Insects and Diseases - A Popular Account of the Way in Which Insects may Spread

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Title: Insects and Diseases - A Popular Account of the Way in Which Insects may Spread - or Cause some of our Common Diseases
Author: Doane, Rennie Wilbur, 1871-
Language: English
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*** Start of this LibraryBlog Digital Book "Insects and Diseases - A Popular Account of the Way in Which Insects may Spread - or Cause some of our Common Diseases" ***

Transcriber's Note

The punctuation and spelling from the original text have been faithfully
preserved. Only obvious typographical errors have been corrected.

[Illustration: An artificial lake, nearly dry and partly filled with
rubbish, has become a breeding-ground for dangerous mosquitoes.]

American Nature Series

Group IV. Working with Nature

INSECTS AND DISEASE

A POPULAR ACCOUNT OF THE WAY IN WHICH
INSECTS MAY SPREAD OR CAUSE SOME
OF OUR COMMON DISEASES

WITH MANY ORIGINAL ILLUSTRATIONS FROM
PHOTOGRAPHS

RENNIE W. DOANE, A.B.

_Assistant Professor of Entomology
Leland Stanford Junior University_

LONDON

CONSTABLE & COMPANY LIMITED

1910

HENRY HOLT AND COMPANY

_Published August, 1910_

THE QUINN & BODEN CO. PRESS

RAHWAY, N.J.

PREFACE

The subject of preventive medicine is one that is attracting world-wide
attention to-day. We can hardly pick up a newspaper or magazine without
seeing the subject discussed in some of its phases, and during the last
few years several books have appeared devoted wholly or in part to the
ways of preventing rather than curing many of our ills.

Looking over the titles of these articles and books the reader will at
once be impressed with the importance that is being given to the subject
of the relation of insects to some of our common diseases. As many of
these maladies are caused by minute parasites or microbes the
zoölogists, biologists and physicians are studying with untiring zeal to
learn what they can in regard to the development and habits of these
organisms, and the entomologists are doing their part by studying in
minute detail the structure and life-history of the insects that are
concerned. Thus many important facts are being learned, many important
observations made. The results of the best of these investigations are
always published in technical magazines or papers that are usually
accessible only to the specialist.

This little book is an attempt to bring together and place in
untechnical form the most important of these facts gathered from sources
many of which are at present inaccessible to the general reader, perhaps
even to many physicians and entomologists.

In order that the reader who is not a specialist in medicine or
entomology may more readily understand the intimate biological relations
of the animals and parasites to be discussed it seems desirable to call
attention first to their systematic relations and to review some of the
important general facts in regard to their structure and life-history.
This, it is believed, will make even the most complex special
interrelations of some of these organisms readily understandable by all.
Those who are already more or less familiar with these things may find
the bibliography of use for more extended reading.

My thanks are due to Prof. V.L. Kellogg for reading the manuscript and
offering helpful suggestions and criticisms.

Unless otherwise credited the pictures are from photographs taken by the
author in the laboratory and field. As many of these are pictures of
live specimens it is believed that they will be of interest as showing
the insects, not as we think they should be, but as they actually are.
Mr. J.H. Paine has given me valuable aid in preparing these photographs.

R.W.D.

Stanford University, California,

March, 1910.

CONTENTS

CHAPTER I
PAGE

PARASITISM AND DISEASE 1

Definition of a parasite, 1; examples among various animals,
2; _Parasitism_, 3; effect on the parasite, 4; how a harmless
kind may become harmful, 5; immunity, 6; _Diseases caused by
parasites_, 7; ancient and modern views, 7; _Infectious and contagious
diseases_, 8; examples, 9; importance of distinguishing,
9; _Effect of the parasite on the host_, 9; microbes everywhere, 10;
importance of size, 11; numbers, 11; location, 11; mechanical
injury, 12; morphological injury, 13; physiological effect, 13;
the point of view, 14.

CHAPTER II

BACTERIA AND PROTOZOA 15

_Bacteria_, 15; border line between plants and animals, 15;
most bacteria not harmful, 15; a few cause disease, 15; how
they multiply, 15; parasitic and non-parasitic kinds, 17; how a
kind normally harmless may become harmful, 18; effect of the
bacteria on the host, 18; methods of dissemination, 18; _Protozoa_,
19; _Amoeba_, 19; its lack of special organs, 19; where
it lives, 19; growth and reproduction, 19; _Classes of Protozoa_,
20; the amoeba-like forms, 20; the flagellate forms, 20; importance
of these, 21; the ciliated forms, 22; the Sporozoa or spore-forming kinds,
22; these most important, 23; abundance, 23; adaptability, 23; common
characters, 24; ability to resist unfavorable conditions, 24.

CHAPTER III

TICKS AND MITES 26

_Ticks_, 26; general characters, 27; mouth-parts, 27; habits,
27; life-history, 27; _Ticks and disease_, 28; _Texas fever_, 28; its
occurrence in the north, 28; carried by a tick, 29; loss and
methods of control, 31; other diseases of cattle carried by ticks,
31; _Rocky Mountain spotted fever_, 32; its occurrence, 32;
probably caused by parasites, 32; relation of ticks to this disease,
33; _Relapsing Fever_, 33; its occurrence, 34; transmitted
by ticks, 34; _Mites_, 35; _Face-mites_, 35; _Itch-mites_, 36;
_Harvest-mites_, 37.

CHAPTER IV

HOW INSECTS CAUSE OR CARRY DISEASE 40

Numbers, 40; importance, 41; losses caused by insects, 41;
loss of life, 42; _The flies_, 43; horse-flies, 43; stable-flies, 44;
surra, 45; nagana, 45; black-flies, 46; punkies, 46; screw-worm
flies, 47; blow-flies, 48; flesh-flies, 48; fly larvæ in intestinal
canal, 49; bot-flies, 50; _Fleas_, 52; jigger-flea, 53; _Bedbugs_, 54;
_Lice_, 54; _How insects may carry disease_, 55; in a mechanical
way, 55; as one of the necessary hosts of the parasite, 56.

CHAPTER V

HOUSE-FLIES OR TYPHOID-FLIES 57

The old attitude toward the house-fly, 57; its present standing,
58; reasons for the change, 58; _Structure_, 59; head and
mouth-parts, 60; thorax and wings, 61; feet, 62; _How they
carry bacteria_, 62; _Life-history_, 63; eggs, 63; ordinarily laid in
manure, 63; other places, 63; habits of the larvæ, 64; habits of
the adults, 64; places they visit, 65; _Flies and typhoid_, 65;
patients carrying the germs before and after they have had the
disease, 65; how the flies get these on their body and distribute
them, 66; results of some observations and experiments, 66;
_Flies and other diseases_, 68; flies and cholera, 68; flies and
tuberculosis, 69; possibility of their carrying other diseases, 70;
_Fighting flies_, 71; screens not sufficient, 71; the larger problem,
71; the manure pile, 72; outdoor privies, 72; garbage can,
72; coöperation necessary, 72; city ordinances, 73; an expert's
opinion of the house-fly, 73; _Other flies_, 75; habits of several
much the same but do not enter house as much, 75; the small
house-fly, 75; stable-flies, 75; these may spread disease, 75.

