f_, antennae.]
[Illustration: FIG. 21 A.
FIG. 21 A.--Under surface. _a_, mentum or chin; _b_, ligula
or tongue; _c_, labial palp; these three together forming
the labium, or lower lip; _e_, eye; _f_, antennae. Above the
maxillae, or lower jaws (_d_ _d_), are shown dissected out:
_d^1_, inner or palpiform lobe; _d^2_, maxillary palp; _d^3_,
lacinia or blade; _d^4_, the palpifer or piece that bears the
palp (_d^2_); _d^5_, stipes or stalk; _d^6_, the cardo or hinge.]
Now we may pass to the mouth parts. It will be good practice to dissect
these out, either in air or in water. We may hold a Beetle between the
finger and thumb of the left hand, and separate all the parts with a
needle held in the right. It is a good plan to gum these parts on a
card, for comparison with the figures in our favourite book--whatever
that may be--on Natural History, and also with the mouth parts of
insects of other Orders. For however much these may differ in form,
and in the uses to which they are put, they are really modifications
of the same parts.
In Fig. 21 we have the upper side and in Fig. 21A the under
side of the head represented, so that we may easily get acquainted
with the different parts, and the names given to them. The cut should
be gone over several times, and the parts in the picture compared with
those in the specimen under consideration. It is good practice to
endeavour to draw what is seen from the specimen itself, and then to
compare the result with the work of the trained artist. And the mouth
parts of Dytiscus may be compared with the mouth parts of the Cockroach
(Fig. 33).
[Illustration: FIG. 22.--Disposition of mouth parts.]
Returning to practical work, the first thing is to separate
the _labrum_, or upper lip, from the head. Then the large
_mandibles_ should be dissected out, and cleaned (by soaking in
caustic potash) from the muscles which will come away with them. Behind
these are a smaller pair of jaws, the _maxillae_, furnished with
a pair of palps, called maxillary palps from their position. These are
to be dissected out; and then the lower lip, or _labium_, may be
separated by passing a sharpened needle along the line where it joins
the chin. The palps on the lower lip are called labial palps.
When these parts are cleaned and dried, they should be gummed on card,
as shown in Fig. 22, where the long lines represent the upper and lower
lips respectively, and the shorter ones the mandible and maxilla of
each side.
So much for the head. Now we discover that what appeared to be the
thorax, when we were looking at the upper surface of the Beetle, and
what is called the thorax in descriptions of Beetles, is really but
a portion of that region, which is seen to be divided into segments.
The covering on the upper surface protects only the first segment,
the middle and hinder ones being covered by the wing-cases and the
_scutellum_ (a triangular piece jutting backward from the second
segment, and meeting the suture). This is not represented in Fig. 17;
but we may put in with our pen a tiny triangle, with its base towards
the head, and its apex towards the tail--this will meet the case.
The first segment bears no appendage above, but to the under side is
attached the first pair of legs. The middle segment also carries a pair
of legs, and on its upper surface are the wing-cases, to the under side
of which, and to the body, the winglets are joined. The last segment
bears the wings above, and the last pair of legs below, these being
placed very far back, so as to give them greater power in propelling
the animal through the water.
It will be convenient to examine the legs next. First, however, it
will be well to look at a normal leg of an Insect (the Cockroach), and
learn the names of the different parts. First comes the _coxa_ (_a_) or
haunch, next the _trochanter_ (_b_), then the _femur_ (_c_) or thigh,
the _tibia_ (_d_) or shank, and the _tarsus_ (_e_) or foot, ending in
a pair of claws. There are three pairs of legs in perfect Insects, and
usually the same number in larval forms, though in some of these legs
are entirely wanting.
[Illustration: FIG. 23.--Leg of Cockroach.]
In the males of the Margined Water Beetle and many of its near
relations the first pair of legs deserve special attention. The first
three joints of the tarsus have coalesced to form a disk or cup, which
in our specimen bears two smaller ones on its inner surface. A power
of 20 will show the disk nearly as well as it appears in Fig. 24.
The purpose of this disk, or clasper, which is absent in females, is
obvious. It was formerly supposed to act as a sucker, but Professor
Lowne and Professor Miall[12] have shown that it does not act by
atmospheric pressure, but by a viscid secretion discharged from the
cup-like hairs with which the inner surface is set.
[Illustration: FIG. 24.--Tarsus of Dytiscus (magnified).]
The middle pair of legs in the male also bear cup-like hairs on the
corresponding joints of the tarsus, and in very much greater number.
Professor Miall quotes Simmermacher to the effect that while the large
disk on the fore-leg has 170 sucking-hairs, the enlarged joints of
the tarsus of the middle leg bear no less than 1590. These hairs are
plainly discernible with the half-inch Steinheil, and I have made them
out with the inch, and think that I could show them to anybody else
with that power. I have not looked for these sucking-hairs on the
middle leg of other Beetles of the same family which have disks on
their fore-legs, but they do exist in some other genera.
If we watch a male Dytiscus in life, in a small aquarium, we shall soon
be convinced that Lowne and Miall are correct in their statement that
the cup-hairs discharge an adhesive substance. We shall see this all
the more plainly if there is much floating vegetation. For, in swimming
about, the Beetle will often come in contact with some of this, and it
will adhere to the cup-hairs. His struggles to free himself from the
encumbrance will show that the attachment is not altogether under his
control. The offending weed is rubbed against the spines of one of the
other legs till it is removed.
[Illustration: FIG. 25.--Female Dytiscus swimming.]
The spines with which the legs are set are worthy of a good deal of
attention, and, like the adhesive cup-like hairs, though in different
fashion, they doubtless assist the animal in holding its prey. The
first and middle legs end in strong claws; those of the last pair are
not so well developed.
The last pair of legs are the swimming organs. The tibia and tarsus
are fringed with long stiff hair behind, so as to hold the water
when the Beetle swims. A peculiar arrangement of the first joint of
the tarsus allows the edge to be presented to the water when the limb
is carried forward for the return stroke, thus offering the least
possible resistance. This Dr. Sharp has compared to the action of a
rower in feathering his oar. There is, however, this difference, which
it is well to note. The oar is feathered after the stroke; the Beetle
feathers its legs before the stroke. It is the first motion when it
begins to swim, and the action is not peculiar to the male.
We now come to the third region, the abdomen. Like the thorax it is
visibly divided into segments, though the division between them is
not so great. Much difference of opinion exists as to the number of
segments in the abdomen of a typical insect. Some authorities maintain
there are eleven, while others put the number as low as five. This,
however, is theoretical rather than practical. It is enough for us
to know that the number apparently varies greatly, owing to the
coalescence of two or more of the segments.
[Illustration:
FIG. 26.--Upper surface of abdomen of typical Beetle.]
The head in Insects, we have seen, carries the eyes, antennae, and
feeding organs. The thorax bears the legs and wings. The abdomen bears
no appendages, except in some cases, on the last segment; these are
called _cerci_. It may be, however, that the stings of bees and
the ovipositors of saw-flies and other insects are modified appendages.
On examining the abdomen of Dytiscus we shall probably be struck with
the difference in appearance between the upper and the under surfaces.
The latter is hard, smooth, and shiny; the former, when the wings are
removed, is seen to be covered with felt-like hair.
Our interest is with the upper surface. Along the abdomen on each
side lie spiracles, stigmata, or openings to the breathing tubes. The
first and last are larger than the rest, and their general form can be
readily made out with an inch magnifier, and with the half-inch we may
get some idea of the detail shown in Fig. 27.
[Illustration: FIG. 27.--Spiracle of Dytiscus
(magnified).]
Dytiscus breathes in this way. Floating up to the top of the water,
the end of the abdomen projects above the surface. If one watches the
Beetle the wing-cases will be seen to rise a little. The air retained
by the felted hairs is given off, and a further supply taken in. Then
the wing-cases are lowered again; the Beetle gives two or three strokes
with its swimming legs, and descends below the surface to ramble round
the tank in search of food.
[Illustration: FIG. 28.--Tracheal tubes (magnified).]
This air-supply between the wing-cases and the abdomen is taken in at
the spiracles and distributed through the tracheal tubes throughout
the body. These tubes branch and subdivide till they end in small
twig-like vessels comparable to the capillaries of the human body.
They consist of two layers--the inner strengthened by what probably is
a spiral fibre, though Packard believes that, in some cases at least,
it consists of similar rings. But we must not pursue this subject. It
would lead us beyond our appointed limits.
* * * * *
Another Beetle fairly common in stagnant waters round London and in
the southern counties is that to which the name Great Water Beetle
(_Hydroph´ilus pic´eus_) of right belongs. This name is sometimes
wrongly applied to Dytiscus, with which its rightful owner has
little in common, except its aquatic habitat. Its scientific name is
_Hydrophilus piceus_; but we shall speak of it as Hydrophilus.
It is not a very easy matter to take this Beetle with a net, by
sweeping in the ordinary way, for it likes to get into the middle of
a mass of vegetation, where it is sure of a good food supply, and is
probably safe from the attacks of Dytiscus, who not unfrequently makes
a meal of his larger relation. A good plan is to pass the net under a
mass of weed and then shake it to and fro in the water. By this means
any Beetles in the weed will be dislodged from their hiding-places, and
fall down into the bottle.
They have, in confinement, the same habit of making a snug place
for themselves; and more than once I have fancied that a Beetle of
this species had escaped from the aquarium, when all the time it was
hidden in a thick patch of water-moss. They are practically vegetable
feeders, though Dallas says that they are not such strict vegetarians
as to deny themselves a meal of animal food when they meet with a dead
mollusc or larva in the course of their wanderings. I have never known
them to indulge in animal food, dead or living, but I have known them
refuse it.
Hydrophilus is the largest British Water Beetle, and, with the sole
exception of the Stag-Beetle, the largest British member of the Order.
Its total length is very little less than two inches, and across the
middle of the back it measures about half as much. It is more slenderly
built than Dytiscus, and the contrast in the size and armature of the
legs is very striking (Fig. 29). There is also a great difference in
their method of progression through the water. Dytiscus moves both legs
simultaneously, while Hydrophilus walks rather than swims, moving one
leg after the other.
If we cannot collect this Beetle for ourselves--which we should
endeavour to do, if possible--it may be bought of almost any dealer
in what are called ‘aquarium requisites.’ But prices rule higher
for Hydrophilus than for Dytiscus. Bateman says that this species
is rarer than formerly, and that specimens cost from 1_s._ to
2_s._ 6_d._ a pair, ‘according to the dealer and the season.’
From this I gather that I must have gone to a shop where the prices
were reasonable, for I have never paid more than 6_d._ for a
Hydrophilus, and then have been allowed to pick out a male. At the same
shop I have paid 2_d._ for Dytiscus.
[Illustration: FIG. 29.--Great Water Beetle. _a_,
male; _b_, female; _c_, larva; _d_, pupa.]
In keeping this Beetle we shall need a larger vessel than was required
for Dytiscus. (In both cases the aquarium should be covered, for if
food be scarce, and sometimes for other reasons, both these Beetles
may take to flight.) The aquarium should be well supplied with growing
water-weed, but none that is choice or valuable should be put in, for
in moving about over the weed the animal will damage almost if not
quite as much as it eats. This difficulty can be easily got over by
supplying it with anacharis, water-crowfoot, milfoil, or any other
common plant that grows rapidly and is easily procurable.
The only specimen that I have taken myself was captured a few miles
north of London. It exhibited a strange instance of depraved appetite.
In the large tank into which it was put were growing vallisneria,
frog-bit, and water-crowfoot in plenty. These it was never seen to
touch. The tank, at one time, had been used for newts, and floating
on the surface was a piece of virgin cork. It had served the former
inmates as a kind of island continent, and had never been removed. To
the under side of this the Beetle would moor himself, head downwards,
and nibble away, as if cork were the natural diet of a British Water
Beetle.
In a few days the Beetle died. It was put into spirit, and soon after
became the subject of a post-mortem. But its strange diet was not the
cause of its death, which was sufficiently accounted for by injuries
inflicted before its capture, probably by a larval or an adult Dytiscus.
It would be mere waste of time to go over this Beetle and describe it
point by point, as was done with Dytiscus. If what was there written
was of any value, readers will be able to apply for themselves the
method laid down. There are, however, some points of difference to
which it will be well to invite attention.
It is a good plan to lay specimens of these Beetles side by side
for comparison. Hydrophilus is the larger of the two; and differs in
colour as well as in size. Its hue is black with an olive tinge; and
in certain lights a blue-black metallic gloss may be seen on the outer
margins of the wing-cases. These are marked with faint longitudinal
lines, and each bears three rows of dots running in the same direction.
The greater length and more slender build of the legs of Hydrophilus
are at once apparent. There is also a marked difference in the tarsal
joints of the fore-legs of the male. The disks and cup-like hairs of
Dytiscus are absent in Hydrophilus, but in their stead the last joint
bears a sub-triangular plate, studded on the inner surface with spines,
which probably serve a similar purpose. A great deal of valuable
information about organs of this kind and their functions will be
found in chapter X of Darwin’s _Descent of Man_. Simmermacher’s
paper[13] should be consulted by all who have the opportunity. Our inch
magnifier will show us these spines quite clearly; and also a curious
little bunch of bristles, which Simmermacher says are probably organs
of touch.
It is a good plan to take Hydrophilus out of the water, and lay it upon
its back, so that the difference between it and Dytiscus may be clearly
seen. The Beetle should be handled carefully, for on the thorax is a
kind of keel, ending in a sharp spine, which extends over part of the
abdomen. This spine is free, and may easily wound the hands of those
who do not watch the motions of the creature pretty carefully. The fore
part of the abdomen and the thorax are covered with short close hairs,
and when the Beetle is in the water these parts entangle a layer of
air, which gives it the appearance of being covered with quicksilver.
The two Beetles differ also in their method of exchanging impure for
pure air. Dytiscus, as we have seen, takes in a fresh supply under
its wing-covers behind; Hydrophilus takes in a fresh supply in front,
employing for this purpose the antennae, which apparently do not
function as feelers, as is generally the case.
When Hydrophilus wants to take in a supply of pure air, it rises to
the top of the water, slowly and deliberately. Unlike Dytiscus, it
is never in a hurry. Then one of the antennae is pushed through the
surface film, thus communicating with the air, which descends to the
hair-covered thorax, whence it reaches the spiracles on the upper
surface of the abdomen. To allow of this the wing-cases are slightly
raised in front. The spiracles in Dytiscus are larger at the posterior
end of the abdomen: in Hydrophilus the largest spiracles are in front.
This is what might be expected, from the method adopted in each case
for procuring a fresh supply of air.
These Beetles have frequently bred in confinement; but no better
account than that of Lyonnet has ever been given of the operation of
the female in making her cocoon and depositing her eggs. As his account
is not generally available, a condensed translation of it is inserted
with his illustration.
[Illustration: FIG. 30.--Female Hydrophilus constructing
a cocoon. (After Lyonnet.)]
Lyonnet[14] wanted to find out how the female made the cocoons (Fig.
30), and this is how he set to work. He put some of these Beetles into
a large aquarium, with a good quantity of water and some duckweed.
On May 31 and the following day he noticed that one of the females
was swimming about in every direction, as if in search of something.
Thinking that this was because she had not the proper materials for her
work, he then put into the aquarium some thread-like alga of a kind
which he had seen attached to some cocoons, and on June 3 the Beetle
began to make a cocoon, but soon gave up the task, apparently because
she was troubled by other aquatic insects which had made a home in this
weed. These intruders were removed, and the Beetle set to work once
more. Lyonnet then noticed that, like a spider, she had her spinning
apparatus at the posterior end of the body. She extended the last
segments slightly, and opened the hindmost one, when he saw a nearly
circular opening, in which was a whitish disk (Fig. 30A,
_a_). On this disk were two little brown tubercles side by side,
nearly at right angles to the longitudinal axis of the body. From each
there projected a blackish-brown conical tube, about a line long, stiff
towards the base, but flexible and elastic towards the tip. These tubes
were the spinnerets, which acted together with a parallel movement, and
from each proceeded a separate thread.
And this is how she made her cocoon. She lay near the surface of the
water back downwards, the under part of the body and the second and
third pair of legs buried in the thread-like weed. The front legs were
free, and with these she shaped the weed over her abdomen. Then she
spun a covering of white silk against the under side of the weed. While
she was spinning, from time to time she used her front legs to press
and flatten the work against her body (Fig. 30B), giving it
the shape of a flattened arch, to which her body gave the requisite
curve. This, forming the top of the cocoon, was finished in about half
an hour. Then she turned (Fig. 30C), and spun the bottom of
the cocoon, moulding this, like the top, on the curve of her abdomen,
and uniting the top and bottom with silk which she spun. The work
occupied about an hour and a quarter.
The Beetle then remained nearly in that position for some two hours. At
first she was hidden in the cocoon quite up to the thorax. The body,
however, was withdrawn almost imperceptibly. During this time she was
busy laying her eggs in regular order, with the pointed ends upwards.
After this she came out of the cocoon, and closed the mouth (Fig.
30D), making the opening smaller by degrees. Then she made a
little mast (Fig. 30D, _b_), of the use of which Lyonnet
admits his ignorance, suggesting, however, that its construction may
serve to use up the silky matter remaining after the work is finished,
lest it should acquire harmful qualities in the body of the Beetle. The
true explanation seems to be that it serves to convey air to the eggs
inside the cocoon.
On July 17 Lyonnet was rewarded for his patient watching by seeing
a larva come out of the cocoon, and the next day some fifty more
appeared. What he saw and recorded it is in the power of others to see,
if they will imitate his patient observation.
The Cocktail Beetle, or Devil’s Coach Horse (_Ocypus olens_), is
an excellent specimen of a Land Beetle to examine, for it is of fairly
large size and extremely common. Moreover it does well in captivity, so
that there will be no difficulty in watching its habits in life, and
pickling it for closer examination when dead.
During the day these animals usually lie concealed under stones or
pieces of earth, coming forth at dusk and during the night in search
of food. Occasionally, however, they may be met with in daylight,
leisurely stalking a smaller beetle or a fly; then with a dash seizing
the victim in their powerful mandibles, which are quite capable of
making an impression on the human skin, as those who handle these
Beetles unwarily will discover for themselves.
[Illustration: FIG. 31.--Cocktail Beetle. _a_,
larva; _b_, pupa.]
Nothing of an animal nature comes amiss to them, and if they cannot
capture living prey, they will make a hearty meal off carrion. This is
an advantage to us, for we may feed our captives with dead insects or
with small pieces of meat.
This Beetle is about an inch long, and of a deep dull black colour.
The head is joined to the thorax by a distinct neck, and the abdomen
is naked, owing to the fact that the wing-cases are very short. Its
wing-cases bear about the same proportion to those of the Margined
Water Beetle that a man’s frock-coat bears to a boy’s Eton jacket. And
this Beetle may be taken as a good type of a group--the Beetles with
short wing-cases (_Brachel´ytra_).
The attitude of this animal when irritated or alarmed is well depicted
in Fig. 31. It raises its head menacingly and opens its strong
mandibles to their full extent, at the same time turning up the end of
the abdomen, like a scorpion about to sting. From the last segment it
will often put forth a pair of white vesicles, from which is discharged
a volatile liquid of disagreeable odour, that probably acts as a
defence against insect-eating creatures.
The best way to capture one of these Beetles is to pick it up with what
Kirby calls the ‘natural forceps’--the finger and thumb. It may be
dropped into any convenient receptacle; the small metal boxes in which
vestas are sold will answer the purpose very well.
My specimen was given me by a friend, who kept it with another in a
round tin box. It lived with me for about three months in a four-ounce
bottle, that measured three inches in height, to the neck, and two
inches in diameter. The bottom was covered to the depth of about an
inch with garden soil, and the top tightly corked, to prevent the
prisoner’s escape. This precaution was necessary; for the inside of
the bottle, though cleaned from time to time, soon became covered with
a coating of earthy particles, which afforded the Beetle a pretty firm
foothold.
It was an extremely interesting pet, and its struggles to escape
by climbing up the sides of the bottle often afforded me much
entertainment. It seemed to have a glimmering notion that the only
way out was by the top, and knowing nothing of the cork it would rear
itself up against the side, and try to climb up by vigorous movements
of its fore-legs. It would also take advantage of any little lump of
earth projecting about the rest. It had not intelligence enough to make
anything like a mound for itself, though the inequalities were probably
the result of its burrowing under the surface. Its temper was none of
the best, for if it was disturbed with the forceps it would resent it
fiercely. The mandibles would be opened, the abdomen curled up, and out
would come the two vesicles as a means of defence. If the forceps were
put near the mandibles, they would be seized, and the Beetle would hold
on so tenaciously that it has often been lifted out of its bottle in
this fashion.
It was exceedingly voracious, and was generally fed on garden worms.
After a full meal its increase in size was very evident. This is not to
be taken to mean that insects grow after they have attained the perfect
or imago state, for this is not the case. But when they have had a long
fast, the segments approach each other, and are forced apart when the
creature is gorged with food. If a Beetle of this species were kept
fasting for some days, and then carefully measured, and measured again
after being plentifully supplied with worms or flies, there would be a
difference of some millimetres between the results.
Dallas has an interesting passage in his _Elements of Entomology_
respecting the boldness of the larval form, which is worth quoting.
‘I have seen one engaged in a struggle, which lasted about twenty
minutes, with a worm of some five inches in length, the larva being
scarcely more than an inch long. During this contest the little savage
crept under the worm, fixing his mandibles into the creature’s body in
various places, each bite apparently producing a considerable swelling.
Sometimes he would fasten upon the head of the worm, and retain his
hold with the pertinacity of a thoroughbred bulldog, although twisted
about in every direction by the struggles of his intended victim. At
last, however, he seemed to come to the conclusion that he had been
too ambitious in his desires, and went quietly off amongst the grass,
rather prematurely, as it seemed to me, for when the worm began slowly
to leave the field of battle, about an inch of his tail was attached to
the rest of his body solely by the intestine, a union which the jaws of
the larva would easily have dissolved.’
I have never seen a fight between a larva and a worm, for the few larva
I have kept have been fed on flies. But the adult Beetle which has once
fastened on a worm cannot be shaken off. It will grip its prey with the
first pair of legs, fixing the claws in the skin, and will finish a
worm three inches long at a meal.
A dead specimen should be looked over in the way recommended for
Dytiscus, raising the small wing-covers and unfolding the wings. The
spiracles are to be looked for at the sides of the abdomen, in the
groove formed by the meeting of the upper and under plates of each
segment. The short downy hair with which the body is covered should
be noticed, and the front legs are well worth examination. The tibia
or shank is armed with a strong spine, and between this part of the
leg and that which follows it is a notch, through which the Beetle
passes its antennae to clean them from dirt. The peculiar shape of
the joints of the tarsus or foot is very plainly discernible with the
appliances at our command, and by a careful management we may make out
the different kinds of hairs with which four out of the five of these
joints are furnished; some stout and spine-like, others finer, ending
in a pear-shaped bulb. These last probably serve the same purpose as
the sucking-disks of Dytiscus and the tarsal plates of Hydrophilus.
CHAPTER III
COCKROACHES; EARWIGS; THE GREAT GREEN GRASSHOPPER; THE
WATER SCORPION; THE WATER BOATMAN;
CORIXA.
The next insect to come within range of our pocket lens is the Common
Cockroach (_Blatta orienta´lis_[15]), popularly misnamed the
Black Beetle. We shall have no difficulty in procuring material for
examination. Housekeepers will tell us that these creatures are only
too plentiful.
In the last chapter we dealt with Sheath-winged Insects--the
Coleop´tera. Cockroaches belong to the Orthop´tera, or Insects with
Straight Wings. The mouth-parts resemble those of Beetles. The chief
differences that mark off the Cockroaches and their kin from the
Beetles are the incomplete metamorphosis which the former undergo, and
the character of the wings. Straight-winged Insects, when they leave
the egg, differ little in shape from the adult, except in the fact that
they have no wings; and these appendages are absent, or so small as
to be useless for flight in many species. When wings are present the
first pair are of little or no use for flight. They are not, however,
hard chitinous sheaths, meeting in the middle line--that is, straight
down the centre of the back--but of a flexible leathery or membranous
substance, and they usually overlap each other at the tips. The hinder
wings are large and nearly semicircular. The principal veins radiate
from the centre to the circumference, like the sticks of a fan, and
when the wings are folded up they lie straight along the upper surface
of the abdomen. It is from this fact that the Order derives its name.
There are two great groups, or sections, of Straight-winged
Insects--those that run, like the Cockroaches, and those that leap,
like the Grasshoppers. No Straight-winged Insect is aquatic.
The Common Cockroach, now so abundant, is not a native, but an
importation from Asia; though how it reached this country is not quite
certain, probably by way of Holland. It seems to have established
itself in London by the end of the sixteenth century, and some two
hundred years later we find Gilbert White recording (in or before 1790)
that ‘a neighbour complained that her house was overrun with a kind of
black beetle, or, as she expressed herself, with a kind of black-bob,
when they got up in the morning before daybreak. Soon after this
account I observed an unusual insect in one of my dark chimney closets,
and find since, that in the night they swarm also in my kitchen....
The male is winged, the female is not, but shows something like the
rudiments of wings, as if in the pupa state.... They are altogether
night insects, _lucifugae_, never coming forth till the rooms are
dark and still, and escaping away nimbly at the approach of a candle.’
This description leaves no doubt as to what the ‘black-bobs’ really
were. This name seems to have dropped out of use, and it would be well
if ‘black beetle,’ in the sense of Cockroach, were also allowed to
drop, for the term contains just as many errors as words.
We may make our first acquaintance with these insects by keeping some
specimens in confinement. A tin box, with a glass lid, will make a
capital dwelling for them. Some paper should be put in, for them
to hide in away from the light, and there can be no difficulty in
providing them with food. ‘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. Cucumbers, too, they will eat, though it disagrees with them
horribly[16].’
We have Dr. Sharp’s authority for the statement that in confinement
these insects are rather amusing pets, as they ‘occasionally assume
most comical attitudes, especially when cleaning their limbs. This they
do somewhat after the fashion of cats, extending the head as far as
they can in the desired direction, and then passing a leg or an antenna
through the mouth; or they comb other parts of the body with the spines
on the legs, sometimes twisting and distorting themselves considerably
in order to reach some not very accessible part of the body[17].’
The prejudice against these insects is, however, so strong, that most
people will prefer to examine dead rather than living specimens, on
account of the disagreeable odour of the latter. This odour is due to
a fetid excretion from the mouth, and if the specimens are killed by
dropping them into boiling water, this will be discharged, and after a
little while they may be taken out with a pair of forceps, and put into
spirit for preservation. If they are dropped alive into spirit, the
excretion will communicate its strong scent to the preserving medium,
and this should be changed before the insects are examined.
From Fig. 32 we may get a general idea of the appearance presented by
a male or female, lying back upwards in a small glass dish, ready for
examination with the pocket lens. The female may be distinguished at a
glance by her wingless condition--only rudiments of wing-cases being
present, and no wings--and her broader abdomen. In life she does not
stand so high upon her legs as does the male, and her abdomen trails
along the ground. The male does not acquire his wings till the last
moult.
[Illustration:
_Female._ _Male._
FIG. 32.--Cockroaches.]
As the Cockroach lies back uppermost in a glass dish, the head is
almost concealed. This is especially the case, unless the insect is
flattened out in some way, or pinned down to a piece of weighted cork.
There will thus be, apparently, two, instead of three main divisions.
This arises partly from the fact that the head is deflexed, or bent
down so that the mouth is turned towards the rear, and partly because
the first segment of the thorax bears a chitinous shield, roughly
semicircular, which covers so much of the head as would otherwise be
visible.
The difficulty, however, may be easily got over, by reversing the
position of the insect, and raising the head with a needle. The
antennae will attract attention by their great length. In the male
insect they exceed, while in the female they fall a little short of,
the total length of the body. They are well worth examination. Even a
low power will show that they consist of a number of joints--usually
from seventy-five to ninety. The three basal joints are much larger
than the rest, and in the female the third basal is nearly as long as
the first. All these joints are thickly set with stiff hairs directed
forwards. At the outer side of each antenna is a compound eye, and on
the inner side is a pale spot, the _fenestra_, which in the males
of some foreign Cockroaches is replaced by a simple eye.
If Cockroaches are kept in confinement, and forced out into the light,
the constant motion of the antennae will satisfy the observer that they
are of great use to their owners. By means of these organs they not
only discover their food, but become by some means, probably by the
motion of air-waves, aware of danger that threatens them. Belt, in his
_Naturalist in Nicaragua_ (p. 110), speaking of the Cockroaches
that infest houses in the tropics, says, ‘They are very wary, as they
have numerous enemies--birds, rats, scorpions, and spiders; their long,
trembling antennae are ever stretched out, vibrating as if feeling the
very texture of the air around them; and their long legs quickly take
them out of danger.’ It is not given to every one to visit the tropics,
but we may all use our eyes in observing the common insects that abound
in our country, and in doing this we shall strengthen the habit of
observation, and very often find confirmation of what we read of the
habits of insects in distant lands.
Sir John Lubbock[18], in treating of the sense of smell in Insects,
says that ‘Plateau put some food of which cockroaches are fond on a
table, and surrounded it with a low circular wall of cardboard. He then
put some cockroaches on the table: they evidently scented the food,
and made straight for it. He then removed their antennae, after which,
as long as they could not see the food, they failed to find it, even
though they wandered about quite close to it.’
The large kidney-shaped compound eyes are sure to attract attention.
It is worth while to take out and break up an eye, gently washing out
the pigment. If we do this, and then examine it with the pocket lens,
we shall have some idea of the multiplicity of lenses in the eye of a
Cockroach, each of the six-sided facets being a lens.
Next come the mouth parts, which may be run over very quickly, for
those of Beetles are formed upon the same plan, and from this primitive
plan are derived the mouth parts of all other Insects, of whatever
character they may be. To examine the mouth organs the insect must be
turned on its back, and the _labrum_ (_a_), or upper lip,
raised with a needle, so as to allow of a general view of the rest.
Then the jaws or mandibles (_b_) may be picked out with a needle.
These jaws are strongly toothed, and work from side to side, and it
is easy to see that they are very efficient organs. The lower jaws
(_c_), or _maxillae_, lie below, and are compound organs,
each being made up of several parts--the base, called the _cardo_
or hinge (not shown in the illustration, but connected at right angles
by a joint with the lower part, the _stipes_). From the stipes
rise the _galea_, or helmet, on the outer side; and, on the
inner side, the _lacinia_, to which the name maxilla is often
applied, though it properly belongs to the whole. At the base of the
_galea_ is inserted the five-jointed maxillary palp, thickly set
with hairs, and probably an organ of touch.
[Illustration: FIG. 33.--Mouth parts of a Cockroach.]
By examining the maxillae (_c_) before they are separated, and
comparing them with the _labium_ (_c_) or under lip, which closes the
mouth from below, it will be evident that there is no slight similarity
between them. Nor is this strange: for the under lip consists of the
second maxillae joined at their bases, which form the _submentum_ (_s_)
and _mentum_ (_m_). (The former is the small, the latter the large
white basal portion; the vertical line in the illustration shows the
mental suture, and should be traced in the dead insect.) The organs in
the centre constitute the _ligula_; and on each side of the labium is a
three-jointed palp (_labial_), like that on the maxillae, thickly set
with hairs, and with a similar function. It is well to work over the
mouth parts a few times till the relation between the maxillae and the
labium is seen and understood. The internal tongue (_d_) is attached to
the inner side of the labium.
Now, still working on the under side of the insect, the three segments
of the thorax are to be made out, and one cannot fail to notice the
great size of the first joint (the _coxa_) in all the legs, and
that these joints seem to serve as shields to protect the under side
of the thorax. Then the different parts of the legs should be traced,
and compared with Fig. 23 on p. 44. The spiny armature of the tibiae is
to be noticed, as are the claws, between which is a projecting lobe,
though this is absent in immature specimens. We shall find that the
appendages of the thorax are the same as in the Margined Water Beetle.
It is well to take as little as possible on trust, and to verify
everything that we possibly can.
Now we may reverse the position of our subject, and having cut off
the wing-cases, which are technically called _teg´mina_, examine
the wings. These may be gently unfolded with a needle or a camel’s
hair brush, when the longitudinal method of folding will be clearly
seen, and the difference of the veining from that of the wings of
the Margined Water Beetle will be apparent. A female should also be
examined, and the small tegmina cut off, so as to see that not even the
rudiments of wings are present.
The Cockroach breathes like other adult Insects, and the spiracles
are ten in number--two on the thorax and eight on the abdomen. The
thoracic spiracles may be pretty readily seen, but those on the abdomen
are not so easy to make out. But by cutting away, with a fine pair
of scissors, the edges of the plates that cover the upper and under
surfaces of the abdomen and the membrane that unites them (Fig. 34), we
may discover them as the open ends of small tubes. While dealing with
the insect in this fashion, it will be easy to take out a piece of the
tracheal tube, which may be compared with Fig. 28.
[Illustration: FIG. 34.--Cockroach, showing Spiracles.]
The abdomen consists of a series of rings or segments, the exact number
of which is rather difficult to decide, from the fact that some are
concealed and others altered in form. Dr. Sharp[19] says that ‘it is
considered that ten dorsal and ten ventral plates exist, though the
latter are not so easily demonstrated as the former.’ In the male, ten
above (dorsal) and nine below (ventral), and in the female two less in
each case, may be made out without dissection.
From the sides of the tenth segment two organs, the _cerci_
(Fig. 35, _a_), are given off, one on each side. These may be
distinguished from the styles of the males by their presence in both
sexes. Our inch lens will show that each cercus consists of sixteen
rings. If we use the half-inch, we shall see that each ring is set with
hairs of different lengths.
When we have got so far it may be well to compare the structure of
a _cercus_ with that of an antenna (p. 67). In each we have a
succession of jointed rings giving flexibility to the organ, and the
rings in each case are studded with hairs. It has been shown pretty
conclusively--and we may verify the experiments--that the antennae are
sense-organs. Are we not justified in coming to the conclusion that,
since the antennae and the cerci resemble each other in structure,
they also resemble each other in function? If the Cockroach receives
sensations by means of the antennae, is it not probable that it also
receives sensations by means of the cerci?
Having worked over the Cockroach from the outside, it will be
advantageous to get some acquaintance with its internal anatomy.
This is not a difficult matter. The specimen is to be pinned down,
under water, with its back uppermost. The wings having been removed,
a longitudinal cut is to be made down the centre from the posterior
part of the abdomen to the back of the head, and the two sides of the
integument turned back. Or the junction between the upper and lower
plates on each side may be cut through with a cutting needle, and the
whole integument removed.
The first task is to clear away the fat-body, a whitish substance
which overlies the chief organs of the body. When this is picked to
pieces and floated off the digestive system will be exposed. After
this has been worked over a few times there should be no difficulty in
dealing with similar parts in other Insects. At the back of the head
lies the gullet or oesophagus leading into the crop (_c_), at the
base of which lies the gizzard (_g_). The interior of this organ
is furnished with six strong chitinous teeth, with small ridges of
the same substance between them. Towards the posterior end are six
cushions, all set with fine bristles. Behind this comes the stomach
(_v_), into which open seven or eight tubes, closed at one end,
and between it are the Malpighian tubes, which are concerned in the
process of excretion. The small intestine (_co_) succeeds, and
behind this is the rectum (_r_).
[Illustration: FIG. 35.--Alimentary Canal of Cockroach.]
It will be interesting to separate the gizzard from the crop (_c_)
and stomach (_v_) and break it open with a couple of needles, so
as to examine the teeth, which will be more easily made out if the
opened organ be allowed to soak for a time in a solution of caustic
potash.
Similar teeth-like processes are found in the gizzards of many other
Insects, and their presence has given rise to some strange ideas.
Swammerdam[20] says, ‘I preserve also the threefold stomach of a
locust, which is very like the stomach of animals that chew the cud,
and particularly has that part of the stomach called Echinus[21] very
distinctly visible. I do not, therefore, doubt but locusts chew the
cud, as well as the animals just mentioned. Indeed, I persuade myself
that I have seen this.’
Somewhat similar teeth-like processes exist in the Lobster, the Crab,
and the Crayfish. ‘Professor 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 adopts
Swammerdam’s comparison, but considers them strainers, not dividers of
the food[22].
We may be fortunate enough to meet with some specimens of the American
Cockroach (_Periplane´ta america´na_, Fig. 36), a much larger
species, which has established itself in some few places in this
country. At the Zoological Gardens, Regent’s Park, it is abundant, and
has almost, if not entirely, driven out the common form. Mr. Bartlett
believes that it was introduced in cases in which animals have been
sent over from America. Both sexes are winged. They not only possess
organs of flight, but use them. If one visits the Gardens, there will
be no difficulty in getting specimens; and it is interesting to compare
the points of agreement in and of difference between this animal and
our common form.
[Illustration: FIG. 36.--American Cockroach (male).]
The Earwig (_Forfic´ula auricula´ria_) is common enough to furnish
us with plenty of specimens on which we may employ our pocket lens. Any
garden in the summer months will yield an ample supply. Earwigs, like
Cockroaches, are light-shunning insects, and love to hide themselves in
the corollas of flowers; and it is probably from their habit of seeking
to conceal themselves that they have acquired their bad reputation--by
no means confined to our own country--of creeping into the ears of
persons lying asleep, and causing death by getting into the brain. Such
an occurrence is beyond the bounds of possibility. No insect of this
size could pass the drum of the ear.
We may easily keep these insects and observe their movements, if we
put them into a wide-mouthed glass bottle and supply them with food.
They are extremely fond of the flowers of the dahlia; but a dahlia
would offer too many hiding-places, so we will put into the bottle some
nasturtium flowers, or any others with a bell-shaped corolla.
If we get a colony in spring we may watch the care of the female for
her eggs. According to Kirby and Spence[23], ‘she absolutely sits
upon her eggs, as if to hatch them--a fact which Frisch appears first
to have noticed--and guards them with the greatest care. De Geer
(_Mémoires_, iii. 548) having found an earwig thus occupied,
removed her into a box where was some earth, and scattered the eggs in
all directions. She soon, however, collected them one by one, with her
jaws, into a heap, and assiduously sat upon them as before. The young
ones, which resemble the parents, except in wanting elytra and wings,
... immediately upon being hatched creep like a brood of chickens
under the belly of the mother, who very quietly suffers them to push
between her feet, and will often, as De Geer found, sit over them in
this posture for some hours.’ Mr. Kirby adds: ‘This remarkable fact I
have myself witnessed, having found an earwig under a stone which I
accidentally turned over, sitting upon a cluster of young ones, just as
this celebrated naturalist has described.’
Like the Cockroaches, Earwigs undergo an incomplete metamorphosis.
When the young leave the egg they resemble their parents, as may be
seen from the immature forms represented in Fig. 37. The resemblance
becomes greater at each successive moult.
[Illustration: FIG. 37.--Larva and Pupa of Earwig.]
In working over these insects, the forceps, or pincers, at the end
of the abdomen will attract attention. They are found throughout the
family, but little is known of their function. It is said that they
are used to aid in folding the wings, and tucking them under the
wing-covers. This can scarcely be their only function, for they are
found in species that have no wings. Probably they serve as organs
of defence and, to some slight extent, of offence. When the abdomen
is curled up, these forceps certainly give the insect a threatening
appearance. They cannot, however, do much harm.
These forceps differ in shape in the male (Fig. 38) and female, the
blades being almost close together in the latter. In the males they
differ considerably in size. Of 583 mature males taken in one day in
the Farne Islands, and examined by Messrs. Bateson and Brindley, the
forceps varied in length from 2·5 mm. to 9 mm.[24] These are called
respectively ‘low’ males and ‘high’ males. The latter are in all points
larger than the former, and have been described as a separate species,
‘but it was impossible to get reliable measurements of the total
length, owing to the fact that the abdominal segments telescope into
each other’ (cf. p. 30).
After examining the antennae and dissecting out the mouth organs, the
peculiar overlapping or imbrication of the plates of the abdomen
should be looked for; and on the membrane that connects them the
spiracles may be detected.
The wings and the complex method of folding have led some systematists
to rank the Earwigs as an Order, while some others rank them as a
Sub-order. For the present, at any rate, we need not concern ourselves
about this. It is enough for us to know that they are closely related
to the Orthop´tera.
As we look at the Earwig from above, the wing-cases recall to our mind
those of the Devil’s Coach Horse (Fig. 31), though there is one great
difference. From beneath those of the Earwig project two small leathery
pieces which are absent in the Beetle. These pieces are not, as one
might imagine, at the tips of the wings, but on the front margin, about
halfway down, and is indicated in the illustration by the shading
between the extremity of the wing-case and the crease-mark at _a_.
[Illustration: FIG. 38.--Earwig (male).]
From the illustration we may understand how the Earwig opens and closes
its wings. From the point _a_ veins, which are thickened about
halfway down, radiate to the hinder edge of the wing, and a little
beyond the thickening they are connected by a vein which runs parallel
with the hinder edge. These radiating veins are brought together, so
that there is a fan-like closing, like that of the Cockroach, but from
a different centre. The wing is then folded back at the place where the
veins are thickened, and then there is a second transverse fold at the
point _a_, so that the only part of the wing now visible is the
leathery patch, which projects beyond the wing-case when the wing is
tucked away.
It is not difficult to unfold the wing of a dead specimen, under water,
using a needle and fine brush. Mr. E. A. Butler[25] recommends a simple
but excellent plan for unfolding and preserving the wing, by gumming
it, with the upper surface downwards, to a piece of card, and gradually
unfolding it and fastening it down. This is not so easy as it may seem,
but with patience and perseverance success will be obtained; and a
similar method may be adopted with the wings of other Insects, which
may be mounted in this way without any trouble. Thus they may be easily
preserved for examination at a future time, or for comparison with the
wings of other Insects.
It is rather remarkable that an insect like the Common Earwig, which
very rarely takes to flight, should have such a complex method of
folding its wings. Dr. Sharp says that though the Earwig ‘is scarcely
surpassed in numbers by any British insect, yet it is rarely seen on
the wing. It is probable that the majority of individuals of this
species may never make use of their organs of flight, or go through the
complex process of folding and unfolding them.’
Let us choose our next example from the Leaping Orthop´tera. They may
be distinguished at a glance from their relatives that run, but do not
leap, by the peculiar structure of the third pair of legs. These are
much longer and stouter than the other two pairs, and the thigh is very
muscular. This insect is a very good type of the family Locus´tidae, to
which, however, none of the insects popularly called ‘locusts’ belong.
They are included in another family (_Acridi´idae_), where the
common British Grasshoppers are also placed. The Locustids and the true
locusts may be distinguished by the difference in their antennae: in
the latter these organs are short, in the former they are very long and
delicate.
The Great Green Grasshopper (_Locus´ta viridis´sima_) (Fig. 39) is
fairly common all over the country, but often escapes observation from
the fact that its hue corresponds so nearly to that of the foliage on
or among which it lives. One specimen taken in a Devonshire lane gave
me a great deal of trouble before it was secured and transferred to
a small tube. It was perched on a leaf when I first saw it, and as I
approached it leaped away. Though I was certain it had not gone far,
it was some little time before I discovered it, and got near enough
to grasp leaf and insect, in time to prevent the latter from taking
another jump.
This insect may be kept alive in confinement for a considerable time,
and will do fairly well on a diet of leaves and fruit, though it will
not refuse an occasional meal of flesh. Dr. Sharp says that a specimen
in confinement ‘mastered a humble-bee, extracted with its mandibles
the honey-bag, and ate this dainty, leaving the other parts of the
bee untouched.’ It is said that if two be placed together in a box
they will fight most desperately, and that the victor will make a meal
off the body of its victim. De Geer witnessed a case of this kind in
a closely allied species that is found in Sweden. Its specific name
signifies ‘wart-eater,’ and commemorates the fact that the peasants
incite these insects to bite their warts, firmly believing that warts
once bitten speedily disappear, and do not grow again. Westwood says
that one of these insects actually devoured part of its own leg that
had been broken off accidentally. When the creature was seen at night
the detached leg was whole; in the morning about half of it had been
eaten.
[Illustration: FIG. 39.--Great Green Grasshopper
(female).]
It is well to get specimens of male and female insects. We shall
require the former in order to examine the sound-producing apparatus,
which the females do not possess; and the latter for the sake of the
ovipositor--a long scimitar-like organ by means of which the eggs are
deposited. Let us take the female first. The length, including the
ovipositor, is a little under two inches, and the antennae will measure
about as much more. The wing-cases do not lie flat upon the back, as do
those of the Cockroach, but in a slanting position, like the sides of
a roof, forming a ridge in the centre. The head is not bent back, as
in the Cockroach, nor does it project in front, as in the Beetles, but
the front is almost vertical. The armature of the mouth is strong, and
of the same pattern as that of the Cockroach. The hood--so the upper
covering of the thorax is called--is of a peculiar shape, somewhat like
that of a saddle. The wing-cases and wings, with their folding, will
offer little difficulty. Next we may examine the cerci, and contrast
them with those of the Cockroach and with the forceps of the Earwig.
Last of all, the ovipositor must be examined, and its structure made
out, so far as the means at our command will allow.
Apparatus of this kind for placing eggs in positions favourable to
their development is by no means confined to these insects, for
examples may be found in other Orders. Sirex, the so-called Tailed
Wasp, has a long straight one, which is often supposed to be a sting,
and the insect itself is not unfrequently taken for a gigantic wasp or
hornet.
When the ovipositor of our subject is looked at with the unassisted
eye, it appears to consist of two curved blades placed side by side,
with an internal groove on each. The apparatus, however, is not quite
so simple: it is made up of six chitinous rods, of which four--the two
above, and the two central ones--are developed from the ninth segment
of the abdomen, while the two lower ones spring from the eighth. It is
not difficult to test these statements. Specimens are plentiful; and as
the ovipositor in this insect is large, and easily broken up into its
component parts, it may well serve as an introduction to the study of
these organs in other Insects--the Saw-flies, for example.
When the insect is about to deposit her eggs, she selects a spot where
the soil is light, and bending the ovipositor nearly at a right angle
to her body, thrusts it into the ground as far as possible. Then, by
a muscular effort, the plates of the ovipositor are separated, and
several eggs travel down the passage formed by the central pair of rods
into the hole. This is repeated time after time, till the operation of
egg-laying is completed. This takes place in the autumn, and the young
emerge from the eggs in the spring. When they come out they are tiny
copies of the adults; but they do not acquire wings till after several
months. The ovipositor of the female appears after the second moult,
and till this organ is developed no difference is apparent between the
young insects.