CHAPTER VI

MOSQUITOES 76

Numbers, 76; interest and importance, 76; eggs, 77; always in water, 77;
time of hatching, 77; _Larvæ_, 78; live only in water, 78; head and
mouth-parts of larvæ, 78; what they feed on, 78; breathing apparatus,
79; growth of the larvæ, 80; _Pupæ_, 80; active but takes no food, 80;
breathing tubes, 80; how the adult issues, 81; _The Adult_, 81; male and
female, 81; how mosquitoes "sing" and how the song is heard, 82; the
palpi, 82; _The Mouth-parts_, 83; needles for piercing, 83; _How the
mosquito bites_, 84; secretion from the salivary gland, 84; why males
cannot bite, 84; blood not necessary for either sex, 84; _The Thorax_,
85; the legs, 85; the wings, 85; the balancers, 85; the breathing pores,
86; _The abdomen_, 86; _The digestive system_, 86; _The salivary
glands_, 87; their importance, 87; effects of a mosquito bite, 87;
probable function of the saliva, 88; _How mosquitoes breathe_, 89;
_Blood_, 90; in body cavity, 90; heart, 90; _Classification_, 91;
_Anopheles_, 91; distinguishing characters, 92; eggs, 92; where the
larvæ are found, 93; _Yellow fever mosquito_, 94; its importance, 94;
the adult, 95; habits, 95; habits of the larvæ, 95; _Other species_, 96;
some in fresh water, others in brackish water, 96; Natural enemies of
mosquitoes, 97; how natural enemies of mosquitoes control their numbers,
98; mosquitoes in Hawaii, 98; _Enemies of the adults_, 99; _Enemies of
the larvæ and pupæ_, 100; _Fighting mosquitoes_, 101; fighting the
adult, 102; _Fighting the larvæ_, 103; domestic or local species, 104;
draining and treating with oil, 104; combatting salt-marsh species by
draining, 105; by minnows or oil, 105.

CHAPTER VII

MOSQUITOES AND MALARIA 106

Early reference to malaria, 106; its general distribution, 106;
theories in regard to its cause, 107; insects early suspected, 107;
_The parasite that causes malaria_, 108; studies of the parasite,
108; _Life-history in human host_, 109; its effect on the host, 110;
the search for the sexual generation, 111; _The parasite in the
mosquito_, 112; review of whole life-history, 115; malaria transmitted
only by mosquitoes, 115; _Summary_, 117; experimental proof, 118.

CHAPTER VIII

MOSQUITOES AND YELLOW FEVER 120

A disease of tropical or semi-tropical countries, 120; outbreaks in the
United States, 120; parasite that causes the disease not known, 121;
formerly regarded as a contagious disease, 122; _The yellow fever
commission_, 123; Dr. Finlay's claim, 124; experiments made by the
commission, 125; summary of results, 129; what it means, 130; _results
in Havana_, 131; _the fight in New Orleans_, 132; _In the Panama canal
zone_, 135; _in Rio de Janeiro_, 136; claims of the French commission,
138; _habits of stegomyia_, 139; breeding habits, 139; possible results
of war against the mosquitoes, 139; Danger of this disease in the
Pacific Islands, 140.

CHAPTER IX

FLEAS AND PLAGUE 142

Great scourges, 142; the "black death," 142; old conditions
and new, 143; _How plague was controlled in San Francisco_,
143; _Indian Plague commission_, 144; Dr. Simond's claim,
145; The advisory committee and the new commission, 146;
_Results of Dr. Verjbitski's experiments_, 147; _Results of various
investigations_, 150; _structure and habits of fleas_, 151; feeding
habits, 152; _Common species of fleas_, 153; _Ground squirrels and
plague_, 155; squirrel fleas, 156; _Remedies for fleas_, 157; cats
and dogs, 159.

CHAPTER X

OTHER DISEASES, MOSTLY TROPICAL, KNOWN OR THOUGHT TO
BE TRANSMITTED BY INSECTS 161

_Sleeping Sickness_, 161; its occurrence in Africa, 161; caused by a
Protozoan parasite, 162; the tsetse-fly, 163; _Elephantiasis_, 164;
caused by parasitic worms, 164; their development, 165; how they are
transferred to man, 165; effect on the patient, 166; _Dengue_, 168;
other names, 168; probably transmitted by mosquitoes, 170;
_Mediterranean fever_, 171; cause, 171; may be conveyed by mosquitoes,
171; _Leprosy_, 171; caused by a bacteria parasite, 171; possibilities
of flies, mosquitoes and other insects transmitting the disease, 172;
_Kala-azar_, 173; transmitted by the bedbug, 173; _Oriental sore_, 174;
the parasite may be carried by insects, 174.

BIBLIOGRAPHY 175

Parasites and parasitism, 175; Protozoa, 176; Bacteria, 177; Insects and
disease, 178; Mosquitoes--systematic and general, 179; Mosquito anatomy,
182; Mosquitoes--life-history and habits, 183; Mosquito fighting, 183;
Mosquitoes and disease, 185; Malaria, 186; Yellow fever, 189; Dengue,
192; Filarial diseases and elephantiasis, 193; Leprosy, 193; Plague,
194; Fleas, 198; Typhoid fever, 199; House-flies--anatomy, life-history,
habits, 200; House-flies and typhoid, 202; House-fly and various
diseases, 203; Human myiasis, 207; Stomoxys and other flies, 208;
tsetse-flies, 209; Trypanosomes and Trypanosomiasis, 210; Sleeping
sickness, 211; Rocky mountain fever and ticks, 212; Ticks and various
diseases, 213; Kala-azar and bedbugs, 216; Text or reference books, 216;
Miscellaneous articles, 218.

ILLUSTRATIONS

AN ARTIFICIAL LAKE, NEARLY DRY AND PARTLY
FILLED WITH RUBBISH, HAS BECOME A BREEDING-GROUND
FOR DANGEROUS MOSQUITOES _Frontispiece_