The chief interest of the male insect lies in its wings, for the first
pair (the _teg´mina_) are the organs with which it produces its
‘love-songs.’ Kirby comments on the fact that Lichtenstein, in the
_Linnaean Transactions_ (iv. 51), ‘draws attention to the eye-like
area in the right wing-case of the males of this genus,’ adding that
that author seemed ‘not to be aware that De Geer had noticed it before
him, as a sexual character; and also, with good reason, supposed that
it assists these animals in the sounds they produce.’
This is how De Geer (_Mémoires_, iii. 429) describes the
sound-producing apparatus of the Great Green Grasshopper: ‘In our
male grasshoppers, in that part of the right elytron which is folded
horizontally over the trunk, there is a round plate of very fine
transparent membrane, resembling a little mirror or piece of talc, of
the tension of a drum. This membrane is surrounded by a strong and
prominent nervure, and is concealed under the fold of the left elytron,
which has also several prominent nervures answering to the margin of
the membrane or ocellus. There is every reason to believe that the
brisk movement with which the grasshopper rubs these nervures against
each other produces a vibration in the membrane, augmenting the sound.
The males in question sing continually in the hedges in the months of
July and August, especially towards sunset, and part of the night. When
any one approaches they immediately cease their “song.”’
It is probably unnecessary to do more than remark that the noises made
by Insects do not correspond to the voices of the higher animals. ‘For
no insect, like the larger animals, uses its mouth for utterance of
any kind: in this respect they are all perfectly mute; and, though
incessantly noisy, are everlastingly silent[26].’
Our plan with these wings is to first verify the fact of their bearing
these talc-like spots, the serrated nervures on the right and the
rudimentary file on the left elytron. The sound is produced by rubbing
the base of the left elytron against that of the right. A recent author
compares this insect to a fiddler, and says that the left tegmen is the
bow and the right the fiddle.
The last point to examine is the ear. It seems strange to say that
these insects have ears in their legs; but though some of the older
entomologists regarded these structures in the tibiae of the first pair
of legs as nothing more than resonators or sound-boards to intensify
their chirping, there is no doubt that they are really organs of
hearing. Much interesting information on this subject will be found in
Sir John Lubbock’s _Senses of Animals_.
[Illustration:
FIG. 40.--Tibial ear of Great Green Grasshopper.]
These oval patches are plainly distinguishable by the unassisted eye,
and correspond in function to the tympanum or drum of the human ear.
The air-supply to the tibiae is distinct from that of the rest of the
body, and is derived from a large orifice on each side of the first
segment of the thorax. These orifices may be seen by removing the
‘hood.’ Indeed, they cannot escape observation. From these orifices an
air-tube passes to each leg, dividing into two branches in the tibia
and reuniting below the drums.
Dr. Sharp[27] says that ‘although the tibial ears of the Locus´tidae
are very perfect organs, there is great difficulty in deciding on the
exact nature of their functions. They would appear to be admirably
adapted to determine the precise locality from which a sound proceeds
... for the legs can be moved in the freest manner in every direction,
so as to bring the drum into the most direct line of the vibrations.
But as to what kinds of vibrations may be perceived, and the manner
in which they may be transmitted to the nerves, there is but little
evidence.’
The next Order from which we shall choose examples will be the
Hemip´tera, containing the Land and Water Bugs and some other forms.
Our chief concern, however, is with the Water Bugs. In this Order the
metamorphosis is incomplete; the mouth is adapted for sucking the
juices of plants and animals; and there are usually four wings. In the
Land and Water Bugs, part of the fore wings is harder than the hind
wings; in the other winged members of the Order both pairs of wings are
membranous. The front wings are called hemel´ytra or halfel´ytra, to
distinguish them from the el´ytra or wing-cases of Beetles, which are
chitinous throughout. Fig. 41 shows the hemel´ytron and hind wing of a
Land Bug, and the names given to the different parts of the front wing.
[Illustration: FIG. 41.--A Land Bug (magnified).
_a_, corium; _b_, clavus; _c_, membrane.]
The Water Scorpion (_Nepa cine´rea_) is not difficult to procure,
or to keep in confinement when it is caught. It would be straining
language to call it a handsome creature, yet it well deserves careful
study, on account of the wonderful modification of the first pair of
legs, and it is from the resemblance of these to the pedipalps of the
scorpion that the insect derives its popular name. Its length is about
an inch and a quarter, from the tip of the beak, or rostrum, to the end
of the breathing-tube. Its greatest width is a little over a quarter of
an inch (Fig. 42). The general hue harmonizes well with the mud, but
the upper surface of the abdomen is a warm red, ‘and is thick set with
hair, so as to afford a very agreeable sight.’
[Illustration: FIG. 42.--Water Scorpion.]
It is extremely common in shallow pools, and its favourite haunt is
near the shore, where it will lie, almost buried in the mud, with its
raptorial legs elevated, ready to seize on any passing insect, and its
breathing-tube just pushed through the surface-film. I learnt this
habit of the insect on the first occasion on which I tried to collect
specimens of it. I had been told that a certain piece of water swarmed
with Water Scorpions. This, I afterwards found, was quite correct;
but though I worked the pond from end to end, a single specimen was
all that rewarded my labour. Whilst transferring the insect from the
net-tube to the bottle for transport, a stranger joined me, and kindly
volunteered his assistance. He had no collecting tackle, but in about
a quarter of an hour he brought at least a dozen good specimens in the
bottle he had borrowed.
It was natural to inquire to what his success was due. He told me that
it was his first attempt at collecting, but that just before joining
me he had noticed the ends of the breathing-tubes sticking out of the
water. This excited his curiosity, and on moving the mud with his
walking-stick, the insects were seen to crawl away slowly. When he
saw me transfer the Water Scorpion from the net-tube to the bottle, he
immediately recognized it. Then he courteously offered his help, for
which, of course, I was grateful.
We may keep the Water Scorpion alive for a considerable time in a small
bottle of water, in which is some growing weed. If we watch it moving
about, we shall see that the front legs are used for locomotion as well
as for seizing prey. Some authors doubt this. Any one may settle the
question for himself, if he will put one of these insects into a small
bottle with plenty of weed. Generally, however, the insect uses only
the second and third pairs for walking, the first pair being raised and
directed forward, with the tarsus bent at an angle (Fig. 42). Even when
it does use its front legs for locomotion, the action is not that of
walking; the insect employs these limbs to pull itself along in a sort
of ‘hand-over-hand’ fashion, but on a level surface it uses the first
pair in the same way as it does the other two pairs.
Its habit of burying itself in the mud may also be watched with very
little trouble. A common pudding-basin will make an excellent aquarium
for this purpose. The bottom is to be covered with garden mould and
vegetable _débris_, mixed with a few stones. The whole mass should
be arranged unevenly, so that when water is added it may not form one
sheet, but a series of small shallow pools. Very little duckweed will
serve to keep it sweet. It only remains to drop in the Water Scorpion.
Before long it will accommodate itself to its new surroundings, and so
bury itself that it will be no easy task to discover it.
An aquarium of the kind described stands at present on my
writing-table. Many have been the inquiries as to what kind of creature
lived therein; and more than one old hand at collecting has failed to
find the insect. It is always safe to look round the sides of the basin
for the breathing-tube; if it is not detected in this situation, a
glance along the surface of the tiny pools will probably show a break
in the film. The Water Scorpion will not be far off.
That the insect really does cover itself with mud may be demonstrated
by transferring it from the aquarium described to any shallow vessel,
and allowing a fine stream of water--say, from a dipping-tube--to fall
gently on it. The mud will be washed away, and in a few seconds will
settle at the bottom.
It is well to keep such an aquarium covered, for the Water Scorpion has
no mean power of flight. A circular plate of glass, which may be bought
for a few pence, makes the best cover, but a piece of fine muslin
fastened round the rim of the basin will do. Swammerdam says, ‘These
Water Scorpions live all the day in the water, out of which they rise
about the dusk of evening into the air, and so flying from place to
place, often betake themselves in quest of food to other waters.’ Then
follows a deduction which shows how far he was in advance of his time:
‘This affords us a satisfactory reason for the great number of insects
that immediately appear in the smallest collections of water, since
they may very well get thither when it is dark, so that the opinion
which ascribes to putrefaction the power of forming insects, &c., must,
by this instance of the Water Scorpion’s nocturnal transmigrations,
appear more and more frivolous and unnecessary.’
[Illustration:
FIG. 43.--_A._ Water Scorpion. _a_, rostrum;
_b_, wing-cases; _c_, wings; _d_ _d_,
second and third pairs of legs; _e_, raptorial legs (the
first pair); _f_, spine forming breathing-tube; _g_,
parasitic mite. _B._ Portion of an oviduct containing eggs
(magnified). _C._ An egg (magnified). _D._ Parasitic
mite. _a_, head; _b_, eyes; _c_, legs; _d_
_d_, legs (seen from under surface). (After Swammerdam.)]
No larva or other animal that is of any value should be kept in the
same aquarium with the Water Scorpion. Dr. Hill, who annotated the
English edition of Swammerdam, says, ‘There is not perhaps in all the
animal creation so outrageous or fierce a creature against those weaker
than itself as the Water Scorpion. It destroys, like the wolf among
sheep, twenty times as many as its hunger requires.’ The Rev. G. C.
Bateman placed one in a saucer with a tiny minnow; ‘but no sooner did
the little fish swim within reach of the fore-legs of the scorpion
than it was seized by them, and pressed against the hungry creature’s
rostrum.’ It seems to be particularly fond of Asellus, the water
woodlouse. I have often taken dead specimens of this crustacean, sucked
completely dry, from between the raptorial legs of the insect. One
specimen was so thoroughly cleaned out that it was mounted just as it
was, the only preparation being a brief soaking in spirit.
In examining the head the eyes may be readily distinguished, and on
pressing the rostrum or beak with a needle, the proboscis will be
forced out, just as one may force out the sting by pressing the abdomen
of a bee or a wasp. In this group the antennae are three-jointed and
concealed. When one begins to look for these organs he will probably
come to the conclusion that the concealment is highly effectual.
A very simple method of finding them in a spirit specimen is to take
the insect between the thumb and index finger of the left hand, holding
it up to the light in such fashion as to keep the first pair of legs
well clear of the head. Then, with a needle held in the right hand,
raise the thorax till it lies in the same plane as the body. Each
antenna lies in a groove beneath the eye. Gentle manipulation with the
needle will lift them out, so that they may be clearly seen, not only
with the hand magnifier, but with the unassisted eye. Or the insect may
be pinned down under water instead of being held in the hand, and the
antennae lifted or brushed out of the groove.
It will greatly simplify matters if, before attempting to raise the
wings, the fact is borne in mind that the tip of the right wing-case
lies over that of the left. It is perhaps as well to remove the
wing-case altogether by inserting a needle under it, and gently raising
it, using a little pressure in an outward direction. It will then be
quite time to raise the wing and to see the method of folding and
unfolding.
[Illustration: FIG. 44.--Raptorial leg of Water
Scorpion.]
In Fig. 44 we have a representation of the raptorial (front) leg
of this insect. This leg should be compared with the legs of other
Insects--not in pictures only, but in actual specimens--that we may see
how, while the general plan is preserved, different joints are modified
to suit the special function of this limb--that of taking prey. (See
also Fig. 43.)
The thigh (_f_) is the largest joint, for the obvious reason that
it contains the muscles that work the tibia and tarsus, which together
form a kind of knife-blade, shutting down into a groove in the thigh,
which may very well be compared to the handle of a pocket-knife. This
description, however, is not to be taken on trust. It is not enough
to read about the groove: we must see it for ourselves, raise the
‘knife-blade’ from the groove, and press it down again, and pass the
needle along the groove. If we examine the interior of the groove, we
shall find that there is a projection along the bottom, so that a cross
section would appear like this--W[image]. The inner portion of the
tibia is also grooved; so that when once the prey is seized by this
powerful limb, it has no chance of escape. The tarsus (_tar_)
is not clearly distinguished from the tibia (_tib_) in the
engraving, but it may be distinctly seen as a claw-like appendage in a
living or dead specimen.
The breathing-tube remains to be examined. It may be detached from the
body for more convenient manipulation. When this is done, the tube will
be seen to consist of two pieces, each grooved on the inner side and
set with hairs, which, as they interlock, prevent the entrance of water.
A somewhat similar arrangement occurs in the proboscis or tongue of
butterflies. There is perhaps a closer parallel in the antennae of
the masked crab, which, under certain conditions, form a kind of
breathing-tube, each antenna being joined to its fellow by the hairs
with which it is set.
There are two other Water Bugs which lend themselves to our purpose
very well. Each is popularly called Water Boatman, though that name is
better confined to Notonecta, because the insects of this genus ‘row
themselves about on their backs with their long feathered legs.’ In the
United States they are called Water Cicadae, from the shrill noise they
make, probably by rubbing the fore-legs together.
[Illustration: FIG. 45.--Water Boatman.]
In Fig. 45 the Water Boatman (_Notonec´ta glauca_) is represented
as seen from above--a position in which we shall rarely discover it,
if we keep it in a small aquarium. In Fig. 46 the same insect is shown
swimming on its back, or in the position it assumes when taking in a
supply of air. The end of the abdomen and the tips of the feet rest
against the surface-film; and at the slightest alarm a vigorous stroke
of the last pair of legs sends the insect to a place of safety. From
the way in which these insects habitually swim, Mouffet came to the
conclusion that it was probable men learned the art of swimming on
their backs from them.
[Illustration: FIG. 46.--Water Boatman swimming.]
Active as the Water Boatman is, it often falls a victim to the Water
Scorpion, if both are kept in the same aquarium. I learnt this fact by
experience, for having put two Water Boatmen into a small tank in which
was a Water Scorpion, I found both the former dead in the morning. It
was evident that their destroyer had had a good meal.
The only point to which attention need be called in examining the Water
Boatman is its method of taking in a supply of air. In looking at a
dead specimen we shall see a ridge or keel running down the middle of
the under side of the abdomen, and fringed with hair on each side.
A similar fringe runs along each side of the abdomen, thus forming
two passages along which the air taken in at the end of the body is
conveyed to the spiracles, the largest of which are on the thorax.
Corixa (Fig. 47) swims with its back uppermost, and when kept in the
aquarium may generally be seen foraging for small creatures--larvae or
worms--among the sediment at the bottom. Bateman, who kept a number
of these insects, says that he fed his specimens on garden worms and
pieces of raw meat. Mine have always foraged for themselves, and done
very well without feeding. They may often be seen to rub the short
fore-legs alternately across the front of the head, probably for the
purpose of producing a call-note. I have often watched them at this
practice, but have never been able to detect any sound. The defect is
evidently in my sense of hearing, for the sounds rest on undoubted
authority, and are coincident with the rubbing of the fore-legs across
the head.
[Illustration: FIG. 47.--Corixa, with wings expanded.]
Where sound-producing organs exist it is more than probable the
capacity for receiving sound-impressions also exists. Graber made some
interesting experiments to test the sense of hearing in Corixa. His
results are thus summarized by Sir John Lubbock[28]: ‘He placed some
Water Boatmen (Corixa) in a deep jar full of water, at the bottom
of which was a layer of mud. He dropped a stone on the mud, but the
insects, which were reposing quietly on some weeds, took no notice. He
then put a piece of glass on the mud, and dropped the stone on to it,
thus making a noise, though the disturbance of the water was the same.
The Water Boatmen, however, at once took to flight.’
CHAPTER IV
SPIDERS, MITES, AND MYRIAPODS
In this chapter we shall take examples from two Classes--the
Arachnoi´dea and the Myriap´oda. To the first-named Class belong also
the Scorpions, which, with the Book Scorpions, the Harvestmen, and some
others, may be neglected here. This will leave us only the Spiders and
Mites to deal with.
Every one knows a Spider when he sees one, though not every one can
give offhand a definition that shall include the whole Order. Let us
endeavour to express their characters in simple terms, keeping in mind
the definition of head, thorax, and abdomen in Chapter II. In Spiders
the head and thorax are joined together in one unsegmented portion,
called the cephalothorax, and this is connected with the abdomen, which
is also unsegmented, by a more or less slender stalk or peduncle. So
that while Insects have three regions, Spiders have but two. We may
express the difference thus:
Insects head, thorax abdomen.
\---v---/[image]
Spider cephalothorax abdomen.
The cephalothorax bears six pairs of appendages (Fig. 48A).
Taking these in order, there are--
1. A pair of falces (_an_), representing the antennae of
insects. These have a movable claw-like joint at the extremity,
perforated so as to convey into a wound the secretion from the
poison-glands.
2. A pair of five-jointed pedipalps (_p_), sometimes called
maxillary palpi, from the fact that the parts whence they spring
correspond to the maxillae of insects (Fig. 21A,
_d_). In the female the last joint terminates in a claw; in
the male this joint is specially modified for sexual purposes
(Fig. 48B).
3. Four pairs of walking legs, of which only the coxae
(_c_) are shown in the diagram. The two front legs are
often used as organs of touch.
Breathing is carried on by chambered air-tubes or lung-sacs, and in all
our British Spiders (with the single exception of the native Trap-door
Spider, which, by the way, does not make a trap-door) there are also
air-tubes resembling those of Insects. The lung-sacs open to the
external air by stigmata (_st_) on the abdomen, while the ordinary
air-tubes open near the spinnerets (_sp_), the organs employed in
the production of the silky threads from which are formed webs, nests,
egg-bags, and ropes. By means of these threads, spiders drop from their
webs to the ground, construct flying bridges from one point to another,
and even in some sort travel through the air.
Spiders live upon the juices of their prey, which are drawn up into the
stomach by means of a suctorial apparatus at the end of the gullet.
The young, when they leave the egg, resemble their parents in all
respects but size--that is, they undergo no metamorphosis.
The eyes of spiders are simple, and six or eight in number. These vary
much in size and relative position, and furnish characters of great
importance in classifying species. Those of the Hunting Spiders, which
make no web, but pursue or spring upon their prey, are usually arranged
in three rows; while those which make webs for the capture of prey have
the eyes in a double row. In all cases they are on the front part of
the upper surface of the cephalothorax.
[Illustration:
FIG. 48.--_A._ Scheme of under surface of Wolf
Spider (female). _B._ Pedipalp of male (enlarged). For
other references, see text.]
The Common Garden Spider (_Epei´ra diade´ma_) is a very good
subject, and there can be no difficulty in procuring any number of
specimens. The smallest garden will be sure to yield a plentiful
supply; and even if we have no garden, a very superficial search
among the hedgerows will give us as many as we can possibly want.
Every one knows this spider, and the beautifully regular web which
it makes. First of all, what one may call the outline of the web
is spun--strong threads stretching from point to point, to which
are attached lines radiating from a common centre. This may be
represented diagrammatically by drawing a circle and producing radii
from the centre to the circumference in all directions, or by making a
rectangular figure and drawing lines to the boundaries from the point
where the diagonals intersect. It must not, however, be supposed that
the outline of the web is of a regular form. In this respect the spider
adapts itself to circumstances, and spins a greater or less number of
supporting threads, as may be necessary.
[Illustration: FIG. 49.--Garden Spider and Web.]
Then the spiral is made (Fig. 49) from the centre to the circumference.
The first few turns are of the same character as the radial lines; but
all the rest of the short lines forming the spiral, and connecting
the radial lines, are coated with a viscid secretion, which is the
essential part of the snare; for the victims are really limed like
birds on a twig, not entangled in the threads. It is well to test the
character of the different parts of the spiral, not only by touch,
which is convincing enough, but with pocket lens. Our low powers will
not give such results as are shown in Fig. 50, but we shall have no
difficulty in distinguishing the sticky parts from those not coated
with the secretion.
[Illustration: FIG. 50.--_A_ Adhesive Threads of
Spiral. _B._ Non-adhesive Radial Threads.]
It may be doubted whether all the Spiders of this genus make the
spiral in the same way, for observers differ in their description of
what they have seen. Some say that a non-adhesive spiral from centre
to circumference is first made, and that the spider then moves ‘in a
closer spiral from the circumference inwards, biting away the former
spiral, replacing it by another, which is viscid and adhesive[29].’ Dr.
Butler, on the other hand, who ‘watched spiders for months together,
petting, feeding, and trying experiments with them every morning,’
after describing the making of the first and non-adhesive part of the
spiral, says, ‘This line is not carried to the boundary, but at some
distance from the centre a second is commenced, formed of extremely
viscid silk, upon which the gummy secretion is distinctly visible,
with the aid of a lens, in the form of closely approximated globules
of amber-coloured glue. It is said that when the viscid lines are
completed the spider cuts away the unadhesive lines, but this I have
never observed[30].’ My own observations lead me to believe that Dr.
Butler is correct in his description. Though I have often watched,
I have never seen a spider ‘biting’ away any part of its web, nor
would the falces appear to be adapted for such an operation. They are
piercing, not cutting, weapons.
The spider may be watched at leisure, if put into a bottle of moderate
size, the top of which should be covered with muslin or calico to
prevent escape. Here it is fairly easy to bring the pocket lens into
play, and to distinguish the different parts of the animal. The eyes,
and their arrangement, should be particularly noticed.
[Illustration: FIG. 51.--Anchorage of Web.]
Blackwall, in support of the position that in making their webs
spiders are guided by touch rather than sight, says, ‘Various species,
when confined in spacious glass jars placed in situations absolutely
impervious to light, construct nets which do not exhibit the slightest
irregularity of plan or defect of structure[31].’ My specimens have
always been kept in the light, and in small bottles rather than
spacious jars, but I have never seen spiders attempt to make a
geometrical web under such conditions.
A bottle which for some months served as a prison-house for a Garden
Spider now stands on my writing-table. Its sides are marked by hundreds
of ‘anchorages’--but the threads cross and recross, forming in some
parts a kind of sheet, and in others a tangled mass. Some of these
threads must have been covered with viscid secretion, for flies were
limed, and so fell a prey to the spider. Their dried skins are dotted
about among the threads, and the spider itself perished long ago from
cold. But I keep the bottle as a curiosity, to show that these spiders
do not always make geometrical webs.
When one has a Garden Spider in a bottle, it may be observed to
practise a curious and very effective method of disabling its prey.
If a bluebottle or any other large fly be dropped and entangled among
the threads, the spider will envelop it in a sheet of web. This is
how Blackwall describes the operation: ‘Causing the victim to rotate
by the action of the third pair of legs and the palps, the first pair
of legs also being frequently employed in a similar manner, they
extend the spinners laterally, and applying to them alternately the
_sustentaculum_ of each posterior leg, they seize and draw out
numerous fine lines in the form of a fillet, which they attach to their
revolving prey, and thus involve it in a dense covering of silk from
one extremity to the other. By means of this stratagem they are capable
of overcoming formidable and powerful insects, such as wasps, bees, and
even large beetles[32].’
The operation does not occupy much time; in a very few minutes the fly
is swathed in a silky covering as completely as an Egyptian mummy in
its linen folds. Of course resistance, much less attack, is out of the
question, and when it is thus rendered powerless for harm the spider
proceeds with its meal.
The _sustentac´ulum_--or support--is a strong movable spine near
the end of the tarsus, on the under side of each of the last pair of
legs. These spines act in opposition to the claws, and thus these
animals are enabled to hold with a firm grasp such lines as they have
occasion to draw from their spinners with the feet of the hind legs,
and such also as they design to attach themselves to.
With regard to this method of swathing prey, Hudson[33] says of
an Argentine spider, ‘that its intelligence has supplemented this
instinctive procedure with a very curious and unique habit. The
Pholcus, in spite of its size, is a weak creature, possessing
little venom to dispatch its prey with, so that it makes a long and
laborious task of killing a fly. A fly, when caught in a web, is a
noisy creature, and it thus happens that when the Daddy longlegs--as
Anglo-Argentines have dubbed this species--succeeds in snaring a
captive, the shrill outrageous cries of the victim are heard for a
long time--often for ten or twelve minutes. This noise greatly excites
other spiders in the vicinity, and presently they are seen quitting
their webs and hurrying to the scene of conflict. Sometimes the captor
is driven off, and then the strongest or most daring spider carries
away the fly. But where a large colony are allowed to continue for a
long time in undisturbed possession of a ceiling, when one has caught
a fly he proceeds rapidly to throw a covering of web over it, then,
cutting it away, drops it down and lets it hang suspended by a line at
a distance of two or three feet from the ceiling. The other spiders
arrive on the scene, and after a short investigation retreat to their
own webs, and when the coast is clear our spider proceeds to draw up
the captive fly, which is by this time exhausted with its struggles.’
In this connexion Hudson notes that spiders are attracted by the sound
of the vibration of a string or wire, thinking it made by an insect
that has been caught; and he says that the stories of tame spiders
are founded on a misunderstanding of the motive that brings the
animal down. We may well doubt if spiders are attracted by music, but
that some spiders possess a sense of hearing, or something analogous
thereto, seems to be proved by the existence of stridulating organs in
certain members of the group.
[Illustration: FIG. 52.--Foot of Garden Spider.]
[Illustration: FIG. 53.--Spinnerets of Garden Spider.]
Two other points remain to be noted. The feet should be examined,
for their structure throws some light on the way in which the Garden
Spider runs so securely to and fro on the radial lines. One of the
legs of a dead specimen should be detached, and its armature of spines
and hairs noted. The foot is armed with three stout claws, which
are pectinated--that is, toothed like a comb, and beneath them are
smaller ones, sometimes described as toothed hairs. It will be easy
to understand what a secure foothold these claws give the animal as it
runs backwards and forwards over the radial lines; for if the web be
shaken by the wind or designedly its owner can take a firm hold with
each foot, and thus have eight separate points of attachment. An inch
hand lens is quite sufficient to give a general idea of the hairy and
spinous clothing of the legs and the position of the claws; but to see
the teeth a higher power--a half-inch--will be necessary.
It is a good plan to choose a freshly killed specimen for the
examination of its spinnerets. If the spider is laid on its back in a
glass dish, gentle pressure on the abdomen away from the cephalothorax
will cause the material secreted by the spinning-glands to ooze out.
This, however, cannot be done if the specimen has been for some time
in spirit. We shall be able to make out six tubes (four of them larger
than the other two) grouped round the anal aperture; but, for the
present, we must take for granted the fact that these are made up of
a number of smaller tubes, so that the end of each spinneret is not
unlike the rose of a watering-can. A good half-inch will give some
indications of this rose-like appearance.
The Jumping Spider (_Sal´ticus sce´nicus_) belongs to a family
the members of which make no web or snare for the capture of prey, but
trust for their food-supply to their skill in stalking flies and other
insects, which they capture by a sudden spring. It is from this habit
that the type-genus and the family have received their scientific and
popular names.
The common British species is extremely abundant, and wherever flies
are plentiful these spiders will not be very far off. This is the case
not only in the country, but also in London and other large towns. It
is a noticeable spider from its coloration--black marked with white.
The eyes are eight in number; though the artist in our illustration
has only represented four. The centre two of the front are the largest
of all, and have been compared not inaptly to bull’s eyes. Two others
are placed on each side of the cephalothorax, so that the eyes form,
roughly, three sides of a square, and the central eyes in the lateral
lines are the smallest. We may represent them thus [image], while the
eyes of the Garden Spider are disposed in this fashion [image].
[Illustration: FIG. 54.--Jumping Spider. (Line shows
natural size.)]
[Illustration: FIG. 55.--Falces of Male Jumping Spider.]
It requires some little practice to detect the eyes of spiders and to
remember their position in the different genera, but by examining every
spider met with, and consulting some handbook to the group, one will
soon be able to determine the commoner British species.
With a couple of needles we may remove the falces (Fig. 55) for
examination; and there is no difficulty in mounting them, as shown in
the cut, on a piece of card, so that they may be compared with the same
weapons in other species. For example, the falces of the Garden Spider
differ from those of the Jumping Spider in that they are short and
stout, and the teeth on the basal joint are more in number.
The following account of the habit of this spider in capturing prey is
from Kirby and Spence’s _Introduction to Entomology_ (ed. 1870, p.
453):--‘When these insects spy a small gnat or fly upon a wall, they
creep very gently towards it with short steps, till they come within a
convenient distance, when they spring upon it suddenly like a tiger.
Bartram observed one of these spiders that jumped two feet upon a
humble-bee. The most amusing account, however, of the motions of these
animals is given by the celebrated Evelyn in his _Travels_. When
at Rome, he often observed a spider of this kind hunting the flies
which alighted upon a rail on which was its station. It kept crawling
under the rail till it arrived at the part opposite to the fly, when
stealing up it would attempt to leap upon it. If it discovered that
it was not perfectly opposite, it would immediately slide down again
unobserved, and at the next attempt would come directly upon the
fly’s back. Did the fly happen not to be within a leap, it would move
towards it so softly, that its motion seemed not more perceptible
than that of the shadow of the gnomon of a dial. If the intended prey
moved, the spider would keep pace with it as exactly as if they were
actuated by one spirit, moving backwards, forwards, or on each side
without turning. When the fly took wing, and pitched itself behind the
huntress, she turned round with the swiftness of thought, and always
kept her head towards it, though to all appearance as immovable as one
of the nails driven into the wood on which was her station: till at
last, being arrived within due distance, swift as lightning she made
the fatal leap and secured her prey.’
This spider employs a clever precaution against falling, when about to
spring upon its prey. It applies the end of the abdomen to the surface
on which it stands, and opening its spinnerets, makes an ‘anchorage’
(Fig. 56). Then, by the act of springing, it draws from the spinnerets
a line attached to the spot from which it started. This line is strong
enough to support the spider in case it misses its prey.
[Illustration:
FIG. 56.--_A._ Foot or Jumping Spider. _B._
Scopula. (Much enlarged.)]
The foot of the Jumping Spider is extremely interesting, and shows
a very ingenious arrangement, by means of which the animal can run
with difficulty on smooth polished upright surfaces, or retain its
footing when it alights on such surfaces after a spring. We can see
this arrangement in Fig. 56. Just behind the claws is a bundle of
coarse hairs, technically called a _scop´ula_, or little brush.
From these hairs adhesive matter flows, and in this fashion the spider
literally sticks on. With this brush of hairs may be compared the
tarsal cushions of many flies, and the adhesive hairs of Dytiscus
and other beetles (see Chap. II)--though these last have a different
function.
Diving Spiders (_Argyrone´ta aquat´ica_) are especially suitable
for our purpose. They are very common in most ponds, and in some
places are so abundant as to be almost a nuisance to the collector who
is in search of other things. Three of them are represented in Fig. 57.
One is swimming; another is just entering the bell-shaped web under
water; and the third is just climbing out of the water on to the broad
floating leaves of the water crowfoot.
[Illustration: FIG. 57.--Diving Spiders.]
De Geer’s account of these spiders is extremely interesting[34], and
we may verify it for ourselves, for these creatures may be kept without
any trouble. They certainly agree, when a number are kept in captivity,
much better than do other spiders. De Geer kept several in the same
aquarium, and says that when they met they felt each other with their
palps, and opened their falces, but he never saw them do any harm to
each other. I have kept them under similar conditions with the same
result.
He tells us that these spiders spin under water a cell of strong,
close, white silk, the shape of which he compares to a skull-cap, the
half of a pigeon’s egg, or a diving-bell. In September, 1736, he first
became acquainted with these creatures, and kept one in an aquarium for
four months. It made its cell against the side of the aquarium, and the
top of it rose above the surface of the water. (This was due to its
being inflated. The web was not spun above the surface.) The walls of
the cell were very thin, but it was filled with air, and the spider was
resting inside, head uppermost, with its legs pressed against the body.
About three months later he found that the mouth of the cell was
closed, and the spider was comfortably settled in its winter quarters.
When pressure was applied the cell burst and the air escaped, mounting
up to the top in bubbles. The spider made its way out, and took an
Asellus that was offered, and made a meal of it.
De Geer then came to the conclusion that these cells under water were
constructed for the purpose of affording the spider a retreat when the
water was frozen over, so that they could not come to the surface
for a supply of air. He found, however, by observation, that they
were also made in summer by both sexes. In a cell of this kind the
female deposits her eggs--from eighty to a hundred in number, enclosed
in a cocoon of white silk--and keeps guard over them, with her head
defending the entrance to the cell.
He succeeded in finding out the method by which the Diving Spider fills
its cell with air. He noticed that when the creature was moving about
in the water, its body was covered with a layer of air, and that this
air was renewed from time to time when the animal came to the surface
and raised its abdomen above the water. Loaded, so to speak, with air
in this fashion, the spider descended, and entered the cell backwards,
leaving an air-bubble. Having repeated this several times, at last she
removed all the water from the cell, introducing in its place an equal
quantity of air.
It is very easy to watch the Diving Spider making its dwelling under
water, and filling it with air. First of all the web is woven in a
bell or thimble shape between the sprays of water-weed, or against the
bottom or side of the aquarium. It is curious to notice how practices
that must be necessary when the creature is at liberty are continued
in captivity where they are useless. A web constructed in running
water, or even in a pond or ditch, is liable to be swept away or to be
emptied of air by a very slight current, so its owner has recourse to
a system of guys and supporting threads, which are not required when
the spider is safely housed in a small aquarium. Nevertheless, the guys
are made. In an eight-ounce bottle I have now a male Diving Spider,
which has lived there for about seven months. Its cell is made between
the whorls of a spray of milfoil, and guy threads have been carried to
no less than five whorls--two above and three below the opening. Now
that it is filled with air, the cell gleams in the water like a great
bubble of quicksilver. The air may be expelled by shaking or tilting
the bottle, and if the web be not damaged the spider will generally
refill it with air, though sometimes it prefers to make a new dwelling.
Fig. 58 shows the cell of a Diving Spider; the white lines represent
supporting threads attached to the water plants.
In examining dead specimens we shall find that, contrary to what is
usual, the male exceeds the female in size. I have a slide of a male,
with the legs spread out before and behind, and the measurement from
the claws of the first pair of legs to those of the fourth pair is 1¾
inches. The body is ¾ inch long. The whole surface is more thickly
clothed with hair than is the case with other spiders, and the reason
for this is obvious. This hairy body-covering serves to carry down into
the water a layer of air, and the fringe of hair on the legs makes them
efficient swimming organs.
[Illustration: FIG. 58.--Cell of Diving Spider.]
The Order of Mites will yield us subjects for our pocket lens. Mites
are related to Spiders, but form a distinct Order. Like the Spiders,
some are aquatic, though the most of them live on land. Many are
parasitic, during the whole or part of their lives subsisting on
the juices of their hosts: the food of others consists of organic
_débris_, and these seem to be of benefit to man, since they act
as scavengers. If we turn to page 96 we shall there find noted the
points of difference in the arrangement of the main divisions of the
body in Insects and in Spiders. In Mites the distinction between the
cephalothorax and abdomen is lost, and the body is more or less oval or
globular. In the perfect forms there are eight legs; but some, in their
earlier stages, have only six. The mouth may be adapted for biting,
though it is usually suctorial. In the Cheese Mites and some others
breathing seems to be carried on through the skin, for there are no
air-tubes; but in most Mites air-tubes, with two stigmata, are present.
If we take a dip with the collecting-net in almost any pond we shall
be pretty sure to capture some specimens of Water Mites of the genus
Hydrach´na, easily recognizable by their bright coloration, their eight
swimming legs thickly fringed with hair, and their unceasing activity.
They may be kept in a bottle, or other small vessel, with some
water-weed, and will forage for themselves. In Fig. 59 we shall see
the points we have to look for in examining a Water Mite with a pocket
lens. There should be no difficulty in making out in the specimen all
the details shown in the cut.
It may be that they will breed: if so we should avail ourselves of
the opportunity of watching their development. Their life-history is
somewhat curious, and is specially interesting from the fact that
while Swammerdam had some faint perception of the true meaning of what
he saw, De Geer, writing a hundred years later, drew entirely wrong
conclusions from similar observations. It was left for Dugès to clear
up the matter in the _Annales des Sciences Naturelles_, 1834.
Before summarizing the account of the French naturalist it may be well
to quote what Swammerdam and De Geer have said on the subject:--
‘There is nothing more remarkable in this insect [the Water Scorpion]
than that it constantly appears covered with a prodigious number of
nits of different sorts and sizes, though perhaps we may with more
reason consider them as so many little creatures, which live and grow
by sucking the Scorpion’s blood. These are somewhat of an oblong
figure, approaching to round, and have a shining, and as it were
bloated, surface, without any of the rings observable in most insects.
The neck is oblong and shaped like a pear, with the small end sticking
in the Scorpion’s body. The colour of this insect is a mean between
that of vermilion and purple; and when it is pretty well grown there
appears within it an elegant transparent spot or particle (Fig. 59).
[Illustration: FIG. 59.--Red Water Mite (nat. size, and
under surface magnified).]
‘This spot or particle induced me to consider with more attention
this minute and hitherto unregarded insect, and even to undertake the
dissection of it. But who would imagine that on this examination it
should prove a perfect and surprising insect? This is, however, a
certain fact; and thus in that infinite variety of works, by means of
which God is pleased to make Himself known to us, we ever meet with new
matter of admiration and astonishment.
‘This little creature being extracted from the shell that covered it,
looks like a young spider before it has left its egg. On the fore part
is the head (Fig. 43D, _a_) and on its head are the eyes, _b_: under
the eyes are placed its little legs elegantly coiled and folded, _c_
_c_; but they appear much more distinctly on turning the insect on its
back, _d_ _d_; and in this situation also it best appears with what art
these legs are laid up in the shell, and all are covered with hair.
The colour of this little creature is, as I have already observed, a
mean between that of vermilion and purple; and this colour shows itself
through the coat or shell, which is transparent. I cannot determine to
what species of insects this is to be referred; nor can I say to what
size it grows, or by what kind of creature it is thus deposited on the
Water Scorpion in the form of an egg, there to receive life and growth.
Nevertheless, I cannot but look on the discovery I have made as very
interesting, since it proves that there are in the nature of things
eggs which acquire a sensible growth by an entraneous nourishment,
unless perhaps some naturalist should choose to consider this as a
complete insect, rather than as an egg; nor shall I strenuously oppose
his opinion, seeing that, in all cases, the egg is in reality no
other than the insect itself, which remains in that state till it has
acquired sufficient strength to break its prison, and live without such
a covering[35].’
Having quoted Swammerdam, let us see what De Geer has to say on the
subject:--
‘On the body and legs of many aquatic insects, such as Dytiscus and
Water Scorpions, may be frequently seen little oval, seed-like bodies,
of a bright red colour, firmly attached, and, as it were, implanted
in the skin, by a little stalk. I have had Water Scorpions with the
upper surface so covered with these red bodies that there was scarcely
a vacant space on the skin. They are most frequently to be seen in
the spring; but the insects on which there was such a great number
did not live long with me. Having crushed some of these seed-like
bodies, I found them filled with red liquid matter. I am convinced, by
experiment, that they are the eggs of Water Mites, since there came out
of them little red Mites with round bodies and long legs, which swam
about with great swiftness.
‘These red Water Mites, then, attach their eggs to the bodies and legs
of larger aquatic insects, and there they remain till the young are
hatched. And since we find eggs of many different sizes, we may be
sure that they grow and increase in size, doubtless owing to a certain
nutritive juice which passes from the body of the insect into the egg.
Hence it is, as I have seen myself, that Water Scorpions loaded with
these eggs become weak and feeble, because they are obliged to furnish
their hangers-on with nourishment from their own bodies. Moreover,
these eggs appear to cause the Water Scorpions some irritation or
uneasiness, since I have often seen them rub with their feet those
parts of the body where the eggs were; and perhaps they did this with
the view of rubbing them off, but they were too securely fastened[36].’
Dugès watched the development of the common Red Water Mite (Fig. 59),
and tells us that towards the end of May the females deposit their eggs
in the leaves of pondweed, which they puncture with their beaks. The
larva (Fig. 60), red in colour, with six legs, is free-swimming, and
has a large beak, which looks like a great head, and terminates in a
narrow mouth. It is not known how long this larval stage continues;
but in the next stage (Fig. 61) the Mite becomes parasitic on aquatic
beetles and bugs, fastening its beak into the body of its host,
from which it derives its nourishment. The legs and palps are often
retracted or absorbed, so that it is not difficult to understand
how it was Swammerdam and De Geer took these parasitic nymphs to
be the eggs of the Mite. During their parasitic condition they
increase considerably in size, at last emerging as adult eight-legged
free-swimming Mites. It was just before the emergence of the Mite
that Swammerdam examined the parasitic nymph, for he figures the
‘insect,’ which he extracted from the egg, as having eight legs (Fig.
43D).
[Illustration: FIG. 60.--Larva of Water Mite.]
[Illustration: FIG. 61.--Nymph of Water Mite.]
I once found a Water Mite in the body of a Dytiscus[37]. I was
breaking up the beetle, and had removed the elytra and the wings. I
only wanted the external skeleton; so a slit was made between the
plates of the dorsal and ventral surface, and the intestines removed.
The Mite was embedded in the fat-body. I could find no mention in the
literature of Beetles or Mites of any similar occurrence; and should
scarcely have mentioned it here, had I not been unexpectedly confirmed
by my friend Mr. G. E. Mainland, F.R.M.S., who once had a similar
experience, and who kindly allows me to quote from a letter he sent me
on the subject:--
‘I am sorry to say I can find no documentary evidence as to the
Arachnid I found embedded in the tissues of Dytiscus, but a good deal
has come back to my recollection. On removing the right elytron and
slitting up the dorsal surface, I found it in the tissue close up to
the thorax. I cannot recollect what ultimately became of it, after I
had shown it to friends at the Hackney Microscopical Society.... I know
that I carefully measured the Hydrachna (which was abnormally large)
and its relative size to that of its host, and made a comparison in a
lecture (to the Senior Band of Hope at St. Michael’s, Hackney) of a man
with a creature as large as a guinea-pig under his shoulder-blade.’
The occurrence of the Mite _inside_ the Beetle was, of course,
quite exceptional. It probably found its way in through one of the
abdominal spiracles.
The Beetle Mite (_Gam´asus coleoptrato´rum_) (Fig. 62) is
extremely common, and is parasitic on the Dung Beetle and on the Humble
Bee, so that in order to examine the parasite we must capture the host.
There can be no difficulty about this, for Dung Beetles and Humble Bees
are plentiful enough. This Beetle Mite, apparently, does not infest
other species of beetles. I have kept the Devil’s Coach Horse in a
bottle with the common Dung Beetle for some months, and though the
latter swarmed with these parasites, they never left their host for
the other beetle. Even when removed by means of a small brush from one
beetle to the other, they left the Devil’s Coach Horse of their own
accord, and soon made their way back to the Dung Beetle.
[Illustration: FIG. 62.--Beetle Mite.]
These parasites, with their host, came into my possession in a strange
way. A friend, who knew my hobbies, told me that he had managed to
procure for me some young beetles just born. I ventured to suggest that
beetles were not born as beetles, but in quite a different shape. My
suggestion was received unsympathetically, and I was told that I should
alter my opinion when the creatures were sent me. But I did not. The
box contained a Dung Beetle, over which were swarming scores of these
little Mites, and I had some difficulty in convincing the gentleman
who sent them to me that these Mites were not the young of the beetle.
We should compare this Beetle Mite with the Water Mite, and notice the
difference in the mouth parts and the legs, which have a large pad
between the claws.
We may find another Beetle Mite, closely allied to this species, on the
Devil’s Coach Horse, and some of its near relations. This Mite was also
known to De Geer[38], whose remarks upon it are worth quoting, in a
condensed form.
He found a beetle covered with these Mites, and on examining them with
a hand lens saw that they were attached to their host by a long thread
or stalk, which came from the posterior end of the body. Several Mites,
he tells us, were joined together by one thread which fastened them all
to the beetle; and he came to the conclusion that the parasites were
nourished at the expense of the beetle, the thread serving to convey
the juices of its body to them.
‘It is very singular,’ he says, ‘to see living insects planted on the
body of larger insects, from which they draw their subsistence by means
of a thread or stalk.’ And then he goes on to compare these ‘vegetative
Mites,’ as he calls them, with the ‘eggs’ of the Water Mites, which
he found on Dytiscus and the Water Scorpion. The thread exists, and
the Mites are attached by it to their host, but they do not draw
nourishment through it from the beetle, for it is composed of their
excrements.
The Myriapods are worm-shaped creatures, breathing by means of
air-tubes, and furnished with a number of limbs closely resembling
each other. There are two groups: the Centipedes and the Millepedes.
The former have the body flattened, with one pair of appendages to each
segment, the first pair being modified into piercing poison-organs, and
they feed on living prey. The body of the Millepedes is round, with two
pairs of appendages to each segment; they have no poison-organs, and
their food consists chiefly, if not entirely, of vegetable matter.
There seems to be some doubt, however, as to whether _Ju´lus_, one
of the commonest Millepedes, does not occasionally indulge in animal
food. In _Nature Notes_ (Jan. 1896) there was a review of the
_Cambridge Natural History_ (vol. v). The reviewer, in a brief
summary of Mr. Sinclair’s part of the book (the Myriapods), said,
after describing the Centipedes: ‘The millepedes, on the contrary, are
sluggish vegetarians, with hard, cylindrical bodies, &c.’ On this a
correspondent wrote in the March number: ‘Some time ago my attention
was attracted to a large earthworm, writhing and twisting about on
the garden path, as though in pain, or through having received some
injury. On examining it more closely to ascertain the cause of its
unusual movements, I found that a millepede had fastened itself to the
side of the worm, and appeared to be boring or eating its way into
the body, whilst the most violent efforts on the part of the worm
were ineffectual in shaking off its antagonist. If the millepede is
a vegetarian, what could be its object in attacking so harmless and
defenceless a creature as the earthworm? The above, which I take to
be a millepede, is the black or dark-coloured creature “with hard,
cylindrical body” ordinarily found coiled up in a spiral under stones
or rubbish.’
The editor, as a matter of course, referred the matter to the writer
of the review. His reply was as follows: ‘If there is no mistake
about the identity of the aggressor in the account cited above, the
observation is one of considerable interest; for, so far as we are
aware, it is the only case on record of a millepede being guilty
of such conduct. But were it not for the positive statement that
the species was the dark-coloured creature with a hard, cylindrical
body, which is ordinarily found coiled up in a spiral under stones
or rubbish--a description which exactly applies to the millepedes of
the genus _Julus_--we should have concluded without hesitation
that the struggle in question was merely one of those that habitually
takes place between the centipedes of the genera _Litho´bius_ or
_Geoph´ilus_ and the earthworms upon which they feed.’
Both the Centipedes and Millepedes are shy, light-shunning animals,
and if we turn over some stones in the garden or in a walk through the
fields we shall probably find specimens enough to serve us in getting
some idea of both groups.
The Centipedes are sometimes called ‘Hundred-legs,’ but this implies
the possession of many more legs than the creatures really have. In
Norfolk and Suffolk the people call them ‘Forty-legs,’ and this is much
nearer the mark.