PAGE

FIG. 1. A LAMPREY 2

FIG. 2. _Sacculina_ 2

FIG. 3. _Trichina spiralis_ 2

FIG. 4. AN EXTERNAL PARASITE, A BIRD-LOUSE (_Lipeurus
ferox_) 3

FIG. 5. AN INTERNAL PARASITE, A TACHINA FLY (_Blepharipeza
adusta_) 3

FIG. 6. WORK OF AN INTERNAL PARASITE, PUSS-MOTH LARVA
PARASITIZED BY A SMALL ICHNEUMON FLY 3

FIG. 7. TYPHOID FEVER BACILLI 20

FIG. 8. _Amoeba_ 20

FIG. 9. _Euglina virdis_ 21

FIG. 10. _Spirocheta duttoni_ 21

FIG. 11. _Paramoecium_ 22

FIG. 12. _Vorticella_ 22

FIG. 13. PATHOGENIC PROTOZOA; A GROUP OF INTESTINAL
PARASITES 22

FIG. 14. CASTOR-BEAN TICK (_Ixodes ricinus_) 28

FIG. 15. TEXAS FEVER TICK 28

FIG. 16. TEXAS FEVER TICK (_Margaropus annulatus_) 29

FIG. 17. _Amblyomma variegatum_ 29

FIG. 18. _Ornithodoros moubata_ 36

FIG. 19. THE FOLLICLE MITE (_Demodex folliculorum_) 36

FIG. 20. ITCH-MITE (_Sarcoptes scabiei_) 37

FIG. 21. HARVEST-MITES OR "JIGGERS" 37

FIG. 22. HORSE-FLY (_Tabanus punctifer_) 44

FIG. 23. STABLE-FLY (_Stomoxys calcitrans_) 44

FIG. 24. A BLACK-FLY (_Simulium sp._) 45

FIG. 25. SCREW-WORM FLY (_Chrysomyia macellaria_) 45

FIG. 26. BLOW-FLY (_Calliphora vomitoria_) 45

FIG. 27. BLUE-BOTTLE FLY (_Lucilia sericata_) 50

FIG. 28. FLESH-FLY (_Sarcophaga sp._) 50

FIG. 29. "THE LITTLE HOUSE-FLY" (_Homalomyia canicularis_) 51

FIG. 30. HORSE BOT-FLY (_Gastrophilus equi._) 51

FIG. 31. OXWARBLE FLY (_Hypoderma lineata_) 51

FIG. 32. SHEEP BOT-FLY (_Gastrophilus nasalis_) 51

FIG. 33. CHIGO OR JIGGER-FLEA, MALE (_Dermatophilus penetrans_) 54

FIG. 34. CHIGO, FEMALE DISTENDED WITH EGGS 54

FIG. 35. BEDBUG (_Cimex lectularis_) 55

FIG. 36. BODY-LOUSE (_Pediculus vestimenti_) 55

FIG. 37. ONE USE FOR THE HOUSE-FLY 57

FIG. 38. THE HOUSE-FLY (_Musca domestica_) 58

FIG. 39. HEAD OF HOUSE-FLY SHOWING EYES, ANTENNÆ AND
MOUTH-PARTS 60

FIG. 40. PROBOSCIS OF HOUSE-FLY, SIDE VIEW 60

FIG. 41. LOBES AT END OF PROBOSCIS OF HOUSE-FLY SHOWING
CORRUGATED RIDGES 61

FIG. 42. WING OF HOUSE-FLY 61

FIG. 43. WING OF STABLE-FLY (_Stomoxys calcitrans_) 62

FIG. 44. WING OF HOUSE-FLY SHOWING PARTICLES OF DIRT
ADHERING TO IT 62

FIG. 45. LAST THREE SEGMENTS OF LEG OF HOUSE-FLY 62

FIG. 46. FOOT OF HOUSE-FLY 63

FIG. 47. LARVA OF HOUSE-FLY 63

FIG. 48. BARN-YARD FILLED WITH MANURE 64

FIG. 49. DIRTY STALLS 65

FIG. 50. PUPA OF HOUSE-FLY 76

FIG. 51. HEAD OF STABLE-FLY 76

FIG. 52. MASS OF MOSQUITO EGGS (_Theobaldia incidens_) 76

FIG. 53. MOSQUITO EGGS AND LARVÆ (_T. incidens_) 77

FIG. 54. MOSQUITO LARVA (_T. incidens_), DORSAL VIEW 77

FIG. 55. EGGS, LARVÆ AND PUPÆ OF MOSQUITOES (_T. incidens_) 78

FIG. 56. LARVA OF MOSQUITO (_T. incidens_) 78

FIG. 57. MOSQUITO LARVÆ AND PUPÆ (_T. incidens_) 79

FIG. 58. ANOPHELES LARVÆ (_A. maculipennis_) 79

FIG. 59. MOSQUITO PUPÆ (_T. incidens_) 80

FIG. 60. MOSQUITO PUPA (_T. incidens_) 80

FIG. 61. MOSQUITO LARVÆ AND PUPÆ (_T. incidens_) 80

FIG. 62. A FEMALE MOSQUITO (_T. incidens_) 81

FIG. 63. A MALE MOSQUITO (_T. incidens_) 81

FIG. 64. HEAD AND THORAX OF FEMALE MOSQUITO (_Ochlerotatus
lativittatus_) 82

FIG. 65. HEAD AND THORAX OF MALE MOSQUITO (_O. lativittatus_) 82

FIG. 66. HEAD OF FEMALE MOSQUITO 83

FIG. 67. CROSS-SECTION OF PROBOSCIS OF FEMALE AND MALE
MOSQUITO 83

FIG. 68. WING OF MOSQUITO (_O. lativittatus_) 86

FIG. 69. END OF MOSQUITO WING HIGHLY MAGNIFIED 86

FIG. 70. DIAGRAM TO SHOW THE ALIMENTARY CANAL AND
SALIVARY GLANDS OF A MOSQUITO 87

FIG. 71. SALIVARY GLANDS OF MOSQUITOES 87

FIG. 72. HEADS OF CULICINÆ MOSQUITOES 90

FIG. 73. HEADS OF ANOPHELINÆ MOSQUITOES 90

FIG. 74. WING OF _Anopheles maculipennis_ 90

FIG. 75. WING OF _Theobaldia incidens_ 90

FIG. 76. A NON-MALARIAL MOSQUITO (_T. Incidens_), MALE,
STANDING ON THE WALL 91

FIG. 77. FEMALE OF SAME 91

FIG. 78. A MALARIAL MOSQUITO (_A. maculipennis_), MALE,
STANDING ON THE WALL 91

FIG. 79. FEMALE OF SAME 91

FIG. 80. EGG OF _Anopheles_, SIDE VIEW 92

FIG. 81. EGG OF ANOPHELES, DORSAL VIEW 92

FIG. 82. ANOPHELES LARVÆ 92

FIG. 83. ANOPHELES LARVÆ 93

FIG. 84. ANOPHELES LARVA, DORSAL VIEW 93

FIG. 85. ANOPHELES PUPÆ RESTING AT SURFACE OF WATER 93

FIG. 86. SALT-MARSH MOSQUITO (_Ochlerotatus lativittatus_);
MALE 98

FIG. 87. SALT-MARSH MOSQUITO (_O. lativittatus_); FEMALE 98

FIG. 88. TOP-MINNOW (_Mollienisia latipinna_) 99

FIG. 89. DRAGON-FLIES 99

FIG. 90. THE YOUNG (NYMPH) OF A DRAGON-FLY 100

FIG. 91. THE CAST SKIN (_exuvæ_) OF A DRAGON-FLY NYMPH 100

FIG. 92. DIVING-BEETLES AND BACK-SWIMMERS 101

FIG. 93. KILLIFISH (_Fundulus heteroliatus_) 102

FIG. 94. STICKLEBACK (_Apeltes quadracus_) 102

FIG. 95. AN OLD WATERING-TROUGH, AN EXCELLENT BREEDING-PLACE
FOR MOSQUITOES 103

FIG. 96. HORSE AND CATTLE TRACKS IN MUD FILLED WITH
WATER 108

FIG. 97. A MALARIAL MOSQUITO (_Anopheles maculipennis_)
MALE 108

FIG. 98. A MALARIAL MOSQUITO (_A. maculipennis_) FEMALE 109

FIG. 99. DIAGRAM TO ILLUSTRATE THE LIFE-HISTORY OF THE
MALARIAL PARASITE 110

FIG. 100. MALARIAL MOSQUITO (_A. maculipennis_) ON THE WALL 111

FIG. 101. MALARIAL MOSQUITO (_A. maculipennis_) STANDING ON
A TABLE 111

FIG. 102. SALT-MARSH MOSQUITO (_O. lativittatus_) STANDING ON
A TABLE 118

FIG. 103. ANOPHELES HANGING FROM THE CEILING 118

FIG. 104. YELLOW FEVER MOSQUITO (_Stegomyia calopus_) 122

FIG. 105. RAT-FLEA (_Læmopsylla cheopis_); MALE 152

FIG. 106. RAT-FLEA (_L. cheopis_); FEMALE 152

FIG. 107. HEAD OF RAT-FLEA SHOWING MOUTH-PARTS 153

FIG. 108. HUMAN-FLEA (_Pulex irritans_); MALE 153

FIG. 109. HUMAN-FLEA (_P. irritans_); FEMALE 156

FIG. 110. MOUSE-FLEA (_Ctenopsyllus musculi_); FEMALE 156

FIG. 111. TRYPANOSOMA GAMBIENSE 164

FIG. 112. TSETSE-FLY 164

INSECTS AND DISEASE

CHAPTER I

PARASITISM AND DISEASE

PARASITES

The dictionary says that a parasite is a living organism, either animal
or plant, that lives in or on some other organism from which it derives
its nourishment for a whole or part of its existence. This definition
will serve as well as any, as it seems to include all the forms that
might be classed as parasites. As a general thing, however, we are
accustomed to think of a parasite as working more or less injury to its
host, or perhaps we had better say that if it does not cause any
irritation or ill effects its presence is not noted and we do not think
of it at all.

As a matter of fact the number of parasitic organisms that are actually
detrimental to the welfare of their hosts is comparatively small while
the number of forms both large and small that lead parasitic lives in
or on various hosts, usually doing no appreciable harm, often perhaps
without the host being aware of their presence, is very great indeed.

Few of the higher animals live parasitic lives. The nearest approach to
a true parasite among the vertebrates is the lamprey-eel (Fig. 1) which
attaches itself to the body of a fish and sucks the blood or eats the
flesh. Among the Crustaceans, the group that includes the lobsters and
crabs, we find many examples of parasites, the most extraordinary of
which is the curious crab known as _Sacculina_ (Fig. 2). In its early
stages this creature is free-swimming and looks not unlike other young
crabs. But it soon attaches itself to another crab and begins to live at
the expense of its host. Then it commences to undergo remarkable changes
and finally becomes a mere sac-like organ with a number of long slender
root-like processes penetrating and taking nourishment from the body of
the unfortunate crab-host.

The worms furnish many well-known examples of parasites, whole groups of
them being especially adapted to parasitic life. The tapeworms, common
in many animals and often occurring in man, the roundworms of which the
trichina (Fig. 3) that causes "measly" pork is a representative, are
familiar examples. These and a host of others all show a very high
degree of specialization fitting them for their peculiar lives in their
hosts.

[Illustration: FIG. 1--A lamprey. (After Goode.)]

[Illustration: FIG. 2--_Sacculina_; _A_, parasite attached to a crab;
_B_, the active larval condition; _C_, the adult removed from its host.
(After Haeckel.)]

[Illustration: FIG. 3--_Trichina spiralis_ encysted in muscle of a pig.
(From Kellogg's Elementary Zoöl.)]

[Illustration: FIG. 4--An external parasite, a bird-louse (_Lipeurus
ferox_).]

[Illustration: FIG. 5--A tachina fly (_Blepharipeza adusta_) the larva
of which is an internal parasite.]

[Illustration: FIG. 6--Work of an internal parasite, puss-moth larva
parasitized by a small ichneumon fly.]

From among the insects may be selected interesting examples of almost
all kinds and degrees of parasitism, temporary, permanent, external,
internal (Figs. 4, 5, 6). Among them is found, too, that curious
condition known as hyperparasitism where one animal, itself a parasite,
is preyed upon by a still smaller parasite.

"The larger fleas have smaller fleas
Upon their backs to bite um,
These little fleas still smaller fleas
And so _ad infinitum_."

Coming now to the minute, microscopic, one-celled animals, the Protozoa,
we find entire groups of them that are living parasitic lives, depending
wholly on one or more hosts for their existence. Many of these have a
very remarkable life-history, living part of the time in one host, part
in another. The malarial parasite and others that cause some of the
diseases of man and domestic animals are among the most important of
these.

PARASITISM

Among all these parasites, from the highest to the lowest the process
that has fitted them for a parasitic life has been one of degeneration.
While they may be specialized to an extreme degree in one direction they
are usually found to have some of the parts or organs, which in closely
related forms are well developed, atrophied or entirely wanting. As a
rule this is a distinct advantage rather than a disadvantage to the
parasite, for those parts or organs that are lost would be useless or
even in the way in its special mode of life.

Then, too, the parasite often gives up all its independence and becomes
wholly dependent on its host or hosts not only for its food but for its
dissemination from one animal to another, in order that the species may
not perish with the host. But in return for all this it has gained a
life of ease, free from most of the dangers that beset the more
independent animals, and is thus able to devote its whole time and
energy to development and the propagation of the species.

We are accustomed to group the parasites that we know into two classes,
as harmful or injurious and as harmless, the latter including all those
kinds that do not ordinarily affect our well-being in any way. But such
a classification is not always satisfactory or safe, for certain
organisms that to-day or under present conditions are not harmful may,
on account of a great increase in numbers or change of conditions,
become of prime importance to-morrow. An animal that is well and strong
may harbor large numbers of parasites which are living at the expense of
some of the host's food or energy or comfort, yet the loss is so small
that it is not noticed and the intruders, if they are thought of at all,
are classed as harmless. Or we may at times even look upon them as
beneficial in one way or another. "A reasonable amount of fleas is good
for a dog. They keep him from brooding on being a dog."

But should these parasites for some reason or other increase rapidly
they might work great harm to the host. Even David Harum would limit the
number of fleas on a dog. Or the animal might become weakened from some
cause so that the drain on its resources made by the parasites, even
though they did not increase in numbers, would materially affect it.

Perhaps the most serious way in which parasites that are usually
harmless may become of great importance is illustrated by their
introduction into new regions or, as is more often the case, by the
introduction of new hosts into the region where the parasites are found.
Under normal conditions the animals of a given region are usually immune
to the parasites of the same region. That is, they actually repel them
and do not suffer them to exist in or on their bodies, or they are
tolerant toward them. In the latter case the parasites live at the
expense of the host, but the host has become used to their being there,
adapted to them, and the injury that they do, if any, is negligible.

But when a new animal comes into the region from some other locality the
parasites may be extremely dangerous to it. There are many striking
examples of this. Most of the people living in what is known as the
yellow fever belt are immune to the fever. They will not develop it even
under conditions that would surely mean infection for a person from
outside this zone. Certain of our common diseases which we regard as of
little consequence become very serious matters when introduced among a
people that has never known them before. The cattle of the southern
states are immune to the Texas fever, but let northern cattle be sent
south or let the ticks which transmit the disease be taken north where
they can get on cattle there, and the results are disastrous.