_Litho´bius forfica´tus_, about an inch long and rufous brown in
colour, is extremely abundant under stones and the bark of trees, and
in cellars and outhouses. These animals run with great rapidity when
disturbed, so that one needs to be on the alert to seize them when
they are driven from the places in which they lurk. The body has nine
principal and six subsidiary or smaller rings, and there are fifteen
pairs of walking legs, besides the first pair, which are modified to
serve as poison-organs. De Geer says that he never dared to pick up
these Centipedes with ungloved hands, because he had seen a fly, which
had been bitten by one, die on the spot, ‘which seems to be a sign that
their bite is venomous.’ He examined their modified legs with a good
microscope, but could not distinguish any opening. There is, however,
an opening, as De Geer suspected, though he could not distinguish it;
it lies near the point, and we may also trace the canal through the
claw down to the poison-glands which lie, one on each side, at the base
of the claws. The mouth parts resemble those of insects, and may be
dissected out in the same way. When this Centipede walks, says De Geer,
it does not use the last four pairs of legs, but drags them after it;
but when it walks backwards, which it does as well as forwards, it then
makes use of these four pairs of legs in the same way as the others. If
we keep Lithobius alive we shall see that it can walk backwards, though
it can scarcely be said to go as well one way as the other. From the
same old writer some useful hints as to the method of keeping these
animals may be gathered. Those that he kept in a vessel without any
moisture soon died, and were quite dried up in twenty-four hours, which
will teach us to keep them in a vessel with damp earth, shaded from
light and heat.
Dr. Sharp[39] gives some interesting details about the breeding habits
of Lithobius, and describes, for the first time, the uses of the two
hooks on the under surface of the body of the female.
He experimented with Centipedes and Millepedes. Keeping them in large
shallow glass vessels, the bottom of which was covered with a layer
of earth, he fed the specimens of Lithobius on insects and worms, and
sometimes on raw chopped meat, but they did not thrive on this as they
did on prey which they could kill for themselves.
[Illustration: FIG. 63.--_A._ _Lithobius
forficatus._ _B._ Mouth parts seen from below (After
Graber.)]
Lithobius, he tells us, lays but few eggs compared with the number
deposited by Julus. Each egg, as it leaves the oviduct, is received
by the hooks mentioned above, and by means of them it is rolled on
the soil till a covering of earth adheres to the viscous material
with which the egg is coated. The male considers the eggs special
delicacies, and devours them whenever he has the opportunity. It is to
prevent this that the female covers them with earth, so that the male
may not recognize them.
Geophilus is a much longer animal than Lithobius, for its body may
consist of from 80 to 180 rings. The species have no eyes. Several of
them are common, especially in the south of England, and possess, as
do many genera of the same family, the property of phosphorescence,
whence have arisen the stories of ‘luminous earthworms’ current from
time to time. _Geophilus crassipes_ is the form most frequently
captured when displaying its light. It is from one inch to two inches
long, of a reddish-orange colour, and somewhat worm-like in shape. Mr.
Pocock, of the British Museum (Natural History), says: ‘The property
of luminosity lies in an adhesive fluid secreted by glands which open
upon the lower surface of the body, and the power of discharging or
retaining the fluid appears to be entirely under the centipede’s
control. The phenomenon is observable during the autumn months, from
about the middle of September to the end of November, and although its
significance is not clearly understood, it is generally believed to be
connected with the pairing of the sexes[40].’
During a visit to the seaside it is well to look out for specimens of
a marine Centipede, which is, apparently, not very common. It is said
to occur ‘under stones and sea-weeds on the shore at or near Plymouth’;
and in 1895 I had the good fortune to meet with one at Bexhill.
This Centipede does not live in the sea, but will survive prolonged
immersion in salt water. It is far too valuable for us to pick to
pieces, so that, if we should have the good fortune to meet with one,
we should carefully examine it, making what notes are necessary, and
then pickle the specimen and send it to the British Museum, Cromwell
Road, S.W. The tube should be labelled with the place and date of
capture, and it should be stated on the label whether the animal was
taken above or below high-water mark.
The Common Millepede--often miscalled the wireworm--is readily
distinguished by the absence of poison-claws, and its cylindrical
worm-like appearance. De Geer, who of course adopted the Linnaean
definition of ‘Insects,’ says of the Millepedes of the genus Julus, to
which our Common Millepede (_Ju´lus terres´tris_) belongs: ‘They
form, as it were, the last link of the chain which unites the class of
Insects to that of Worms, for the body is elongated and cylindrical;
and though they have a great number of feet, these are so short, that
when these animals walk, they seem rather to glide along after the
fashion of legless worms[41].’
[Illustration: FIG. 64.--The Common Millepede.]
Dr. Sharp says that these animals do very well in confinement, and he
found that sliced apples and grass formed the best food for them. He
watched the process of nest-making and egg-laying, and these creatures
are so abundant, and the necessary appliances so simple, that we may
follow his example and see it for ourselves. His arrangements were the
same as for Lithobius, and he saw the female make a hollow sphere of a
bit of earth, stuck together by the secretion from the salivary glands,
and smooth on the inside. A small hole was left on the top, and through
this she passed in from 60 to 100 eggs, closing the aperture with earth
moistened with the salivary secretion. The eggs were hatched in about
twelve days. The young of all the Myriapods when they leave the egg
have but three pairs of legs, but the number of limbs and segments is
increased at each successive moult.
Having watched our Millepedes in confinement, it will be well to take a
preserved specimen and examine it carefully with the pocket lens, so as
to compare it with the Centipede; then to compare both with the common
earthworm, and to note the points of likeness and of difference. The
dark spots on each segment in the illustration show the stigmata.
[Illustration: FIG. 65.--Segments of Millepede
(magnified).]
CHAPTER V
CRUSTACEANS.--PRAWN, SHRIMP, MYSIS, CRABS;
AMPHIPODS; ISOPODS
The next group of Arthropod animals with which we have to deal is that
of the Crusta´cea. Some or other of the members of this class are
well known to everybody, if only in the shape of toothsome food--the
Prawn, the Shrimp, the Lobster, the Crayfish, and the Crab. The great
characteristic of this class of the Arthropod phylum is the so-called
‘shell,’ which differs greatly from true shell in being composed of
chitine, hardened with salts of lime. Most of the species live in the
water and breathe by means of gills or through the skin. In dealing
with these creatures, some long words must be employed, if our present
work on them is to be a stepping-stone to something more advanced. The
difficulty is more apparent than real, and if boldly faced will soon be
overcome.
Our first division, or sub-class, of the Crusta´cea is that of the
Malacos´traca, or animals with soft shells--a name originally adopted,
as Mr. Stebbing tells us[42], ‘to distinguish such creatures as crabs
and crawfish and prawns from such others as oysters and clams; not
because of the absolute, but because of the comparative softness
of their shells.’ Under this sub-class are grouped two Orders--the
Stalked-eyed and the Sessile-eyed Crustaceans, the technical names for
which are the Podophthal´ma and the Edriophthal´ma.
To the Stalked-eyed Crustaceans belong the Prawn, the Shrimp, Mysis,
or the Opossum Shrimp, and the Crabs, to mention only those forms with
which we are dealing here. The reason for scientific and popular names
will be evident if living or spirit specimens are examined, for it will
be seen that the eyes are elevated on stalks. Mr. Stebbing[43] relates
an amusing story of a very intelligent student, who, on being told that
the eyes (of the shrimp) were stalked, candidly confessed to having
always thought that this appearance was due to their having been forced
out of the head by boiling.
The general shape of a Prawn (_Palaemon serratus_) is fairly familiar
to everybody. The body is divided into two principal regions--the
carapace, or cephalothorax, as it used to be called (formed by the
union of the head and thorax), and the pleon, or swimming part. The
carapace has a projecting beak or rostrum, and is unsegmented; the
pleon is divided into segments, and the whole may be represented thus:--
---- -- -- -- -- -- -- )
_c_ 1 2 3 4 5 6 _t_
where the long stroke (_c_) stands for the carapace, the shorter ones
(1–6) for the segments of the pleon, and the ) for the telson or tail.
The carapace consists of fourteen united segments, and this will give
twenty or twenty-one segments in all, according as we reckon the telson
an appendage of the sixth segment of the abdomen, or as a distinct
segment. The carapace bears the eyes, two pairs of antennae, six pairs
of mouth appendages, and five pairs of walking legs or perei´opods,
normally with seven joints--in all, fourteen pairs of appendages, that
is, one pair for each of the fourteen segments of which the carapace
is composed. The segments 1–5 of the ple´on bear swimming feet, or
ple´opods, and the female uses these for retaining the eggs, which she
bears about with her. In this fashion the ‘hen’ lobster carries her
‘berries.’
The Prawn is a capital inmate of the aquarium, and as it does well
in confinement, specimens should be kept in order to get a general
acquaintance with their form and external anatomy, and to watch their
habits. The Common Prawn will answer the purpose, but still better is
_Palaemone´tes varians_, an exceedingly common species. It has
this advantage, that it ‘seems to be equally at home in salt water
and fresh.’ The only condition necessary is a good supply of food,
and this may be furnished by putting into the aquarium from time to
time a quantity of water-fleas. If these Prawns are well fed they will
shed their skins at frequent intervals, and this operation will give
us material for examination, for the cast skin will serve our purpose
almost as well as a spirit specimen.
Some of these Prawns are now living in one of my aquaria. They were
taken in a brackish dyke or cut near Newhaven, in Sussex, and in
the mud which was brought back with them were a number of small
bivalves of the genus _Sphaerium_. Most people know Mr. Kew’s
exceedingly interesting book, _The Dispersal of Shells_[44].
In it he relates some extraordinary instances of the way in which
species of shells are carried short distances, and may be carried
from one district or country to another. These Prawns offered a good
illustration of this, and practically confirmed some of the statements
in his book, for on several occasions they were seen with the bivalve
shells attached to their walking legs. The molluscs lay half buried
in the mud and vegetable _débris_ at the bottom of the tank,
and as the Prawns walked about they sometimes trod between the open
valves, which, as they closed, fastened on to the intruding limb. On
one occasion the molluscs did not relax their grasp for days; and had
this incident occurred when the creatures were at liberty the molluscs
might have been carried for a considerable distance. If specimens of
_Sphaerium_ are put into an aquarium containing Prawns of this
kind, it is probable that before very long the crustaceans will have
one or two attached to some of their limbs.
Prawns are exceedingly beautiful, and if we get hold of live specimens,
from salt water or fresh, they should be put into an aquarium--the
smaller, in reason, the better--so that their motions may be watched
with the hand lens. If much weed be put in, the Prawns will use their
walking legs in preference, while if there is little vegetation the
powerful tail-fan will be employed for motion backwards, while the five
pairs of limbs on the abdomen enable their owners to move forwards
through the water.
From Fig. 66 one may get a good notion of a Prawn, and of the points
in which Prawns, in the zoological sense of the word, differ from
Shrimps. The head of the Prawn is armed in front with a long blade-like
beak, studded along its upper and lower edges with a series of teeth
like those of a saw, and the second leg is chelate, that is, armed with
pincers, resembling, in miniature, that of a lobster or crab. In the
Shrimp, on the contrary, there is scarcely a trace of the beak, and
the first leg is incompletely chelate, or sub-chelate[45] (Fig. 67),
its last joint folding back upon the one that supports it, just as the
blade of a pocket-knife closes on its handle. These two distinctions
hold good between all Prawns and all true Shrimps.
[Illustration: FIG. 66.--Prawn.]
Now let us go over our Prawn--a spirit specimen--in detail. The
antennae may be separated, and examined, and the appendages of the
inner pair distinguished, for at first it may be thought that there
are more than two pairs. This, however, is not the case, as should be
ascertained by actual investigation. A needle inserted at the base of
the outer antennae will separate the first three segments, bearing
respectively the eyes, and the first and second pairs of antennae.
The eye should be carefully looked at to make out that it is really
compound. Then the joints of the antennae, each with its circle of
sense-hairs, are to be noticed. Last of all, the inner pair of antennae
deserve attention, for these carry in the basal joint an organ of
hearing. This joint is large and sac-like, and contains an opening
through which grains of sand are introduced by the animal itself. The
grains serve to transmit the vibrations of the water in the sac to the
auditory hairs, to each of which a branch is sent off from the auditory
nerve. If the joint is opened the sand will be found. The first
antennae of a lobster or crayfish may also be examined and compared.
The mouth organs, of which there are six pairs, will offer some
difficulty, and for this reason it may be well to pass them over in
this case and to deal with these organs generally when treating of the
Crab.
Beneath the outer foot-jaws are the first pair of walking feet, which
are used as cleansing organs. Gosse describes them as ‘beset with hairs
which stand out at right angles to the length of the limb, radiating in
all directions like the bristles of a bottle-brush.’ If we watch our
Prawn in life, we shall frequently see these limbs in active operation.
They are brought to bear on every part of the body within reach.
Sowerby says[46]: ‘The prawn loves to be clean, and he takes surprising
pains to keep himself so. Drawing up his tail and abdomen, he subjects
their under surface to the most careful revision, scrubbing and poking
between the lappets of the shell and body, diving into every crevice,
and with the pincer-hand picking out every speck too large to brush
away.’ The next pair of legs are also chelate; but the three following
pairs are armed with claws, and it is upon the points of these that the
animal walks on the bottom. The pincers of the second pair of legs are
used to pick up food and bring it up to the mouth organs, where it is
taken by the outer foot-jaws, and passed into the mouth. The swimming
feet carry two branches, finely fringed with hairs.
[Illustration: FIG. 67. First walking leg of Shrimp
(enlarged).]
If the carapace be removed the gills at the base of the walking feet
will be exposed. These consist of thin leaf-like plates attached to a
central stalk, and they are aërated by water passing in behind and out
in front.
After what has been said of the Prawn, little space need be devoted
to the Shrimp, for it may be gone over in precisely the same way. It
will be sufficient to call attention to the difference in the antennae,
to the rudimentary rostrum or beak, and to emphasize the distinction
between the terminal joints of the first leg in the two creatures. The
leg shown in Fig. 67 corresponds to the limb used for cleansing by the
Prawn.
There is a great difference in their habits, for Shrimps burrow in the
sand for concealment. In doing this the swimming feet, as well as the
walking legs, are brought into action, and when the Shrimp is settling
down, sand is swept over its back by the antennae, to render the
concealment complete.
In many of the rock-pools round the coast, and also in brackish water,
Mysis, or the Opossum Shrimp, may be met with. It is not, so far as my
experience goes, a good inmate of the aquarium, but it is extremely
interesting from the fact that, unlike its higher relations, the
auditory apparatus is not situated in the antennae, but in the plates
of the telson (Fig. 68E).
[Illustration: FIG. 68.--Mysis, or the Opossum Shrimp.]
Mysis is shrimp-like in general appearance but differs from Shrimps
in the structure of the legs, in the absence of gills, and in other
particulars.
The telson consists of five pieces. In each of the two inner and
smaller pieces is an oval sac, like that described in the basal joints
of the first antennae of the Prawn, containing a single lens-shaped
otolith, consisting of chalky matter embedded in some organic substance.
‘The vibration of the hairs [in this sac] is mechanical, not depending
on the life of the animal. Hensen took a Mysis, and fixed it in such
a position that he could watch particular hairs with a microscope. He
then sounded a scale; to most of the notes the hairs remained entirely
passive, but to some one it responded so violently and vibrated so
rapidly as to become invisible. When the note ceased the hair became
quiet; as soon as it was re-sounded, the hair at once began to vibrate
again. Other hairs, in the same way, responded to other notes. The
relation of the hairs to particular notes is probably determined by
various conditions; for instance, by the length, thickness, &c.[47]’
We shall not be able at present to repeat Hensen’s experiment, but we
may break up the sac and extract the otolith, which may be seen with
the lenses at our command.
Small specimens of the Shore Crab (_Car´cinus mae´nas_) are fair
game for us. They will interest us while living in the aquarium, and
when dead we can put them into pickle, and break them up at our leisure.
The broad shell of the Crab--the crab-cart of children--corresponds
to the carapace of the Lobster, the Prawn, and the Shrimp, and bears
the same number of appendages--fourteen pairs. To make out the pleon
or swimming part, it is only necessary to lay the crab on its back,
and, with a needle, or small knife, turn back the flap--or ‘apron,’
as fishermen call it--which lies in a groove on the under surface.
Here we shall find the pleopods, or swimming feet, though they are not
really used for that purpose. The eyes, the two pairs of antennae,
and the five pairs of walking legs will offer no difficulty. It is
only necessary to remark that the terminal joints of the last pair of
walking legs are flattened and fringed with hair, showing some approach
to the swimming crabs, which use those organs to swim with.
Now we may examine the mouth organs, of which there are six pairs.
To do this, the crab may be fixed, with the back downwards, or held
lightly but firmly in the left hand. The latter plan is perhaps the
more convenient. The index and middle fingers should support the
carapace, and the thumb should be placed on the pleon. The outer pair
of mouth organs are the third maxillipedes, or jaw-feet. These close
the area of the mouth, somewhat after the fashion of the double-doors
of a cupboard, though the hinging, of course, is different. To open
these jaw-feet, a needle should be inserted at the top, with a gentle
pressure downwards and outwards. The back of the crab is turned away
from us, so that the left jaw-foot should be pressed outwards to the
right, and the right jaw-foot to the left.
Theoretically these limbs consist of the same number of joints as
the perei´opods or walking legs; and this is to be borne in mind,
even if we do not succeed--and we probably shall not--in tracing the
full number of seven joints. But we may notice and count the terminal
joints, and observe the fringing of the limb with hair.
A similar method of using the needle will enable us to raise the second
and first pairs of maxillipedes, which are of smaller size and softer
structure.
Having raised these organs, it is well to replace them--to close the
doors, as it were--and then to raise them again, to observe how they
work. They may then be detached and fastened to a small piece of card,
for comparison with similar organs in the lobster and the crayfish, and
with the mouth organs of insects.
Beneath the maxillipedes are the second and first maxillae--thin,
leaf-like organs. The first-named are furnished with spoon-like scoops,
which serve to carry out from the gill-chamber the water that has
parted with its oxygen in aërating the gills.
Immediately below the maxillae lie the mandibles, with hard, cutting
edges, by means of which the food is broken up. Each carries a palp.
[Illustration: FIG. 69.--Maxillipedes and Maxilla
(_b_) of Shore Crab. (_d_) First maxillipede. (After
Savigny.)]
These inner three pairs should also be detached, and the whole of the
mouth organs arranged on a card thus:--
MANDIBLES.
FIRST MAXILLAE.
SECOND MAXILLAE.
FIRST MAXILLIPEDES.
SECOND MAXILLIPEDES.
THIRD MAXILLIPEDES.
The first attempt will certainly be unsuccessful; and the first few
attempts will probably be unsatisfactory; but we shall gain knowledge
with each successive trial. And knowledge is worth the winning.
The stomach is interesting, and the gastric mill may be easily
examined. When the mouth organs are removed, there will be no
difficulty in taking out the stomach. This should be cut open with a
needle, and then we shall see the gastric teeth (_g_ _g_)
which grind up the food against the fixed calcareous plate (_b_
_b_). The lower end of the stomach is set with fine hairs, which
prevent the passage of food into the intestines until it has been
ground fine between these living millstones. A similar arrangement is
found in all the higher Crustacea. The time spent in comparing the
gastric mill of the Crab with the ‘gizzard’ of the Cockroach will not
be thrown away.
The Broad-clawed Porcelain Crab (_Porcella´na platyche´les_) is
also worth keeping, for it is a droll little creature. These crabs
are generally to be found clinging to the under surface of stones or
of ledges of rocks overhanging small pools. The chief interest of
these crabs, for us, lies in the exceedingly beautiful arrangement for
procuring food with which the outer pair of foot-jaws is furnished.
[Illustration:
FIG. 70.--Stomach of Crab laid open. _b_ _b_
_b_, fixed plate, against which the gastric teeth _g_
_g_ are opposed; _b´_ _b´_, gastric teeth
enlarged.]
‘Watching a Broad-claw beneath a stone close to the side of my tank, I
noticed that his long antennae were continually flirted about; these
are doubtless sensitive organs of touch, or some analogous sense,
which inform the animal of the presence, and perhaps of the nature,
of objects within reach. At the same time I remarked that the outer
foot-jaws (pedipalps) were employed alternately in making casts, being
thrown out deliberately, but without intermission, and drawn in,
exactly in the manner of the fringed hand of a Barnacle, of which both
the organ and the action strongly reminded me. I looked at this more
closely with the aid of a lens: each foot-jaw formed a perfect spoon
of hairs, which at every cast expanded and partly closed. That you may
understand this better, I must say that the foot-jaw resembles a sickle
in form, being composed of five joints, of which the last four are
curved like the blade of that implement. Each of these joints is set
along its inner edge with a row of parallel bristles, of which those
of the last joint arch out in a semicircle, continuing the curve of
the limb; the rest of the bristles are curved parallel or concentrical
with these, but diminish in length as they recede downwards. It will be
seen, therefore, that when the joints of the foot-jaw are thrown out,
approaching to a straight line, the curved hairs are made to diverge;
but as the cast is made they resume their parallelism, and sweep in, as
with a net, the atoms of the embraced water[48].’
All this description may be verified from a spirit specimen, if the
foot-jaws be carefully removed. And the examination with the lens will
also show that these hairs are plumose, that is, set with smaller hair,
like the barbs of a feather.
At this point we may conveniently take leave of the Stalk-eyed, and
pass on to the Sessile-eyed, Crustacea. Leaving the Cuma´cea out
of the question, we have two Sub-orders from which to choose our
subjects--the Amphip´oda and the Isop´oda--conveniently Englished,
Am´phipods and I´sopods. We learn from Mr. Stebbing[49] that ‘the
Amphip´oda, which are common in fresh as well as in salt water, were
so named by the French naturalist Latreille, as having feet extending
in all directions, their limbs at the same time having much diversity
of form, in correspondence with diversity of function. The Isop´oda,
or equalfooted animals, besides being found both in fresh and salt
water, have more decidedly than the Amphip´oda extended their range
to the dry land. The name was invented by Latreille in ignorance of
the great number of species, since investigated, in which the feet are
strikingly unlike and unequal. Nevertheless, the name may stand, just
as a rose remains a rose even when it is not rose-coloured.’ These last
two sentences must be borne in mind, for they throw great light on a
subject that may give us some trouble--the question of priority in
nomenclature.
The majority of the Amphipods live in salt water, but a few are found
in ponds and streams, and some dwell on the shore, near, but not in,
the sea. The animals of this Sub-order are distinctly segmented, and
three regions may be distinguished thus [image], where C stands for
the Cephalon, or head, Per. for the Perei´on, or body, and Pl. for
the Ple´on (literally, the swimming part), or tail. On the head we
shall find two pairs of antennae, the eyes, and the mouth appendages.
Each segment of the perei´on bears a pair of appendages; the first
two pairs are called respectively the first and second gnath´opods
(or jaw-feet), and the other five pairs perei´opods, or walking feet.
The pleon carries three pairs of ple´opods, or swimming feet, on the
first three segments, and each of the following three has a pair of
uropods or tail appendages. It is well to make out these parts in every
specimen that comes in our way. More is learnt by breaking up one
specimen than by reading the clearest description so often that one
knows it by heart.
[Illustration: FIG. 71.--Gammarus. (After Sars.)]
We may begin with the Fresh-water Shrimp (_Gam´marus pulex_),
which may be taken abundantly in running water where there is plenty
of vegetation. Willow-moss affords these Crustaceans a favourite
hiding-place. It is an excellent plan to gather a quantity of
weed and shake it over a newspaper or a piece of mackintosh. The
‘Shrimps’--which, by the way, are not really Shrimps--will be dislodged
from the weed, and we shall see them wriggling along on their sides,
from which habit they and their near relatives are often called
‘Scuds,’ and ‘Screws.’ They are useful inmates of an aquarium, because
they feed on decaying animal matter, and so keep the water pure and
sweet. Opinions are divided as to whether these animals feed on
water-plants; it is probable that when their natural food fails them,
they take what comes in their way. I have kept marine and fresh-water
species of Gammarus (the genus to which the Fresh-water Shrimp belongs)
in tanks which contained no other animals, but plenty of vegetation,
and both lived and did well for a considerable time. They are by no
means unwilling to make a meal off the dead body of one of their own
species; but it is exceedingly doubtful if they prey on each other, as
some old writers have asserted. This notion probably arose from the
fact that the male carries the female, which is much smaller, about
with him, during the period of courtship, holding her tightly beneath
his body by means of the fingers of its first two pairs of hands. The
habit is not confined to this genus, nor even to this Sub-order; for
some, if not all, the species of Idotea carry on their courtship in
similar fashion, as does also the Water Woodlouse. For the verification
of statements such as these, a small aquarium is necessary, but the
animals will not be under observation long before the observer will be
convinced of their truth.
All species of Gammarus, whether living in the sea or fresh water,
may be readily distinguished by the rows of small spines on the three
hinder segments of the pleon, for this is one of the characteristic
marks of the genus. After we have kept specimens in the aquarium for
a time, so as to become familiar with their general appearance and
habits, we will put them to practical use by breaking them up.
Our first task is to work over the animal as a whole, and to make
out the three regions--ceph´alon, or head; pereī´on, or body; and
plē´on, or swimming part, or tail--into which it is divisible. It
will not be sufficient to do this once, and then to imagine we have
the whole matter fixed in our memory. It should be repeated over and
over again, with every specimen that comes into our hands, till we know
these regions practically, and the number and kind of appendages they
carry. And then the three rows of spines are to be looked for. For all
this the inch lens will be quite sufficient.
Now let us separate the head. When this is done, and if we use the
lens, we shall at once be able to account for the name ‘Sessile-eyed
Crustacea,’ for the difference between the eyes of our specimen and
those of a shrimp or a crab will be evident. Nor can there be any doubt
that they are compound eyes, though the outer integument is not divided
into facets. The antennae are next to be considered. Of these there
are two pairs, the superior, or inner, pair being the longer. These
antennae consist of three basal joints and a many-jointed flagellum,
or lash, and on each of the inner pair is a secondary appendage,
arising from the distal (or outer) end of the third basal joint. We
may represent one of the superior antennae thus: [image]. The dashes
represent the three basal joints, the long row of dots the many-jointed
flagellum, and the slanting row of dots stands for the secondary
appendage. The sensory-hairs on the joints of the flagellum should be
looked for, and may be made out with the inch lens. The same power will
show the denticle, or tooth-like projection at the base of the lower
antennae.
Next come the mouth parts. As compared with Crabs, Amphipods seem badly
off in this respect; for the second and third maxillipedes of the
former become the first and second gnathopods of the latter, so that
instead of six pairs of mouth organs the Amphipods have only four.
It is not an easy matter for a beginner to separate the mouth parts
of an Amphipod, but the difficulty is not insuperable, and will grow
‘small by degrees and beautifully less’ with practice. We have to make
out four pairs of organs arranged in the order given at the side of the
page, the mandibles being the innermost.
MANDIBLES.
FIRST MAXILLAE.
SECOND MAXILLAE.
MAXILLIPEDES.
Of course we must begin with the maxillipedes (Fig. 72). The specimen
may be held between the finger and thumb, and the parts picked out with
a needle. It is, however, better and easier to make the dissection
under water. Then we can remove the second and first maxillae, the
latter easily recognizable by its palp or feeler. Last of all come the
mandibles, also bearing a palp. We shall _feel_ these under the
needle, because of their hardened cutting edges. These are distinctly
toothed. When practice has made the separation of these parts fairly
easy, they should be compared with the mouth parts of other members of
the group, so as to utilize the knowledge we have gained.
[Illustration: FIG. 72.--Maxillipedes of _Gammarus
marinus_ (in centre). _A._ First maxilla. _B._
Second maxilla (magnified).]
Next come the two pairs of gnathopods, and in these we have to find
seven joints--which may be denoted by the numbers 1, 2, 3, 4, 5, 6, 7;
1 being the basal joint, or that nearest the body. The sixth joint is
often called the ‘hand,’ and the seventh, the ‘finger.’ The joints vary
greatly in different genera. The walking legs are next to be examined,
and we may notice that the first and second pairs are turned forwards,
and the third, fourth, and fifth pairs backwards. At the bases of these
legs are the breathing apparatus, and the females have leaf-like plates
on the anterior four pairs, forming a pouch in which the eggs are
hatched, and here she shelters her young, and carries them about with
her.
The following account of this habit is taken from Bate and Westwood’s
_Sessile-eyed Crustaceae_ (i. pp. 380, 381), and was furnished to
the authors of that book by Dr. James Salter: ‘On catching a female
with live larvae, nothing is seen of the progeny till the parent has
become at home in the aquarium, when the little creatures leave her
and swim about in her immediate neighbourhood. The plan I have adopted
to watch this curious habit of maternal protection, has been to place
a single individual in a bottle of sea water. After a time, and that
soon, the little crustacean seems at ease and swims slowly about,
when the young fry leave her and swarm around her in a perfect cloud;
they never leave her for more than half or three-quarters of an inch,
and as she slowly moves about they accompany her. If now one taps the
side of the bottle with one’s finger-nail, the swarm of larvae rush
under their parent, and in a second are out of sight. The parent now
becomes excited, and swims about quickly, as if trying to escape;
but by letting the bottle containing her rest quite still she soon
gets composed, when out come the young larvae again and swim about
as before. This may be repeated as often as the observer wishes, and
always with the same result. I have only seen this in one species, but
it is quite a common species in Poole Harbour, and I have watched the
interesting habit many times.’
The swimming legs are, roughly speaking, [image]-shaped--that is, they
consist of a stem, carrying two many-jointed filaments, fringed with
fine plumose hairs. A hair is said to be plumose when it bears smaller
and finer hairs on each side. ‘By folding the tail beneath the body,
and suddenly striking it out again, those animals which exist in the
water, as well as those which live on the shore, are enabled to dart or
leap to a considerable distance[50].’
[Illustration: FIG. 73.--Nest-building Amphipod. (From
life.)]
Our hand lens may be well employed in watching some of the
nest-building Amphipods at work in the aquarium. There can be no
difficulty in keeping these creatures in captivity, and under
observation, as they build their tubes and rear their families. They
are plentiful in every rock pool round the coast, and it would be a
hard matter to dip the net into any such pool without getting a few
specimens.
They need absolutely no care. The aquarium of the specimen figured
was a four-ounce bottle, tightly corked; and in it was a spray of
Cladophora, on which the animal fed, and the growth of which broke up
the carbon dioxide and set free good store of oxygen. Here it lived for
some months, and built more than one tube for itself against the side
of the bottle.
It is easy enough through the pocket lens to watch the Amphipod at
work. Like other builders, the first thing it does is to get its
materials ready. Lying on its side, with its back against the glass,
it will rake together with its antennae and jaw-feet a good store of
vegetable _débris_, or if there be no supply of this, will break
off branches from the growing weed.
But gathering vegetable _débris_, or even filaments of living
weed, is very far from being tube-building. Something more is needed
to bind the mass into a coherent structure. This the animal itself
supplies. The bases of the first two pairs of walking feet are large,
and contain glands which secrete a glutinous cement, that can be spread
like mortar, or spun out into threads.
An American observer devoted much time to the observation of these
animals. He says[51]: ‘When captured and placed in a small zoophyte
trough, with small branching algae, the individuals almost always
proceeded at once to construct a tube, and could very readily be
observed under the microscope.... The branches were not usually at once
brought near enough together to serve as the framework of the tube, but
were gradually brought together by pulling them in and fastening them
a little at a time until they were brought into the proper position,
where they were firmly held by means of a thick network of fine threads
of cement spun from branch to branch. After the tube had assumed
very nearly its completed form, it was still usually nothing but a
transparent network of cement-threads woven among the branches of the
weed.’
Then he describes the method in which the Amphipod works up bits
of weed and its own droppings into the framework of the tube. In
putting its foecal pellets to this use, it reminds one of a species of
Melicerta (_Melicerta janus_)[52], which employs the same material
to coat its gelatinous sheath.
In breaking up weed and pellets with its foot-jaws and (probably) its
mandibles, the Amphipod recalls the practice of some of the Masking
Crabs, which have been seen to apply to the mouth the material they
were using to deck themselves. Dr. Aurivillius suggests that in the
case of the Crabs there may be an adhesive secretion from the mouth,
as there is possibly in the Amphipods. ‘The spinning was done wholly
with the first and second pereiopods, the tips of which were touched,
from point to point over the inside of the skeleton tube, in a way
that recalled strongly the movements of the hands in playing upon a
piano. The cement adhered at once to the points touched, and spun out
between them in uniform delicate threads. The threads seemed to harden
very quickly after they were spun, and did not seem, even from the
first, to adhere to the animal itself. In one case, in which the entire
construction of the tube was watched, the work was apparently very
nearly or quite completed in little more than half an hour.’
The species we are likely to meet with in rock-pools fashion their
tubes in a similar way. The only difference to be noted is that they
employ less cement, and a larger proportion of broken-down weed and
other matters.
The Sand-hopper (_Tali´trus locus´ta_) and the Shore-hopper
(_Orches´tia littorea_) are so exceedingly plentiful that it may
be well to collect and preserve some during any visit to the seaside.
Both are of fairly large size, and present no great difficulty to us in
making out their several parts. Let us take the Sand-hopper first.
Sand-hoppers swarm on most sandy shores, where they perform the useful
part of scavengers. They are always found above high-water mark, and
do not enter the sea of their own accord. In hunting for them it is a
good plan to turn over decaying masses of sea-weed, for under them the
Sand-hoppers are sure to swarm.
Strange tales have been told of their voracity. Bate and Westwood[53]
record the story of a correspondent who says that at Whitsand he ‘saw
“not millions, but cartloads,” of this species lying piled together
along the margin of the sea. They hopped and leaped about, devouring
each other, as if for very wantonness. A handkerchief, which a lady let
fall amongst them, was soon reduced to a piece of open work by the
minute jaws of these small creatures.’
This statement has been copied into a good many books, without
criticism. At last Mr. David Robertson tried various experiments with
a view to discover if these creatures would feed on each other, or,
failing other food, put up with cambric or muslin. The specimens upon
which he made his observations did neither the one nor the other. Mr.
Robertson embodied the results of his experiments in a paper which he
read before the Natural History Society of Glasgow[54]. And the story
may be read in an abbreviated form in the Rev. T. R. R. Stebbing’s
_Naturalist of Cumbrae_, p. 329.
In Gammarus we have a standard with which to compare our Sand-hopper.
The first thing to notice is the difference in the antennae. Here the
superior pair are very short, and carry no secondary appendage, and
the lower pair have no denticle or tooth-like process. There is also
considerable difference in the gnathopods, or jaw-feet, the sixth joint
of which, in the Sand-hopper, does not form a ‘hand.’ The pleopods, or
swimming feet, are small, and are used for leaping. We shall also find
some difference in the details of the mouth parts, especially in the
toothing of the mandibles.
We now come naturally to the Isop´oda, which are distinguished by the
nearly uniform size of the seven segments which constitute the trunk,
and the seven pairs of limbs borne by these segments. The head is
distinct, and the breathing apparatus is carried on the under side of
the pleon--in these animals not the ‘swimming’ part--five pairs of
plates lying one over another, sometimes covered by a larger outer pair.
A normal I´sopod may be represented [image:--·······--], where the
small dash will stand for the cephalon, or head; the seven dots for the
segments of the perei´on, and the long dash for the pleon.
The Common Asellus (_Asellus aquaticus_) of ponds and ditches
is an excellent subject. It lives well in confinement, and if the
conditions are fairly favourable, will increase and multiply. These
animals will forage for themselves, and pick up a comfortable living
from the vegetable _débris_ that always accumulates at the bottom
of an aquarium, and they are not averse from an occasional meal of
animal food.
[Illustration: FIG. 74. Water Woodlouse.]
While our specimens of Asellus are moving about in any convenient
vessel, we may verify with the hand lens what has been said about the
general form. Then we may notice the antennae, the inner pair being
much the smaller. There can be no difficulty in discriminating the head
and the eyes; the seven segments of the perei´on, each bearing a pair
of limbs; and the pleon with its two terminal appendages. These last
consist of a stalk bearing two longer filaments, armed with spines, and
ending in a small pencil of hairs.
It is easy to see that the segments of the pleon have coalesced, so as
to form a continuous plate or shield on the upper surface.
If we now take our dissecting microscope and place an Asellus in some
water in an excavated 3 in. by 1 in. slip on the stage, examination
with an inch lens will show us a considerable amount of detail. With
the half-inch Leitz lens (see p. 18) one may see quite clearly the
beautifully annulated form of the flagella of the antennae, the sensory
hairs with which these organs are set, and the circulation of the blood
in the limbs and the antennae--the corpuscles moving in a continuous
stream. More than this: we shall be able with the same power to
distinguish tufts of Vorticellids that settle on the Asellus, and the
commensal rotifers that roam about on the body of their host, generally
on the limbs and under surface.
Now we may turn the Asellus on its back, to examine the breathing
apparatus more closely than we were able to do when the creature was
moving about in the bottle. It will be easy to make out the opercular
plates--modified tail appendages--that open and shut to admit water to,
or allow it to flow out from, the true breathing-plates which function
as gills, and correspond to the swimming feet of the Amphipods.
In the female there is a pouch beneath the perei´on, in which the eggs
are carried till they are hatched, and which serves as a nursery and
refuge for the young.
If a good number of these animals be collected they will probably
breed, and then there will be the opportunity of seeing for ourselves
the young carried about in the incubatory pouch.
There are two other aquatic I´sopods which will make good subjects for
us on account of their great abundance, and the ease with which they
may be kept in any improvised aquarium, with a little weed. They may
both be taken in brackish water, and will live and thrive in fresh
water, without any admixture of salt. Indeed, both have lived for some
months in a small bottle of New River water, in which the only weed is
some willow moss. They feed on this and on the vegetable _débris_
that accumulates at the bottom of the bottle, and both species have
bred.
The first is Idot´ea (_I. pelag´ica_), a long, narrow creature,
with very short inner antennae. The last four segments of the pleon
form a plate on the upper surface; and on the under surface the
opercular plates may be opened like tiny folding-doors, to show the
breathing plates.
These vary greatly in colour. Of another species, Spence, Bate, and
Westwood say: ‘According to our experience the colour of the animal is
dependent upon that of the weed on which it lives. Those that live on
the black fucus are generally very dark purple, while those that we
find on the green algae are brightly verdant; and it has always been
our opinion that this change was due to the food[55].’
The other little creature is called Sphaero´ma--it has no English
name--from the fact that it can roll itself into a ball. It is not
difficult to identify, from the fact that all the segments of the pleon
are joined into one plate, the hinder margin of which is entire, thus
[image].
The garden will afford us a hunting-ground for the last specimen of
this group for which we have space--the Woodlice. Enough has been said
of the method of looking over and breaking-up I´sopods generally to
render detailed description unnecessary. The inner pair of antennae,
however, are so small as to be readily overlooked: indeed, on first
sight these creatures seem to have but a single pair. Some have, and
others have not, the power of rolling themselves into a ball; and,
concerning the former, Swammerdam tells the following story:--
‘One of our maidservants had at one time found a great number of
Woodlice in the garden, contracted into round balls ..., and thinking
she had found a kind of coral beads, she began to put them one after
another on a thread, but it soon happened that these little creatures,
which roll themselves up in such a manner only for fear of harm, and
appear as if they were dead, being obliged to throw off their mask,
resumed their motions. On seeing which, the maidservant was so greatly
astonished, that she threw away the Woodlice and the thread, and cried
out, and ran away[56].’
CHAPTER VI
AQUATIC INSECT LARVAE
In this chapter we shall deal with a few aquatic insect larvae. Of
these, some are aquatic also in the perfect condition, while others
emerge from the pupa stage as aërial insects. It requires no great
amount of care to keep these creatures, and some hints on this subject
and on collecting are given in the first chapter.
The larva of Dytiscus is abundant during the greater part of the year,
and is almost sure to be met with by the collector, who will find it
an extremely interesting object for examination and study. No other
creatures should be put in the same bottle with these larvae; and if
there are several of them in one bottle, it is a good plan to put in
plenty of pondweed, which will often keep them from attacking each
other.
When full grown, the Dytiscus larva may attain a length of two inches
or rather more. Its colour is dingy brown, and its aspect forbidding
enough to justify the uncomplimentary names that have been bestowed
upon it--Water-devil and Water-tiger. It certainly rivals the tiger in
fierceness, and its method of stealing up to its prey and attacking it
from behind led Swammerdam to call it the Sicarius or Assassin Worm.
One must not imagine that Swammerdam was ignorant of its nature; ‘worm’
with him was a general term for any larval form. Indeed, he says,
‘It is extremely probable that some peculiar species of the Water
Beetle proceeds from this worm, when, having remained in the water a
sufficient time, it betakes itself to the land to undergo its mutation;
but this is mere conjecture.’ What was conjecture for him is fact for
us.
Now let us put our larva into a small tube, and examine it more
closely. The head is large and joined to the first segment of the
thorax by a distinct neck. There are twelve small eyes, six on each
side, a pair of antennae, two pairs of palps, and a large pair of
sickle-shaped mandibles, which Swammerdam calls ‘teeth,’ and says that
‘it is perhaps to contain the muscles such teeth require that Nature
has made the head so large.’ Behind the head come eleven segments, of
which the first and last are the longest. They gradually increase in
width till the sixth, the rest again decreasing, till the eleventh ends
in a blunt point, from which diverge two appendages, thus [image],
thickly fringed on both sides with hair, as are the tenth and eleventh
segments.
There are six legs, one pair to each of the first three segments.
These also carry fringes of hair, thus increasing their power as
swimming organs; and, in addition, they bear numerous spines, and end
in strong double claws, which must be of service in climbing over
aquatic vegetation, and may assist in holding a struggling victim or in
striking it down, so as to bring it within reach of the mandibles.
Spiracles will be found--seven on each side. These do not, however, in
the larval condition, serve as breathing organs, though they fulfil
their proper office in the pupa. The air-tubes of the larva open at the
extremity of the last segment. When the larva wants to breathe it comes
to the surface without an effort, for it is lighter than the water it
displaces. The tail rises above the surface, and a fresh supply of air
is taken in. When the larva wishes to descend, a stroke of the tail
sends it downwards, and as it reaches the bottom of the tube it will
cling with its claws to any weed we may have put in with it, or hold on
with them to the glass itself.
The chief interest of this animal lies in its mandibles, and the method
in which they are employed. It was formerly believed that the mouth of
this larva was closed, so that it could not take solid food, and that
it lived entirely on the juices of its prey, which it sucked up through
its hollow mandibles.
Swammerdam says of this larva, ‘When about to eat he seizes with the
two teeth (mandibles) the little creatures that come in his way, and
pierces their body with the sharp crooked points. The teeth being
perforated from the points to the roots, he in a surprising manner
sucks through them into his mouth the blood of the unfortunate captive.
This may be easily seen, especially when the blood of his prey is of a
red colour, as the teeth are transparent.’
He then describes how he watched one of these larvae feed, and saw the
blood, mixed with air-bubbles, travel up the mandibles. After this he
tells us how, if we have a Dytiscus larva, we may ‘procure ourselves
a very entertaining and surprising sight, by throwing to it a small
earthworm; for let this last move, twine, and otherwise bestir itself
ever so much, the other keeps its hold, and very calmly sucks the blood
of its prisoner.’
We may, however, go to work in another fashion. We may dissect out the
mandibles from a dead larva and pass a fine hair into the slit near the
point, and it will come out at the orifice near the base. It is through
this orifice that the nutritive juices of the prey are drawn into the
true mouth. This practical proof that the mandibles are pierced is by
no means so difficult as one might suppose.
De Geer[57] seems to have been the first to suggest that there must
be some kind of true mouth, and in support of his suggestion tells us
that he saw this larva eating up the solid parts of a Water Woodlouse
(_cloporte_), after having sucked up its juices. More than this,
he places the mouth in what has proved to be the true position, though
he did not solve the mystery as to why it is kept so firmly closed.
This was done by Mr. Burgess, an American naturalist, from whose
paper[58] the following particulars are condensed:--
‘Authors have described this creature as mouthless; and if we examine
the slit where we should expect the mouth to be, we find that this slit
ends in a perfectly closed seam. The methods of microscopical research
were brought into play, and a longitudinal section of the head cut and
mounted. This showed that the upper and lower lips were locked together
by a peculiar joint--the upper coming over and locking into the under
lip (Fig. 75).’
We may get some idea of this mouth-lock by placing the fingers of the
right hand over those of the left, and then bending them.
Mr. Burgess concludes his paper thus: ‘We find that the Water-tiger,
far from being mouthless, as ordinarily assumed, has in fact a
very wide mouth, though its lips are closely locked together by a
dove-tailed grooved joint developed for this purpose. Whether this
joint can be unlocked by the animal itself is another question, which
I cannot answer, though De Geer’s observation above quoted makes this
probable. It is, at all events, easy to open the mouth by manipulation
with a pair of forceps.’
[Illustration: FIG. 75.--Mouth-lock. _m_, mouth ×
125. (After Burgess.)]
Professor Miall[59] has verified Burgess’s observations, and carried
them a step further. He found by actual experiment that ‘the mouth-lock
acted automatically, opening when the mandibles opened, and closing
when they closed.’
Both these authorities stand very high. Yet, with all respect to them,
it will be well to test these statements before accepting them.
Something of this mouth-lock may be seen in any well-prepared slide
of a Dytiscus larva. If we hold the slide up to the light and examine
with a power of 10, we shall see a dark line--in some cases two nearly
parallel thinner lines--running from the base of one mandible to the
base of the other. The dark line is the closed mouth-lock. The two
nearly parallel thinner lines are the edges of the lips drawn asunder
in preparing the specimen or by the pressure of the cover-glass. If we
get to see this much, we shall have advanced one step.
[Illustration: FIG. 76.--Dytiscus Larvae.]
Next we may verify Professor Miall’s experiment, though not quite in
his way, for such section-cutting as he speaks of is beyond our powers.
Larvae as large as possible should be chosen, and killed by dropping
them into boiling water. The mandibles of one should be allowed to
remain closed, and those of the other plugged open with pith or a small
piece of wood--a bit of a match will serve. By putting each in turn
into an excavated slip with water, carefully covering this with a plain
slip, and holding these slips together with an elastic band, we may
examine the larvae as we did the slide.
The result will be practically the same. Where the mandibles are
closed, we shall see the dark line; and where they are open, we shall
see the two thinner ones nearly parallel.
One caution may be necessary. The mandibles bear at the base a
rounded process, which fits into a chitinous cup. It is not difficult
to mistake this for the mouth-lock, with which, however, it is not
connected.