Another striking example and one that is attracting world-wide attention
just now is the trypanosome that is causing such devastation among the
inhabitants of central Africa. With the advent of white men into this
region and the consequent migration of the natives along the trade
routes this parasite, which is the cause of sleeping sickness, is being
introduced into new regions and thousands upon thousands of people are
dying as a result of its ravages.

DISEASES CAUSED BY PARASITES

Some two hundred years ago, after it became known that minute animal
parasites were associated with certain diseases and were the cause of
them, it rapidly came to be believed that all our ills were in some way
caused by such parasites, known or unknown. Further study and
investigation failed to reveal the intruders in many instances and so it
began to be doubted whether after all they were responsible for much
that had been laid at their doors. Then after it was discovered that
minute plant parasites, bacteria, were responsible for many diseases
they in turn began to be accused of being the cause of most of the ills
that the flesh is heir to.

In later years we have come to adopt what seems to be a more reasonable
view, for we can see and definitely prove that neither of these extreme
views was correct but that there was much truth in each of them. To-day
we recognize that certain diseases, such as typhoid, cholera,
tuberculosis and many others, are caused by the presence of bacteria in
the body, and it is just as definitely known that such maladies as
malaria and sleeping sickness are caused by animal parasites.

Then there is a long list of other epidemic diseases, such as smallpox,
measles and scarlet fever, the exact cause of which has not been
determined. Many of these are believed to be due to micro-organisms of
some kind, and if so they will almost certainly sooner or later be
found. Curiously enough most of the diseases in this last class and many
of those in the first are contagious, while all that are caused by
animal parasites are, as far as is known, infectious but not contagious.

INFECTIOUS AND CONTAGIOUS DISEASES

It is important that we keep in mind this distinction. By contagious
diseases are meant those that are transmitted by contact with the
diseased person either directly, by touch, or indirectly by the use of
the same articles, by the breath or effluvial emanations from the body
or other sources. Small-pox, measles, influenza, etc., are examples of
this group. By infectious diseases are meant those which are
disseminated indirectly, that is, in a roundabout way by means of water
or food or other substances taken into or introduced into the body in
some way. Typhoid, malaria, and yellow fever, cholera and others are
examples of this class. Thus it is evident that all of the contagious
diseases may be infectious, but many of the infectious diseases are not
as a rule contagious, although some of them may become so under
favorable conditions.

Just one example will show the importance of knowing whether a disease
is contagious or infectious. Until a few years ago it was believed that
yellow fever was highly contagious and every precaution was taken to
keep the disease from spreading by keeping the infected region in strict
quarantine. This often meant much hardship and suffering and always a
great financial loss. We now know that it is infectious only and not
contagious, and that all this quarantine was unnecessary. The whole
fight in controlling an outbreak of yellow fever or in preventing such
an outbreak is now directed against the mosquito, the sole agent by
which the disease can be transmitted from one person to another.

EFFECT OF THE PARASITE ON THE HOST

We have seen how a few parasites in or on an animal do not as a rule
produce any appreciable ill effects. This is of course a most fortunate
thing for us, for the parasitic germs are everywhere.

There is perhaps "more truth than poetry" in the following newspaper
jingle:

"Sing a song of microbes,
Dainty little things,
Eyes and ears and horns and tails,
Claws and fangs and stings.
Microbes in the carpet,
Microbes in the wall,
Microbes in the vestibule,
Microbes in the hall.
Microbes on my money,
Microbes in my hair,
Microbes on my meat and bread,
Microbes everywhere.
Microbes in the butter,
Microbes in the cheese,
Microbes on the knives and forks,
Microbes in the breeze.
Friends are little microbes,
Enemies are big,
Life among the microbes is--
Nothing '_infra dig_.'
Fussy little microbes,
Millions at a birth,
Make our flesh and blood and bones,
Keep us on the earth."

While of course most of these microbes are to be regarded as absolutely
harmless and some as very useful, many have the power to do much injury
if the proper conditions for their rapid development should at any time
exist. While the size of the parasite is always a factor in the damage
that it may do to the host the factor of numbers is perhaps of still
greater importance because of the power of very rapid multiplication
possessed by so many of the smaller forms.

Certain minute parasites in the blood may cause little or no
inconvenience, but should they begin to multiply too rapidly some of the
capillaries may be filled up and trouble thus result. Or take some of
the larger forms. A few intestinal worms may cause no appreciable effect
on the host, but as soon as their numbers increase serious conditions
may come about simply by the presence of the great masses in the host
even if they were not robbing it of its nourishment. Many instances are
known where such worms have formed masses that completely clogged up the
alimentary canal. Such injuries as these may be regarded as mechanical
injuries. Some parasites injure the host only when they are laying their
eggs or reproducing the young. These may clog up passages or some of
them may be carried to some more sensitive part of the body where the
damage is done. The guinea-worm of southwestern Asia and of Africa lives
in the body of its host for nearly a year sometimes attaining a great
length and migrating through the connective tissue to different parts of
the body causing no particular inconvenience until it is ready to lay
its eggs when it comes to the surface and then great suffering may
result. The African eye-worm is another example of a parasite causing
mechanical injury only at certain times. It works in the tissues of the
body sometimes for a long while, doing no harm unless it finds its way
to the connective tissue of the eyeball.

It is known that many of the germs which cause diseases cannot get into
the body unless the protecting membranes have first been injured in some
way. Thus the germs that cause plague and lockjaw find their way into
the system principally through abrasions of the skin. Many physicians
have come to believe that the typhoid fever germ cannot get into the
body from the intestine where it is taken with our food or drink unless
the walls of the intestine have been injured in some way. It is well
known that of the many parasites that inhabit the alimentary canal some
rasp the surface and others bore through into the body cavity. This in
itself may not be a serious thing, but if the mechanical injury thus
caused opens the way for malignant germs, baneful results may follow.
Even that popular disease appendicitis is believed to be due sometimes
to the injury caused by the work of parasites in the appendix.

Parasites may cause morphological or structural changes in the tissues
of their hosts. The stimulation caused by their presence may result in
swellings or excresences or other abnormal growths. Interesting examples
of this are to be found in the way in which pearls are formed in various
mollusks. In the pearl oysters of Ceylon occur some of the best pearls.
If these are carefully sectioned there may usually be found at the
center the remains of certain cestode larvæ whose presence in the oyster
caused it to deposit the nacreous layers that make up the pearl. Other
parasites cause similar growths in various shellfish. The great
enlargements of the arms or legs or other parts of the body seen in
patients affected with elephantiasis is an abnormal growth due to the
presence of the parasitic filaræ in some of the lymph-glands where they
have come to rest.

Finally, the parasite may exert a direct physiological effect on the
host. This is evident when the parasite demands and takes a portion of
the nourishment that would otherwise go to the building up of the host.
Sometimes this is of little importance, but at other times it may be a
matter of life or death to the infected animal. The physiological
effect produced may be due to the toxins or poisonous matters that are
given off by the parasite while it is living in the host's body. Thus it
is believed that the malarial patients usually suffer less from the
actual loss of red blood-corpuscles that are destroyed by the parasite
than they do from the effects of the poisonous excretions that are
poured into the circulation when the thousands of corpuscles break to
release the parasites.

One other point in regard to the relation of the parasite to its host
and this part of the subject may be dismissed. From our standpoint we
look upon the presence of parasites in the body as an abnormal
condition. From a biological standpoint their presence there is
perfectly normal; it is a necessary part of their life. We think that
they have no business there, but from the viewpoint of the parasites
their whole business is to be just there. If they are not, they perish.
And when we take a dose of quinine or other drug we are killing or
driving from their homes millions of these little creatures who have
taken up their abode with us for the time being. But they interfere with
our health and comfort, so they must go.

CHAPTER II

BACTERIA AND PROTOZOA

BACTERIA

On the border line between the plant and the animal worlds are many
forms which possess some of the characteristics of both. Indeed when an
attempt is made to separate all known organisms into two groups one is
immediately confronted with difficulties. In looking over the text-books
of Botany we will find that certain low forms are discussed there as
belonging with the plants, and on turning to the manuals of Zoölogy we
will find that the same organisms are placed among the lowest forms of
animals. The question is of course of little actual importance from
certain points of view. It serves, however, to show the close relation
of all forms of life, and from a medical standpoint it may be of very
great importance owing to the difference in the life-habits, methods of
reproduction and methods of transmission of many of the forms that cause
disease. We have already seen that none of the diseases that are caused
by animal parasites is contagious, while many of those caused by
bacteria are both contagious and infectious.

Just over on the plant side of this indefinite border line are the
minute organisms known as bacteria. Their numbers are infinite and they
are found everywhere. The majority of them are beneficial to mankind in
one way or another, but some of them cause certain of the diseases that
we will have to discuss later so attention may be called here to a few
of the important facts in regard to their organization and life-history
in order that we may better understand how they may be so easily
transferred from one host to another.

Although these bacilli are so extremely minute (Fig. 7), some of them so
small that they cannot be seen with the most powerful microscopes, they
differ in size, shape, methods of division and spore-formation. Each
species makes a characteristic growth on gelatin, agar or other media
upon which it may be cultivated. In this way as well as by the
inoculation of animals the presence of the ultramicroscopic kinds may be
demonstrated.

The method of reproduction is very simple. They increase to a certain
point in size, then divide. This growth and division takes place very
rapidly. Twenty to thirty minutes is sufficient time in some cases for
a just-divided cell to attain full size and divide again. Thus in a few
days time the number of bacteria resulting from a single individual
would be inconceivable if they should all develop.

Fortunately for us, however, they do not all multiply so rapidly as this
and besides there are natural checks, not the least of which are the
substances given off by the bacteria themselves in their growth and
development. Such excretions often serve to inhibit further
multiplication. Sometimes, though not often, they form spores which not
only provide for a more rapid multiplication, but enable the organism to
live under conditions that would otherwise prove fatal to it.

Bacteria may be conveniently grouped under two heads: those that live
upon dead organic matter, known as the saprophytic forms, and those that
are found in living plants or animals, the true parasites. Such a
grouping is not always entirely satisfactory, for many of the kinds that
live saprophytically under normal conditions may become parasitic if
opportunity offers, and also many of those that are usually regarded as
parasitic may be grown in cultures of agar or other media, under which
conditions they may be regarded as living saprophytically.

It is this power of easily adapting themselves to different conditions
that makes many of the kinds dangerous. The bacillus which causes
tetanus or lockjaw will illustrate this. It is a rather common bacillus
in soil in many localities. As long as it remains there it is of no
special importance, but if it is introduced into the body through a
scratch or any other wound it becomes a very serious matter.

We may say, then, that the effect the bacillus has on the host depends
largely on the host. Not only does it depend on what the host is, but
the particular condition of the host at the time of infection is of
importance. Children are subject to many diseases that adults seldom
have. Hunger, thirst, fatigue, exposure and other factors may make a
person susceptible to the actions of certain bacteria that would be
harmless under other conditions.

The minute size and great numbers of the bacteria make their
dissemination a comparatively simple matter. They may be carried in the
air as minute particles of dust; they may be carried in water or milk;
they may be carried on the clothing or on the person from one host to
another, or they may be disseminated in scores of other ways. In other
chapters, particularly the one dealing with the house-fly and typhoid,
we shall see how it is that insects are often important factors in
spreading some of the most dreaded of the bacterial diseases.

THE PROTOZOA

The Protozoa, or one-celled animals, belonged to an unknown world before
the invention of the microscope. The first of these instruments enabled
the early observers to see some of the larger and more conspicuous
members of the group and each improvement of the microscope has enabled
us to see more and more of them and to study in detail not only the
structure but to follow the life-history of many of them.

_The Amoeba._ With some, as the common amoeba (Fig. 8), a minute
little form that is to be found in the slime at the bottom of almost any
body of water, the life-history is extremely simple. The organism itself
consists of a minute particle of protoplasm, a single cell with no
definite shape or body-wall and no specialized organs or apparatus for
carrying on the life-functions. It lives in the slime or ooze in fresh
or salt water, takes its food by simply flowing over the particle that
is to be ingested, grows to a certain limit of size, then divides into
two more or less equal parts, each part becoming a new animal that goes
on with its development as did the parent form. This process of growth
and division may go on for many generations, but cannot continue
indefinitely unless there is a conjugation of two separate individuals.
This process of conjugation is just the opposite to that of division.
Two amoeba flow together and become one. It seems to rejuvenate the
organism so that it is able to go on with its division and thus fulfil
its life-mission which is the same for these lowly animals as with the
higher, that of perpetuating the species.

_Classes of Protozoa._ The group or Phylum Protozoa is divided into four
smaller groups or classes. The amoeba belongs to the lowest of these,
the Rhizopoda. Rhizopoda means "root-footed," and the name is applied to
these animals because most of them move about by means of root-like
processes known as pseudopodia or "false feet." This is by far the
largest class and contains thousands of forms, mostly living in salt
water but there are many fresh-water species. They are non-parasitic,
but some of them by their presence in the body may cause such diseases
as dysentery, etc.

[Illustration: FIG. 7--Typhoid Fever bacilli. (After Muir and
Ritchie.)]

[Illustration: FIG. 8--_Amoeba_, showing the forms assumed by a single
individual in four successive changes. (From Kellogg's Elementary
Zoöl.)]

[Illustration: FIG. 9--_Euglina virdis._ (After Saville Kent.)]

[Illustration: FIG. 10--_Spirocheta duttoni_, × 4500. (After Breinl and
Carter.)]

The next class which may be known as the whip-bearers (_Mastigophora_)
includes those Protozoa that move by fine undulating processes called
flagella. One of the common representatives of this class is the little
green _Euglena_ (Fig. 9), whose presence in standing ponds and puddles
often imparts a greenish color to the water. Then in the salt water near
the surface there are often myriads of minute _Noctiluca_ whose
wonderfully phosphorescent little bodies glow like coals of fire when
the water is disturbed at night. Although this class contains fewer
forms than the preceding some of these have within recent years been
found to be of great importance because they live as parasites on man
and other animals. The trypanosome whose presence in the blood and
tissues of the patient causes that dreadful disease which ends in
sleeping sickness belongs here as well as do several other similar kinds
that produce serious troubles for various mammals and birds. The
Spirochæta, about which there has been so much recent discussion, also
belong here. These are simple spiral-like forms (Fig. 10), that are
sometimes classed with the simple plants, bacteria, but Nuttall and
others have shown very definitely that they should be classed with the
simplest animals, the Protozoans. These are the cause of relapsing
fevers in man and of several diseases of domestic animals. It is
believed by certain eminent zoölogists that when the germ that causes
yellow fever is discovered it will be found to belong to this group.