There is still one other method which we may adopt to see the mouth.
The head of one of these larvae may be cut off, dropped into a strong
solution of caustic potash, where it should remain for a day or two.
After washing it well in water, cut it in half lengthways, and turning
either half upon its side, so as to expose the part cut, examine with
the pocket lens.
These experiments are by no means difficult. But if carried out step
by step, it will be easy to understand how the larva can suck out the
juices of its prey, and how it can open its mouth to swallow some of
the solid parts.
The larva does not readily change into a pupa in confinement. If,
however, we wish to rear a beetle, the larva must be plentifully
supplied with food, and removed from a bottle to a flat dish,
where earth can be placed against the side so as to rise above the
water-level. Fig. 77, where a pupa is represented in a bank by the side
of a pond, will give us a hint how to go to work. The time occupied in
pupation will vary according to the temperature of the room--but is
never less than a fortnight. In the open it is probable that the winter
is passed in the pupal condition, the perfect insect emerging in the
spring. Like Land Beetles, it does not assume its dark hue for some
days, but its pale skin darkens by degrees.
[Illustration: FIG. 77.--Pupa of Dytiscus.]
The larva of Hydrophilus does not seem to be often taken in this
country. It would make a capital subject for investigation, and the
food-supply could be arranged easily enough. The repetition of the
experiences of Lyonnet, who reared these larvae from the egg, would
be of great interest. He says that he took about thirty larvae from
the brood, and fed them with very small water-snails. These they
devoured in the same way as the larger larvae do. Having seized the
snail with their mandibles, they bent backwards, and supporting it
on their back, which served them for a table, eat it there, without
making any use of their legs to hold their prey. When the supply of
small snails ran short, they did very well with large ones cut up into
pieces, and with tadpoles. If, however, food was not supplied to them,
they fed on each other. But, except when pressed by hunger, they lived
together peaceably enough, and seemed to take pleasure in each other’s
society[60].
The larva of _Limnobia replicata_, a Crane-fly allied to, but
smaller than, the well-known Daddy-longlegs, is another excellent
subject for investigation. It is not very often taken; perhaps because
it is not often looked for. But it is common enough, for all that.
In general appearance these larvae resemble small caterpillars covered
with spines. Some of these are simple and others forked, not much
unlike the letter [image], with a short stem, and the arms slightly
curved. There are no feet, and the last segment carries two pairs of
hooks, one large and the other small. From their position they are
called anal hooks.
The dykes of the Sussex marshes are an excellent hunting-ground.
Probably the channels of water-meadows, or any other shallow standing
water in which aquatic moss grows, would serve the collector’s purpose
quite as well. And such pieces of water abound all over the country.
For taking these larvae the ordinary net and bottle are of little use,
though a few may be captured by sweeping backwards and forwards among
patches of aquatic moss.
There is, however, a far easier and surer method. A good handful of
the moss should be gathered, and put into a shallow vessel half full
of water--a developing-dish answers capitally--and then shaken to and
fro or gently stirred with a small piece of stick. The larvae will curl
into a ring and fall to the bottom, whence they may be picked up and
dropped into a bottle or other receptacle to be taken home. A fair
quantity of moss should be gathered, for this is their favourite food,
and all larvae are greedy feeders.
Other water-plants, however, do not come amiss to them. Just before
these lines were written I was examining a bottle in which some of
these larvae were kept. It contained a few sprays of willow-moss and
some ivy-leaved duckweed, which floated on the surface. One larva on a
spray of moss was reaching upward, and it was distinctly seen to feed
on the duckweed. This must have been from choice, for there was within
reach plenty of what all observers consider to be its natural food.
This, too, might have been more easily obtained; for, to reach the
duckweed, the larva had to hold on to the moss by the anal hooks, and
extend its body in a fashion analogous to that of the caterpillar of a
geometer moth, which will hold on to a branch with its fore-legs and
claspers and maintain the body at an angle of 45°, sometimes for hours
together. I have also seen them feed on hornwort.
The larva of this small Crane-fly is not at all difficult to keep. It
is exceedingly hardy, and will survive a great deal of rough treatment.
In November, 1895, I sent three in a tube--securely packed, as I
thought--to a friend in Yorkshire. A few days afterwards I heard, with
regret, ‘that the bottle was broken in transit, and that the larvae
arrived dead.’ Three days later I was gratified by receiving a letter,
from which the following is quoted: ‘The Limnobia larvae have come to
life again. I put them into water as soon as they arrived, and after
lying motionless for many hours they have begun to creep about and
feed.’
This is excellent testimony to their powers of endurance, but it is
weak in comparison to that which De Geer supplies[61].
He was Marshal of the Court of Sweden, and one November, before leaving
his country house for his official duties at Stockholm, he put four
of these larvae into a vessel of water, and left them to take their
chance. The water froze into a solid mass. When he returned in the
following May he found about half the water thawed, and two of the
larvae dead. The others, though they had been enclosed in ice all the
winter, were lively and in good condition. He put them into another
vessel with fresh water and some aquatic moss, and at once they began
to move about and commenced to feed. Both pupated by the fifteenth of
the month, and the perfect fly emerged after six days in the pupal
stage.
[Illustration: FIG. 78.--Larva of _Limnobia
replicata_.]
The following description of this larva is principally condensed from
De Geer, whence the figures are also taken. The body is long and
cylindrical, and divided into eleven segments, of which the first and
largest is sub-triangular. The second and third segments are shorter
than the rest.
The head (_t_) is very small and completely retractile within
the first segment, the anterior margin of which completely closes
the orifice, so that, in this condition, the creature appears to be
headless. The body is covered with spines, some simple and others
branched. On the first three segments there are only simple spines;
but from the fourth to the tenth segment inclusive there are also on
each segment two forked spines--that is, fourteen in all. On the last
segment the spines are simple, and here are also four chitinous hooks,
one pair much larger than the other. These simple spines carry a white
vessel, which extends throughout their whole length; but in the forked
spines there are two such vessels placed side by side in the stem, and
diverging one to each branch.
He comes to the conclusion that these spines are probably the breathing
apparatus of the larva, for they are similar to those which he observed
in an aquatic caterpillar. Walker[62] calls these spines ‘long
filamentous processes, which appear to be internally supplied with
air-tubes,’ but he does not seem to have taken the trouble to break up
a specimen, or he would have been in no doubt as to their real nature.
This is shown by the fact that the larva never comes to the surface to
take in a supply of air, but contents itself with the oxygen dissolved
in the water.
[Illustration:
FIG. 79.--Forked spine of Limnobia, enlarged; the dark
lines show air-tubes.]
The pupa is quite as remarkable as the larva, though the breathing
apparatus does not assume the form of spines, but consists of two
‘trumpets,’ one on each side of the head, as is the case with the
pupae of gnats, using that term in a wide sense. The colour is a
greenish-brown, dotted with black. The abdomen is capable of a good
deal of motion from side to side; and by this means the pupa can raise
itself to the surface of the water to take in a supply of air.
De Geer remarked that when the pupa rose for this purpose it lay with
its body horizontal, having the lips of the trumpets a little above the
surface. It appeared not to like being placed on its back, because in
that position the trumpets cannot be raised above the surface. When he
tried the experiment of putting the pupa back downwards, it wriggled
over by bending the abdomen.
[Illustration: FIG. 80.--Pupa case of Limnobia.
(From a photograph taken at the Yorkshire College, Leeds.)]
On the abdomen there are several chitinous hooks, which serve in some
sort as substitutes for limbs. By their means the pupa can moor itself
to the stems of aquatic plants; and this is necessary, since its
specific gravity being less than that of the water it would be always
at the surface, if it had no such means of anchorage. And of course,
at the top of the water, it would be exposed to the danger of being
snapped up by birds.
De Geer’s specimen passed six days in the pupal state, and then emerged
as a perfect Crane-fly. My specimens did not emerge till after ten and
eleven days of pupahood, which seems strange, as they were plentifully
supplied with food in their larval stage.
The fly is a little more than half an inch long, and may be
distinguished from the common Daddy-longlegs by the character of the
wings, which are folded somewhat like those of a wasp, but with this
difference: that the wings of this fly are folded outward, while those
of the wasp are folded inwards. When the insect wishes to fly, it opens
the fold so that the whole wing presents a plane surface, but the fold
reappears directly the Crane-fly ceases its flight (Frontispiece).
[Illustration: FIG. 81.--Fore-wing of Bee, showing
marginal fold (× 7).]
De Geer’s allusion to the wing of the wasp might have been extended
to most of the Hymenoptera, as we may see by examining the fore and
hind wings of a bee or a sawfly. Dr. Sharp[63] says, ‘The wings [of
the Hymenoptera] are remarkable for the beautiful manner in which the
hinder one is united to the anterior one, so that the two act in flight
as a single organ. The hind wing is furnished with a series of hooks,
and the hind margin of the front wing is curled over so that the hooks
catch on to it. In some of the parasitic forms the wings ... have no
hooks. The powers of flight, in these cases, are probably but small.’
If we were taking our subjects in consecutive order, the larva of
Ptychoptera would properly come here, as being that of a Crane-fly. But
since it will be convenient to examine another larva which resemble
Limnobia in its breathing apparatus, we will take next the larva of
_Parap´onyx stratiota´ta_, one of the China Marks, for it is
extremely interesting and by no means hard to come by.
The China Marks are small moths, with white wings bearing dark
markings, which have been supposed to resemble Chinese characters.
Their larvae are aquatic in varying degree--that is, some breathe air
in the usual way, by means of spiracles; while others, by means of
gills, breathe the oxygen dissolved in the water.
Many collectors have, no doubt, taken these larvae, and cast them away
in the belief that they were caddis-worms. Such was the experience of
the Rev. Gregory Bateman, the author of _Fresh-water Aquaria_,
who says (p. 259): ‘While hunting for fresh-water animals, one not
seldom comes across an insect wrapped up in two or more green leaves,
or pieces of leaves, of some aquatic plant. The leaves and the animal
have somewhat the appearance of a caddis-worm in its case; in fact, for
a time, before I knew what it was, I mistook it (and I daresay others
have done so also) for a caddis-worm.’
The cases are usually, but not invariably, made from the food-plant
of the insect. Mr. Bateman has noted that these larvae ‘do not
always confine themselves to the same weed, either for food or for
building material.’ This has also been my experience. A larva of the
Brown China Mark, recently taken on the Norfolk Broads--an excellent
collecting ground for aquatic larvae--was put into a tube. The case
had been damaged, and the only vegetation in the tube was a spray of
bladderwort. On examining the tube, after some days, I failed to find
the larva. The reason was evident on removing the cork, a small part
of which had been gnawed away to procure material for the repair of
the larval case, which was affixed to the under side. The larva was
dead and too much decayed to be put into pickle, a circumstance I much
regret, as I should like to have preserved the larva in such a strange
dwelling. As it is, I have only been able to keep the house without its
tenant.
Pondweed is the usual home of the larva of the Brown China Mark, and
from the leaves of this plant the first larval case is generally
fashioned. This was the species upon which Réaumur made his interesting
observations, most of which have been confirmed by succeeding
observers. In well-grown larvae the contrivance by which the animal
is protected from contact with the water in which it lives should be
noted, as it may be easily, with the hand lens. The skin is thickly set
with tiny protuberances between which the water cannot penetrate, the
surface film stretching from tip to tip of these prominences, just as
it does over the hairs that cover the body of a water-spider.
De Geer[64] describes an aquatic larva of one of the China Marks
(_Paraponyx stratiotata_), which has its breathing apparatus
fashioned on a similar plan to that of Limnobia, though there is some
difference in the details. He found his specimens on the leaves of the
Water Soldier, and his interesting account recalls the fact to memory
that this remarkable plant was at one time called the Marsh Aloe--an
exceedingly appropriate name.
He describes the filaments on the body of the larva, and concluded
that they were probably breathing-organs, because of the dark-coloured
vessels within them. These he traced, as we will presently do, to their
union in the stem of the gill, and thence to the air-vessels running
down each side of the body of the larva. He fed them on leaves of the
Water Soldier, and kept them through the winter. In the following June
he found them preparing to undergo their transformation into the pupal
stage, and at the end of the month the moths came out. He was gratified
by seeing the congress of these insects. The females deposited their
eggs on the floating vegetation and on the sides of his aquarium, a
little below the surface of the water, and in about eight days the
young larvae were seen.
[Illustration: FIG. 82.--Larva of _Paraponyx
stratiotata_ (enlarged).]
These larvae must be very abundant, though they do not seem to be often
taken by collectors. In describing an allied (American) species, which
is found on Vallisneria and pondweed, Mr. Hart says[65], ‘They feed
at first exposed on the leaf, but later two or even three leaves are
loosely webbed together, face to face, by each larva, between which it
remains concealed while feeding. They are, therefore, hard to discover
unless their hiding-places are broken up by seining, or the like, when
the larvae may be seen swimming about.’ This is, no doubt, the reason
why these larvae are not more often taken. Anything like a seine net is
of course out of the question for us; but masses of vegetation may be
readily broken up by vigorously working the bottle and net backwards
and forwards amongst them. Specimens I have seen were taken among
duckweed; and Mr. Hart mentions one instance of part of the larval case
being constructed of ivy-leaved duckweed, ‘which was abundant there at
that time.’
Now let us bring our lens to bear, so that we may make out the external
structure, and recognize the similarity of the breathing-organs of this
Moth-larva to those of the Crane-fly larva already treated of (p. 168).
In order to make out the scheme of the gills, which is somewhat
complicated, one should first of all distinguish the spiracles,
remembering that they are not functional. And it is best to begin with
those on the middle segments of the body. They may be detected as
little dark spots, sometimes enclosed in a ring. The head, the first
segment of the thorax, and the last segment of the body, bear no gills;
the second segment of the thorax has but two pairs on each side; and
there is but a single gill on each side of the ninth segment of the
body. On the other (nine) body-segments there are the full number
of five gills on each side, arranged two above, and three below the
spiracle. The upper pair are called supra-stigmals, or gills which lie
_above_ the spiracles; the lower pair are called infra-stigmals,
or gills which lie _below_ the spiracles; and the single one, the
lowest, is known as the pedal or foot gill. These technicalities need
not give us any trouble here, in our examination of the larva; nor do
they present any real difficulty. But it is worth while to master the
arrangement as soon as we get hold of one of these larvae, and then we
shall be able to take up and understand technical descriptions of this
aquatic caterpillar and its allies, in so far, at least, as they refer
to the breathing apparatus.
The gills differ in their character: some few are simple, while most of
them are more or less branched. In Limnobia the branching of the gills
is into a simple fork; in Paraponyx this kind of division also occurs,
and in others most of the gill branches are also given off from the
main stem below one of the branches of the fork.
In Fig. 84 we have a representation of one of these branched gills.
It will not be difficult for us to make out the details as there
shown. But the vessels that run down into the filaments, constituting
them breathing-organs, are smaller than those of Limnobia, and will
consequently require a little more care and patience before we can
distinguish them.
[Illustration:
+---------------+
| GILL GILL |
| SPIRACLE |
| GILL GILL |
| GILL |
+---------------+
FIG. 83.--Diagram of segment of Paraponyx, showing
arrangements of tracheal gills.]
[Illustration:
FIG. 84.--Gill of Paraponyx larva. _a_, _b_,
stem; _c_, _d_, _e_, _f_, branches;
_g_, attachment of the air-vessel of the gill to the main
tracheae of the body. (After De Geer.)]
One would think that, with such an array of gills, this larva ought
to be in good case for its air-supply. It may, perhaps, be doubted
whether this is so. At any rate, the creature adopts the same plan as
the larva of Chironomus, which has no gills at all, for driving away
from its case water that has parted with its oxygen. Water charged with
oxygen pours into the case, and so the air-supply is renewed. This plan
is nothing more than keeping the fore-part of the body in undulating
motion, the tail in both the larvae serving as a point of attachment.
One or two that I have kept made their cases against the side of the
bottle, and so afforded an excellent opportunity of seeing them in this
motion. The Tanypus larva does the same thing. Against the side of one
of my small aquaria a Tanypus larva and a Chironomus larva have both
made tubes; and as I look up from writing these lines I can see them
both hard at this work.
The larva of the Alder-fly (_Si´alis luta´rius_) is also furnished
with tracheal gills, seven on each side. So little is known of the
life-histories of common insects that it may be profitable to introduce
the account of an observer who watched the deposition of the eggs and
the emergence of the young larvae:
‘On April 25 I found, on the rushes round the margin of a small pond,
a great many patches of eggs, and shortly after observed many of the
_Sialis lutarius_ depositing them.
‘They form large patches of from two to three inches in length,
generally encircling the whole rush near the top, but sometimes
deposited on one side only, and extended to about a line in breadth.
‘I counted 100 in a square line, so that each batch may be fairly
considered to contain from 2,000 to 3,000 eggs; the greater portion of
which must consequently perish either in the egg or larva state; as,
common as the insect is, and widely distributed throughout the country,
we should be perfectly overwhelmed with the swarms of the perfect
insect if such were permitted, when it is considered that round this
one small pond there could not have been less than 100 patches of them.
‘The eggs are of a very singular form, and placed in a slanting
position.
‘The females, while depositing them, appeared perfectly motionless on
the rush, and varied considerably in size, being from five lines to
nearly double that in length. Some parts of the patches of eggs are of
a much lighter colour than the rest.
‘On May 3 I found many of the eggs hatching, the little larvae tumbling
about in great numbers, with their bodies erected like [the larvae of]
the _Staphylinidae_.
‘On putting them into water they swam about with the greatest activity,
wriggling and undulating their bodies about much like a serpent or the
tadpoles, and working their legs at the same time[66].’
The author draws attention to the disproportionately large head of the
larvae, which, however, he did not describe, as he had ‘brought some of
them alive, and some eggs for exhibition.’
Sialis larvae occur in most ponds with muddy bottoms. They may be taken
by scooping up some of the mud in a long-handled spoon--a most useful
instrument for the collector--and washing it, or by throwing in the
drag, and bringing to land a mass of water-weed, roots and all. A few
may generally be detected near the roots. They may be picked up with a
small pair of forceps, or with a brush, and dropped into a bottle; or,
better still, into separate tubes; for they are fierce and voracious,
and, failing other food, by no means indisposed to prey on each other.
Their general appearance, and especially their powerful mandibles, give
them some resemblance to the larvae of a water beetle, for which a
celebrated naturalist not unnaturally took them, when he began to study
them. And this would seem to be the opinion of some mounters, for I
have a slide of the mouth parts of this larvae, labelled ‘Mouth parts
of the larvae of a water beetle.’ It was not till I had broken up a
good many Sialis larvae that I found out what the slide really was.
These larvae may be kept alive in small bottles of water, if they
are supplied with food. They will eat Chironomus larvae and those of
Tanypus. Professor Miall has found that they will eat caddis-worms and
May-fly larvae. Probably, no small aquatic creatures that they can
overcome are safe from them.
A larva that is full-fed, and ready to change to become a pupa, will
measure about an inch in length or a little more (Fig. 85). The general
colour is brownish, with dark markings. The legs are powerful, and our
lens will show us that they end in two strong curved claws. From each
of the first seven segments of the abdomen are given off a pair of
jointed tracheal gills or breathing-organs, which are directed upwards
and backwards when the larva is at rest--a rare occurrence--but which
wave to and fro in the water when the creature is swimming.
From this fact has arisen the statement found in most books that the
larva uses these gills not only for respiration, but for locomotion.
Professor Miall has come to a contrary conclusion, and, as he has
kindly informed me, is confirmed in his opinion by the weakness of the
muscles.
[Illustration:
FIG. 85.--Larva of Sialis (enlarged).]
It will be well to make repeated observations till we are satisfied
on the subject. When these larvae are kept, the conditions necessary
to provide them with food and to keep the water aërated by means of
growing vegetation are unfavourable to close observation. It will,
therefore, be necessary to remove one or more of these larvae to a
bottle in which there is nothing but pure water.
The work is now rendered much easier. There is nothing to obstruct. As
soon as the larvae reach the bottom they will walk round and round,
giving us a good opportunity of watching them. In swimming--which may
be backwards as well as forwards--the abdomen is waved from side to
side. To see the backward motion one need only put a dipping-tube or a
pencil, or the like, in front of the larva, so as to bar its progress.
The creature will retreat a step or two, and then, with a flourish of
the abdomen, dart back through the water. The larva will sometimes wave
the abdomen up and down, just as one may see a Chironomus larva do when
it has affixed its dwelling to the side of the glass, and this motion
probably assists the process of respiration.
When the larvae have been watched under the conditions above described,
I have never been able to detect independent motion of the gills. But
it is better that every one should observe for himself, and draw his
own conclusions from what he sees.
Now let us examine a specimen more closely with the lens, or under the
dissecting microscope. The mouth parts may be broken up separately, or
a little careful manipulation will enable us to see the chief features
without injuring the specimen. The head is strong and massive, and
the group of ocelli, or simple eyes, may be clearly made out. The
antennae bear a small pencil of hairs, no doubt sensory in function,
at the extremity, but careful management of the light will be required
to distinguish them. The mandibles are extremely business-like
instruments, and each bears two strong teeth on the inner side. Next
come the maxillae, with their palps, each of which has an appendage,
while each maxilla carries three strong spines. The labium, with its
palps, and the mentum, with its saw-like notchings, may be plainly seen.
The three segments of the thorax offer no difficulty. The legs are
worth more than a cursory examination from their apparatus of spines
and double fringe of hairs. Nine body-segments succeed to the thorax,
and behind these is a long tail-like organ, which some authors consider
represents a tenth segment.
[Illustration:
_h_ _t_ 1 2 3 4 5 6 7 8 9 tail
-- ----- -- -- -- -- -- -- -- -- -- ------
| | | | | | | |
FIG. 86.--Diagram of Sialis larva, showing arrangement
of gills (represented by vertical lines). _h_, head;
_t_, thorax; 1–9, segments of abdomen.]
The gills are seven on each side, and are given off from the spaces
in front of the first seven segments of the abdomen. Each gill is
five-jointed--an unusual arrangement, for the gills of the larvae of
Limnobia and Gyrinus are without joints. With the half-inch Leitz the
branching tracheal tube in the gill may be seen, as well as the double
fringe of hairs and the long hairs at the extremity. The tail-like
organ, though without joints, bears a close resemblance to the gills.
It has two tracheal tubes running through it, and it is fringed on both
sides with hair. Indeed, Professor Miall, F.R.S., compares it to ‘two
ordinary tracheal gills completely fused together.’ The first glance
will convince the observer that the comparison is just.
[Illustration: FIG. 87.--Pupa of Sialis.]
The pupa (Fig. 87) need not detain us, for the larva undergoes its
transformation in the ground, not in the water, where it could be
watched. But it is interesting to notice that the legs and wings are
enclosed in separate cases, and that the segments of the abdomen bear
spines. These spines are extremely serviceable to the pupa when making
its way out of its cell to emerge as a perfect insect, which is well
known, especially to fishermen, as the Alder-fly. It may be found near
streams, and rarely uses its wings.
_Ptychoptera paludosa_ is a small Crane-fly, with an aquatic larva
which will repay observation. It is one of the group generally called
‘rat-tailed maggots,’ from the peculiar character of the breathing
apparatus, which consists of a retractile tube at the end of the
abdomen. It is, I believe, better known to some dealers than the larva
of the Drone-fly, the rat-tailed maggot of Réaumur. A few months ago I
wanted some Drone-fly larvae, and asked a dealer to supply me. When the
larvae arrived and were turned out for examination, they proved to be
Ptychoptera larvae--which I did not want. I naturally wrote to point
out the mistake; and was told, in reply, that the larvae sent were the
only ‘rat-tails’ known to my correspondent.