The members of the class Infusoria, so called because they were early
found to be abundant in various infusions, are characterized by numerous
fine cilia or hair-like organs by means of which the organism moves
about and procures its food. The well-known "slipper animalcule"
(_Paramoecium_) (Fig. 11), and the "bell-animalcule" (_Vorticella_)
(Fig. 12) are two common representatives. The _Paramoecia_ were the
animals mostly used by Jennings in his wonderfully interesting
experiments on the behavior of these lowly forms of life. He showed that
they always reacted in a certain definite way in response to particular
stimuli, and he was led to believe that he could see "what must be
considered the beginnings of intelligence and of many other qualities
found in the higher animals." A species of _Vorticella_ was probably the
first Protozoan that was ever observed. An old Dutch microscopist, Anton
von Leeuwenhoek, in 1675, while studying with lenses of his own
manufacture, discovered and described forms which undoubtedly belong to
this genus. Few if any of the Infusoria are pathogenic, although some
are said to be associated with certain intestinal diseases both in man
and the lower animals (Fig. 13).

[Illustration: FIG. 11--_Paramoecium._ (From Kellogg's Elementary
Zoöl.)]

[Illustration: FIG. 12--_Vorticella_, one individual with the stalk
coiled, the other with the stalk extended. (From Kellogg's Elementary
Zoöl.)]

[Illustration: FIG. 13--Pathogenic Protozoa; a group of intestinal
parasites. _A_, _B_, _Megastoma entericum_, _C_, _Balantidum entozoon_.
(After Calkins.)]

The last class, the Sporozoa, or the spore-forming animals, while small
in the number of known species, only about three hundred kinds being
known, is extremely important. A number of diseases in man and other
animals are due to the presence of these Sporozoans, for they are all
parasitic. Few if any animals are exempt from their attacks. They even
attack other minute Protozoa. One hundred and fifty-seven species have
been recorded as attacking insects, one hundred species attack birds,
fifty-two reptiles, eighty crustaceans, twenty-two fish, and so through
the list. Ten have been recorded as attacking man. In some instances the
parasite is always present in the host and some hosts may harbor several
different species of Sporozoa.

Very little work had been done on this group of parasites prior to 1900.
Since that time most of the species that we now know have been
discovered, and within the last few years the life-histories of many of
these have been worked out quite completely. No other group of animals
is being studied more to-day by both the physicians and biologists.

The Sporozoa vary greatly in appearance, organization and life-history.
They are so very plastic that they can adapt themselves readily to their
various hosts, hence we have a great variety of forms. But they all
agree in certain characters; all take their food and oxygen and carry on
excretory processes by osmosis, _i.e._, through the body-wall; all are
capable of some kind of locomotion, some have one or more flagella,
others move by a pseudopod movement. Some are capable of moving from
cell to cell in the body as do the white blood-corpuscles. They all
agree in the production of spores--hence the name.

At certain stages in their development the nucleus within the body of
the organism divides again and again until there are a great many
daughter nuclei, each accompanied by a small mass of protoplasm, often
inclosed in a little sac or cyst of its own. This is the process of
spore-formation and we see that from a single individual we may have by
division, not two animals as in the amoeba, but a score or more of
them. The little cysts or capsules that inclose them enable them to
resist without injury many vicissitudes that would otherwise destroy
them. They may dry up or freeze or lie for a long time in the ground or
water until the time comes when they are introduced into another host.

The class Sporozoa is divided into five small groups or orders. Nearly
all of these contain forms that are of more or less importance, but the
ones that live in the blood-cells (_Hæmosporidiida_) are of the most
interest to us because the parasites that cause the malarial fevers and
various other diseases belong here. These are dependent on two hosts for
their existence, the sexual generation usually occuring in an insect or
other invertebrate and the asexual generation in some vertebrate.

CHAPTER III

TICKS AND MITES

The other group or Phylum of animals with which we will be particularly
concerned is known as the Arthropoda, which means "jointed-feet" and
includes the crayfish, crabs, spiders, mites, ticks and insects. Of
these only the last three are of interest to us now. It is customary to
speak of spiders, mites and ticks as insects, but as they have four
pairs of legs, instead of three pairs, in the adult stage, and as their
bodies are not divided into three distinct regions as in the insects,
they are placed in a different class.

GENERAL CHARACTERS OF TICKS

The ticks are all comparatively large, that is, they are all large
enough to be seen with the unaided eye even in their younger stages and
some grow to be half an inch long. When filled with blood the tough
leathery skin becomes much distended often making the creature look more
like a large seed than anything else (Fig. 14). This resemblance is
responsible for some of the popular names, such as "castor-bean tick,"
etc.

The legs of most species are comparatively short, and the head is small
so that they are often hardly noticeable when the body is distended. The
sucking beak which is thrust into the host when the tick is feeding is
furnished with many strong recurved teeth which hold on so firmly that
when one attempts to pull the tick away the head is often torn from the
body and left in the skin. Unless care is taken to remove this, serious
sores often result.

Ticks are wholly parasitic in their habits. Some of them live on their
host practically all their lives, dropping to the ground to deposit
their eggs when fully mature. Others leave their host twice to molt in
or on the ground. The female lays her eggs, 1,000 to 10,000 of them, on
the ground or just beneath the surface. The young "seed-ticks" that
hatch from these in a few days soon crawl up on some near-by blade of
grass or on a bush or shrub and wait quietly and patiently until some
animal comes along. If the animal comes close enough they leave the
grass or other support and cling to their new-found host and are soon
taking their first meal. Of course thousands of them are disappointed
and starve before their host appears, but as they are able to live for a
remarkably long time without taking food their patience is often
rewarded and the long fast ended.

Those species which drop to the ground to molt must again climb to some
favorable point and wait for another host on which they may feed for a
while. Then they drop to the ground for a second molt and if they are
successful in gaining a new host for the third time they feed and
develop until fully mature and the female is ready to lay her eggs. The
Texas fever tick, and some others, as we shall see, do not drop to the
ground to molt but once having gained a host remain on it until ready to
deposit their eggs.

The young ticks have only six legs (Fig. 15) but after the first molt
they all have eight.

TICKS AND DISEASE

_Texas Fever._ Ever since stockmen began driving southern cattle into
states further north it has been noted that the roads over which they
were driven became a source of great danger to northern cattle. Often
80% to 90% of the native cattle died after a herd of southern cattle
passed through their region and the losses became so great that both
state and national laws were passed prohibiting the driving or shipping
of southern cattle into northern states.

[Illustration: FIG. 14--Castor Bean Tick (_Ixodes ricinus_) not fully
gorged.]