This larva is a mud-dweller, and is best captured by scooping up
surface mud near the banks of pools and ditches, just where the water
shallows on to the shore. This should be washed in a small dish or
saucer, so as to carry away the mud and leave the larvae wriggling
on the bottom. They may be picked up with a brush and dropped into a
bottle for transport home.
There is not the slightest difficulty about keeping them for
observation. A bottle of the capacity of six or eight fluid ounces
will make a good aquarium for a dozen or even twenty. The bottom should
be covered to the depth of about an inch with mud fairly rich in
organic matter. My own plan has been to use the accumulation from the
bottom of a large aquarium. In this the larvae will bury the body, and
feed, the tail protruding and thrust up to the surface of the water, of
which there should be about two inches above the mud.
This is a liberal allowance of space. A couple of these larvae lived
with me for some months in a glass capsule two inches in diameter and
three-quarters of an inch in height. The mud at the bottom and the
water covering it together measured about half an inch. Both pupated,
and in due time from the pupa cases a perfect insect came out.
But that larvae may pupate, they require to be well fed. How shall
we know when the bulk of the nourishment has been extracted from the
mud? From the castings of the larvae; and these, though of a different
shape, are as easy to be distinguished as the castings of the earthworm
in the garden or those of the lobworm on the seashore. All the mud that
passes through the bodies of the larvae is discharged in the form of
tiny hard, cylindrical pellets; and when the mass consists of these
pellets it should be changed, or the larvae will go short of food. They
will, however, support long fasts.
From Fig. 88 we may get a good idea of the appearance of the larvae
when kept in confinement. The figures are rather less than natural
size, and all the attitudes were sketched from life. One is seen
extended on the bottom; two are partially buried in the mud, with the
breathing-tube protruding. The larva on the mud, and bent into curves,
is just about to rise to the surface; others are shown in the act of
rising, while one has its breathing-tube raised above the surface, and
another is attached by the breathing-tube to the side of the glass
vessel. The larva with the star-like process at the end of the tail is
that of Odontomyia, a large bee-like fly.
[Illustration: FIG. 88.--Larvae of _Ptychoptera
paludosa_ (from life).]
A larva of good size, like that of Ptychoptera, is especially easy to
examine; and by reason of its transparency the tracheal tubes may be
clearly traced. The under surface of the larva should be first looked
at, and its adaptation for existence in the mud of a pond-bottom will
be evident. The creature is legless, but possesses three pairs of
false legs armed with dark-coloured hooks, and each body-segment bears
a circle of stiff hairs, which enable the larva to travel through
the mud, in the same way that the earthworm moves through the soil.
Moreover, the segments between these circles are pretty thickly set
with hairs.
The tracheal tubes run down on each side of the body, not in a direct
line, for there is a most ingenious arrangement by which contraction
and expansion of the larva, and the protrusion and retraction of
the tail, are provided for. One can easily discern that in most of
the segments the tubes are large, and that these large portions are
connected by smaller tubes, whence others are given off into the body.
These connecting-tubes are loop-like when the larva is of the normal
length, but are straightened out, thus adding to their length, when the
larva is extended.
The opening and closing of these loops may be observed at leisure if
a larva be put in a long excavated slip, with some water, and then
covered with a plain glass slip. The two slips, fastened together with
small elastic bands, should then be laid on the stage of the dissecting
microscope for examination; or they may be held in the hands, and the
movements of the larva watched with the hand lens.
In the posterior segments of the body the tracheal tubes run side by
side, while in the tail itself they are, so to speak, intertwined. When
the tip of the tail pierces the surface-film a fresh supply of air is
taken in.
At the base of the extensile portion are two processes which diverge,
one on each side, at an angle of 45 degrees. These, according to a
German observer, are tracheal gills, and they are absorbed just before
the larva enters the pupal condition.
Réaumur found these larvae plentifully in the Bois de Boulogne, and
gives a figure[67]. He was not, however, successful in rearing the fly.
Lyonnet not only took the larvae and kept them in an aquarium, but
watched their change into the pupal condition, and saw them emerge as
perfect insects. An abstract of this description will probably be of
interest.
[Illustration:
FIG. 89.--_A._ Ptychoptera larva (enlarged).
_B._ Tail, showing air-vessels (still more enlarged).
(After Lyonnet.)]
He tells us that the larvae showed signs of changing into pupae in
June. The change was made without the larvae leaving the water, and
they underwent all their transformations in less than a fortnight.
At the approach of the change the larvae became whiter in colour,
but less transparent. Then they cast their skin, leaving therein the
air-vessels, or rather their external covering. After this last moult
was over, he was surprised to find that the tube which formed the tail
of the larva, and by which it took in a supply of air, though it serves
the same purpose in the pupa, is attached to the thorax, near the top
of the head[68]. Lyonnet appears to have overlooked the fact that there
was a second and shorter tube given off from the thorax, which most
observers consider to be functionless (Fig. 90).
[Illustration: FIG. 90.--Pupa of Ptychoptera. (After
Lyonnet.)]
Strange as is the larva, the pupa is stranger still, and seems even
better adapted for existence in the mud. The hinder part of each
segment of the abdomen is thickened and studded with chitinous
projections. This thickening is more marked, and of greater extent in
each succeeding segment, reaching its greatest development in the last
segment, which is armed with hooks. The body part of each segment bears
rows of smaller spines, so that this pupa should have little or no
difficulty in moving through pretty thick mud.
Several of these Crane-flies have passed through all their stages with
me, and in nearly every case the transformation from pupa to perfect
insect was made in water--in a tube three inches long, with a diameter
of about an inch. The larvae were taken about the middle of September.
My notes show that the first pupated on November 17, and the first fly
came out on November 25.
The long breathing-tube of the pupa was several times purposely
displaced from its position on the surface. The creature was evidently
incommoded, and twisted itself into strong curves; the head was thrown
from side to side till part of the breathing-tube was raised above the
surface and adhered to the side of the glass tube. Nor did the efforts
cease till a considerable portion of the tube was in free communication
with the air. This would seem to show that the air-supply is taken
through the bladders which appear at irregular intervals in the
breathing-tube, and not at the tip, where the keenest observers have
failed to find an opening.
INDEX
Acridiidae, 80.
Alder-fly, larva of, 177;
pupa of, 182.
American Cockroach, 74.
Amphipods, 141;
Rev. T. R. R. Stebbing on, 141.
Aquaria, description of, 20.
Arachnoidea, 96.
_Argyroneta aquatica_, 108;
De Geer on, 109;
nest of, 110.
Arthropods, 27;
divisions of, 28;
description of, 29.
Ascidians, Darwin on, 13.
_Asellus aquaticus_, description of, 153.
Assassin Worm, 158.
Aurivillius, Dr., on Crabs, 150.
Bate and Westwood on Gammarus, 147;
on Talitrus, 151.
Bateman, Rev. G. C., on Hydrophilus, 50;
on Nepa, 91;
on China Marks, 171.
Beakers, 21.
Beetle Mite, 119.
Belt on Blatta, 67.
Black Beetle, 63.
Black-bobs, 64.
Blackwall on Spiders, 101, 102.
_Blatta orientalis_, description of, 63, 66;
Gilbert White on, 64;
Dr. Sharp on, 65;
Belt on, 67;
Sir J. Lubbock on, 68;
anatomy of, 68;
breathing of, 70;
Swammerdam on, 74;
Professor Plateau on, 74.
Book Scorpions, 96.
_Brachelytra_, 59.
Broad-clawed Porcelain Crab, 139.
Brown China Mark, 172.
Brushes, 23.
Bugs, 86.
Burgess, Mr., on larva of Dytiscus, 160.
Butler, Dr., on Spiders, 100.
Butler, Mr. E. A., on Forficula, 79.
Capsules, 21.
_Carcinus maenas_, description of, 136.
Centipedes, 122;
marine, 125.
Cerci, 47.
Chelate, meaning of word, 132 _n._
China Marks, 171;
Rev. G. C. Bateman on, 171;
De Geer on, 172.
Cockroach, 63.
Cocktail Beetle, 57.
Coleoptera, 32.
Collecting, method of, 25.
Corixa, 94;
Graber on, 95;
Sir J. Lubbock on, 95.
Crab, Shore, 136;
Porcelain, 139.
Crane-fly, 165.
Crustacea, 128.
Daddy-longlegs, 103.
Dallas on Ocypus, 61.
Darwin on microscopes, 12;
on Ascidians, 13.
Devil’s Coach Horse, 57.
Dipping-tubes, 23.
Dissecting microscope, 18.
Dissection, mode of, 37.
Diving Spiders, 108.
Dugès on Water Mites, 117.
Dung Beetle, 119.
_Dytiscus marginalis_, 31;
etymology of, 33 _n._;
breaking of shells by, 33;
Sowerby on, 34;
description of, 35;
food of, 36;
anatomy of, 37;
habits of, 41;
Kirby on, 41;
Sir J. Lubbock on, 42;
Professors Lowne and Miall on, 45;
Dr. Sharp on, 47;
abdomen of, 47;
breathing of, 48;
larva of, 157;
Swammerdam on, 158, 159;
Mr. Burgess on, 160.
Earwig, 75.
Edriophthalma, 129.
Elytra, 41.
_Epeira diadema_, description of, 98;
web of, 98;
J. A. Thomson on, 100;
Dr. Butler on, 100;
Blackwall on, 101, 102;
mode of taking its prey, 102;
Hudson on, 103;
feet of, 104;
spinnerets of, 105.
Evans, W., on Sialis, 177.
_Field_, the, quoted, 33, 35.
Forceps, 22.
_Forficula auricularia_, 75;
Kirby and Spence on, 76;
De Geer on, 76;
young of, 76;
forceps of, 77;
anatomy of, 77;
Butler on, 79;
Dr. Sharp on, 79.
Formalin, 24.
Forty-legs, 122.
Fresh-water Shrimp, 142.
_Gamasus coleoptratorum_, 119.
_Gammarus marinus_, 146;
Dr. J. Salter on, 147.
_Gammarus pulex_, 142;
anatomy of, 144.
Geer, De, on Forficula, 76;
on Locusta, 80;
on Water Mites, 116;
on Beetle Mites, 120;
on Lithobius, 123;
on Julus, 126;
on larva of Limnobia, 167;
on Paraponyx, 172.
Geophilus, 124.
_Geophilus crassipes_, 125;
Mr. Pocock on, 125.
Glass block, 22.
Glass box, 22.
Glass slips, 22.
Gosse on Prawns, 133.
Graber on Corixa, 95.
Grasshopper, Great Green, 80.
Great Water Beetle, 49.
Hart, Mr., on Paraponyx, 173.
Harvestmen, 96.
Hemiptera, 86.
Hill, Dr., on Nepa, 90.
Hudson on Spiders, 103.
Hundred-legs, 122.
Hunting Spiders, 98.
Hydrachna, 113.
_Hydrachna globula_, 118 _n._
Hydrophilus, larva of, 164;
Lyonnet on, 164.
_Hydrophilus piceus_, 49;
Bateman on, 50;
legs of, 53;
Simmermacher on, 53;
breathing of, 54;
cocoons of, 54;
Lyonnet on, 54.
Hymenoptera, wings of, 170.
Idotea, 143.
_Idotea pelagica_, 155.
Illinois State Laboratory, mode of collecting in, 25.
‘Insects,’ 27;
divisions of, 32.
Invertebrates, 28.
Isopods, 152.
Julus, 121, 124.
_Julus terrestris_, 126;
De Geer on, 126;
Dr. Sharp on, 126.
Jumping Spiders, 105.
Kew, Mr., on dispersal of shells, 130.
Kirby on Dytiscus, 41;
on Ocypus, 59;
on Forficula, 76;
on Locusta, 83;
on insect noises, 84;
on Salticus, 107.
Knives, 24.
Land Bugs, 86.
Leaping Orthoptera, 79.
Leitz, lenses of, 16.
Lichtenstein on Locusta, 83.
Limnaea broken by Dytiscus, 33.
_Limnobia replicata_, larva of, 165;
De Geer on, 167;
pupa of, 168.
_Lithobius forficatus_, 122;
De Geer on, 123;
Dr. Sharp on, 123.
_Locusta viridissima_, 80;
Dr. Sharp on, 80, 85;
De Geer on, 80, 83;
Westwood on, 81;
Kirby on, 83;
Lichtenstein on, 83;
ear of, 85.
Locustidae, 80.
Lowne, Professor, on Dytiscus, 45.
Lubbock, Sir J., on Dytiscus, 42;
on Blatta, 68;
on Corixa, 95;
on Opossum Shrimp, 135.
Lyonnet on larva of Hydrophilus, 164;
on Ptychoptera, 186.
Magnification, power of, 15 _n._
Mainland, Mr. G. E., on Water Mites, 118.
Malacostraca, 128;
Rev. T. R. R. Stebbing on, 128.
Margined Water Beetle, 31.
Masking Crabs, 150.
_Melicerta janus_, 150.
Miall, Professor, on Dytiscus, 45;
on Sialis larvae, 179, 181.
Microscopes, use of, 11;
description of, 15.
Millepedes, 121, 125.
Mites, description of, 112.
Model, a calico, 30.
Myriapods, 96, 120.
Mysis, 135.
_Nature Notes_ on Millepedes, 121.
Needles, 23.
_Nepa cinerea_, 86;
mode of keeping, 88;
Swammerdam on, 89;
Dr. Hill on, 90;
Rev. G. C. Bateman on, 91;
anatomy of, 91.
Noises made by insects, 84.
_Notonecta glauca_, 93.
_Ocypus olens_, 57;
Kirby on, 59;
as a pet, 59;
Dallas on, 61;
larva of, 61.
Opossum Shrimp, 135;
Sir J. Lubbock on, 135.
_Orchestia littorea_, 151.
Orthoptera, 63.
Ovipositors, 82.
_Palaemon serratus_, 129.
_Palaemonetes varians_, 130.
_Paraponyx stratiotata_, 171;
De Geer on, 172;
Mr. Hart on, 173;
anatomy of, 174.
Peripatus, 30;
Professor Sedgwick on, 31.
_Periplaneta americana_, 74.
Pholcus, 103.
Phylum, definition of, 27.
Planorbis broken by Dytiscus, 33.
Plateau, Professor, on Blatta, 74.
Pocket lens, description of, 13.
Pocock, Mr., on Geophilus, 125.
Podophthalma, 129.
_Porcellana platycheles_, 139.
Prawns, 129;
Gosse on, 133;
Sowerby on, 133.
Ptychoptera, larva of, 182;
pupa of, 187;
Réaumur on, 185;
Lyonnet on, 186.
Quekett Microscopical Club, incident at, 20.
‘Rat-tails,’ 182.
Réaumur on Ptychoptera, 185.
Robertson, Mr., on Talitrus, 151.
Salter, Dr. J., on Gammarus, 147.
_Salticus scenicus_, 105;
Kirby and Spence on, 107;
foot of, 108.
Sand-hopper, 151.
Scorpions, 96.
Sedgwick, Professor, on Peripatus, 31.
Sharp, Dr., on Dytiscus, 47;
on Blatta, 65;
on Forficula, 79;
on Locusta, 80, 85;
on Lithobius, 123;
on Julus, 126;
on wings of Hymenoptera, 170.
Shells broken by Dytiscus, 33;
Mr. Kew on dispersal of, 130.
Shore Crab, 136.
Shore-hopper, 151.
Shrimp, 132.
_Sialis lutarius_, larva of, 177;
W. Evans on, 177;
anatomy of, 180;
pupa of, 182.
Sicarius, 158.
Simmermacher on Hydrophilus, 53.
Sirex, 82.
Sowerby, G. B., on Dytiscus, 34.
Sphaerium, 130.
Sphaeroma, 155.
Spiders, general description of, 96;
Trap-door, 97;
Hunting, 98;
Garden, 98;
Jumping, 105;
Diving, 108.
Stebbing, Rev. T. R. R., on etymology of Dytiscus, 33 _n._;
on Malacostraca, 128;
on meaning of _chelate_ and _sub-chelate_, 132 _n._;
on Amphipods, 141.
Stewart, Professor, incident of, 20.
Stilopyga, 63.
Swammerdam on Blatta, 74;
on Nepa, 89;
on Water Mites, 114;
on Woodlice, 156;
on Dytiscus larva, 158, 159.
Tailed Wasp, 82.
_Talitrus locusta_, 151;
Bate and Westwood on, 151;
Mr. Robertson on, 152.
Tanypus, larva of, 176.
Tegetmeier, Mr. W. B., on destruction of shells, 34.
Thomson, J. A., on Spiders, 100.
Trap-door Spider, 97.
Vertebrates, 28.
Wart-eater, 81.
Water Beetle, Margined, 31;
the Great, 49.
Water Boatman, 93.
Water Bugs, 86.
Water-devil, 157.
Water Mites, 113;
Swammerdam on, 114;
De Geer on, 116;
Dugès on, 117;
Mainland on, 118.
Water Scorpion, 86.
Water-tiger, 157.
Westwood on Locusta, 81.
White, Gilbert, on Blatta, 64.
Wing-cases, 41.
Wireworm, 126.
Wood, Rev. J. G., on lens stand, 15.
Woodlice, 155;
Swammerdam on, 156.
Worms, 29.
Wright, Mr. L., on microscopes, 16.
Zeiss, lenses of, 15.
THE END.
FOOTNOTES:
[1] Darwin, _Descent of Man_ (2nd ed.), p. 159, note 23.
[2] The power of magnification of a lens is the ratio of its focal
distance to 10 inches. Thus a lens of 1 inch focus (or focal distance)
magnifies 10 times (written × 10, or ten diameters); one of ½ in. focal
distance, 20 times, and so on.
[3] _A Popular Handbook to the Microscope_, p. 39.
[4] _Journal of the Quekett Microscopical Club_, v. 148.
[5] _Ponds and Rock Pools_, p. 17.
[6] _Bulletin of the Illinois State Laboratory of Natural
History_, iv. 158.
[7] ‘Dytiscus’ is written of set purpose. It is not, as some people
tell us, a miswriting for Dyticus; but a properly formed diminutive,
from the Greek _dutēs_ = a diver; like _paidiskos_ = a little boy.
Linnaeus consistently calls the genus Dytiscus from 1735 onwards.
Dyticus only dates from Geoffroy’s _Histoire Abrégée des Insectes_,
first published anonymously in 1762. On this question of nomenclature I
am glad to have the support of the Rev. T. R. R. Stebbing, F.R.S., who,
in answer to my inquiries, kindly wrote, ‘Darwin uses “Dytiscus” in the
_Origin of Species_, and I should decidedly recommend its being upheld.’
[8] April 4, 1896.
[9] _Popular History of the Aquarium_, p. 258.
[10] May 2, 1896.
[11] International Science Series, No. lxv.
[12] _Aquatic Insects_, pp. 55, 56.
[13] _Zeitschrift f. wiss. Zoologie_, Bd. xl. S. 481.
[14] _Mémoires du Muséum d’histoire naturelle_, xviii. 454 sqq.
[15] I have purposely given _Blatta_ as the generic name, rather
than _Stilopyga_, which should properly be used, as the former is
only employed in very recent literature.
[16] Miall and Denny, _The Cockroach_, p. 20.
[17] _Cambridge Natural History_, v. 231.
[18] _The Senses of Animals_, p. 44.
[19] _Cambridge Nat. Hist._ v. 223.
[20] _Book of Nature_, p. 94.
[21] This refers to the gizzard. _Echinus_ was used to denote the
third stomach of Ruminants (now called the manyplies), because it was
thought to resemble a hedgehog rolled up.
[22] Miall and Denny, _The Cockroach_, p. 118 (note).
[23] _Introduction to Entomology_, letter xi.
[24] _Proceedings of the Zoological Society_, 1892, p. 586.
[25] _Our Household Insects_, pp. 159–163.
[26] Kirby and Spence, _Introd. to Entomology_, ed. 1870, p. 484.
[27] _Cambridge Natural History_, v. 318.
[28] _The Senses of Animals_, p. 75.
[29] J. Arthur Thomson, _Outlines of Zoology_, p. 288.
[30] _Science for All_, ii. 178.
[31] _British Spiders_, p. 10.
[32] _British Spiders_, p. 359.
[33] _Naturalist in La Plata_, p. 188.
[34] _Mémoires_, vii. p. 304 sqq.
[35] _Book of Nature_, pp. 101, 102.
[36] _Mémoires_, vii. 144, 145.
[37] The specimen was kindly identified for me, by Dr. Trouessart of
Paris, as a nymph of _Hydrachna globula_ (Dugès), and has been
deposited in the British Museum (Natural History).
[38] _Mémoires_, vii. 123–128.
[39] _Cambridge Natural History_, vol. v. ch. ii.
[40] _Nature_, Dec. 12, 1895.
[41] _Mémoires_, vii. 569.
[42] _Crustacea_, p. 7.
[43] _Crustacea_, p. 225.
[44] International Science Series, No. lxxv.
[45] ‘A limb is _chelate_ when it has joints that will act
together like a pair of tongs. Generally this character is produced by
the hinging of the seventh joint a considerable way down on the side
of the sixth. When the seventh joint, or finger, can be folded back
upon the sixth, although the latter is not produced into any thumb-like
process to oppose it, the limb is then said to be _sub-chelate_,
the claw being in that case partial, though often extremely efficient.’
Stebbing, _Crustacea_ (International Science Series, lxxiv), p. 45.
[46] _Popular History of the Aquarium_, p. 223.
[47] Lubbock, _Senses of Animals_ (International Science Series,
lxv.), p. 93.
[48] _Aquarium_ (ed. 1856), pp. 41, 42.
[49] _Crustacea_ (International Science Series, lxxiv), pp. 8, 9.
[50] Bate and Westwood, _British Sessile-eyed Crustacea_, i. 8.
[51] _Trans. Connecticut Academy_ (1882), iv. 274, 275, note.
[52] _Ponds and Rock Pools_, p. 118.
[53] _British Sessile-eyed Crustacea_, i. 21.
[54] _Proceedings Nat. Hist. Soc. Glasgow_, vol. i. pt. ii. n. s.
pp. 130–132.
[55] _British Sessile-eyed Crustacea_, ii. 381.
[56] _Book of Nature_, i. 174.
[57] _Mémoires_, iv. 386.
[58] _Proceedings Boston Society of Natural History_, xxi. 223–228.
[59] _Natural History of Aquatic Insects_, p. 47.
[60] _Mémoires du Museum_, xviii. 442, 443.
[61] _Mémoires_, vi. 352–55.
[62] _Diptera_, iii. 281.
[63] _Cambridge Natural History_, v. 494.
[64] _Mémoires pour servir_, i. 577 sqq.
[65] _Bulletin of the Illinois State Laboratory of Natural
History_, iv. 167.
[66] W. Evans, _Trans. Entomol. Soc._ (London), iv. 261.
[67] _Mémoires pour servir_, t. vi. Plate 31.
[68] _Mémoires du Muséum_, t. xix. pp. 103, 104.
Transcriber’s Notes:
1. Obvious printers’, punctuation and spelling errors have been
corrected silently.
2. Some hyphenated and non-hyphenated versions of the same words have
been retained as in the original.
3. Superscripts are represented using the caret character, e.g. D^r.
or X^{xx}.
4. Italics are shown as _xxx_.
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