[Illustration: FIG. 15--Texas fever tick, just hatched; has only six
legs.]

[Illustration: FIG. 16--Texas fever tick (_Margaropus annulatus_) young
adult not fully gorged.]

[Illustration: FIG. 17--_Amblyomma variegatum_ several ticks belonging
to this genus transmit _Piroplasma_ which cause various diseases of
domestic animals.]

But for years the cause of this fever, which came to be known as the
Texas fever, was not known. The southern cattle themselves seemed
healthy enough and it was difficult to understand how they could give
the disease to the others. It was early noticed, too, that it was not
necessary for the northern cattle to come in direct contact with the
others in order to contract the disease. Indeed the disease was not
contracted in this way at all. All that was necessary for them was to
pass along the same roads or feed in the same pastures or ranges. Still
more puzzling was the fact that these places did not seem to become a
source of danger until some weeks after the southern cattle had passed
over them and then they might remain dangerous for months.

In 1886 Dr. Theobald Smith of the Bureau of Animal Industry, United
States Department of Agriculture, found that the fever was caused by the
presence in the infected cattle of a minute Sporozoan parasite
(_Piroplasma bigeminum_). Further investigations and experiments proved
conclusively that this parasite was transmitted from the infected to the
well animal only by the common cattle tick now known as the Texas fever
tick (Fig. 16).

The infection is not direct, that is, the tick does not feed on one
host then pass to another carrying the disease germs with it. Unlike
many other ticks the Texas fever tick does not leave its host until it
is fully developed. When the female is full grown and gorged she drops
to the ground and lays from 2,000 to 4,000 eggs which soon hatch into
the minute "seed-ticks" which make their way to the nearest blade of
grass or weed or shrub and patiently wait for the cattle to come along.

If the mother tick had been feeding on an animal that was infected with
the Texas fever parasite, her body was filled with the minute organisms
of which some found their way into the eggs so that the young ticks
hatching from them were already infected and ready to carry the
infection to the first animal they fed upon.

It took many years of hard patient work to learn all this, but the
knowledge thus obtained cleared up much of the mystery in connection
with the occurrence of the fever in the north and, as we shall see,
suggested the possibility of other diseases being communicated in the
same way.

It was found that the southern cattle in the region where the ticks
occur normally, usually have a mild attack of the disease when they are
young and although they may be infected with the parasite all the rest
of their lives it does not affect them seriously. These cattle are
almost always infected with ticks, and when taken north where the ticks
do not occur naturally and where the cattle are therefore non-immune,
some of the mature ticks drop to the ground and lay their eggs which in
a few weeks hatch out and are ready to infect any animal that passes by.
The northern cattle not being used to the disease soon sicken and die.

It is estimated that the annual loss due to this disease and the ravages
of the tick in the United States is over $100,000,000, so of course most
determined efforts are being made to stamp it out. Formerly various
devices for dipping the tick-infested cattle into some solution that
would kill the ticks were resorted to, but it was always expensive and
never very satisfactory. The immunizing of the cattle by inoculating
them when they were young with infected blood has been practised. Very
recent investigations have shown that it is possible and practicable to
rid pastures of ticks by a system of feed-lots and pasture rotation. The
aim is to have as many of the ticks as possible drop to the ground on
land where they may be destroyed and to so regulate the use of the
pasture that the ticks in all of them may eventually be left to starve.

Several similar diseases of cattle, many of them probably identical
with Texas fever, occur in other parts of the world where the losses are
sometimes appalling. Horses, sheep, dogs, and other animals are also
affected with diseases caused by the same group of Protozoan parasites.
Most of them have been shown to be transmitted by various species of
ticks (Fig. 17) so that from an economical standpoint these little pests
are becoming of prime importance. Not only do they transmit the disease
germs that infect domestic animals but they are known to be responsible
for at least two diseases of men, Rocky Mountain spotted fever and the
relapsing fevers.

_Spotted Fever._ The first of these is a disease that for some years has
been puzzling the physicians in Idaho and Montana and other mountainous
states. A few years ago certain observers recorded finding Protozoan
parasites in the blood of those suffering from the disease, and although
more recent investigations have failed to confirm these particular
observations it is now quite generally believed that the disease is
caused by some such parasite and that the organism is transferred from
one host to another by certain species of ticks that live on wild
mammals of the region where the disease exists. Dr. H.T. Ricketts, who
has made a special study of the disease, has shown:

"1. That the period of activity of the disease is limited to the
season during which the adult female and male ticks attack man.

"2. That in practically all cases of this disease it can be shown
that the patient has been bitten by a tick.

"3. That the period between the tick bite and the onset of the
disease in the many animals he has experimented with corresponds
very closely to this period as observed in man.

"4. That infected ticks are to be found in the locality where the
disease occurs.

"5. That the virus of spotted fever is very intimately associated
with the tissues of the tick's body as is shown by the fact that
the female passes the infection on to her young through her eggs,
and further, by the observation that in either of the two earlier
stages of the life cycle the disease may be contracted by biting a
sick animal and communicated to other animals after molting or even
after passing through an intermediate stage."

Professor R.A. Cooley of Montana, from whose report the above quotation
is taken, has also made studies of the habits of the tick and believes
there can be no doubt that it is the disseminator of the disease.

_Relapsing Fever._ The relapsing fever is an infectious disease or
possibly a group of closely related infectious diseases occurring in
various parts of the world. Occasionally it is introduced into America,
but it does not seem to spread here. It has been shown that the disease
is communicated from one person to another by means of blood-sucking
insects. In Central Africa where the disease is very prevalent a certain
common tick (_Ornithodoros moubata_) (Fig. 18) is known to transmit the
disease. This tick lives in the resting places and around the huts of
the natives and has habits very similar to the bedbug of other climes,
feeding at night and hiding during the day. It attacks both man and
beast and is one of the most dreaded of all the African pests.

Nathan Bank, our foremost authority on ticks, in summing up the evidence
against them says:

"It is therefore evident that all ticks are potentially dangerous.
Any tick now commonly infesting some wild animal, may, as its
natural host becomes more uncommon, attach itself to some domestic
animal. Since most of the hosts of ticks have some blood-parasites,
the ticks by changing the host may transplant the blood-parasites
into the new host producing, under suitable conditions, some
disease. Numerous investigators throughout the world are studying
this phase of tick-life, and many discoveries will doubtless
signalize the coming years."

MITES

The mites are closely related to the ticks, and although none of them
has yet been shown to be responsible for the spread of any disease their
habits are such that it would be entirely possible for some to transmit
certain diseases from one host to another, from animal to animal, from
animal to man, or from man to man. A number of these mites produce
certain serious diseases among various domestic animals and a few are
responsible for certain diseases of men.

_Face-mites._ Living in the sweat-glands at the roots of hairs and in
diseased follicles in the skin of man and some domestic animals are
curious little parasites that look as much like worms as mites (Fig.
19). Such diseased follicles become filled with fatty matter, the upper
end becomes hard and black and in man are known as blackheads. If one of
these blackheads is forced out and the fatty substance dissolved with
ether the mites may be found in all stages of development. The young
have six legs, the adult eight. The body is elongated and transversely
wrinkled. In man they are usually found about the nose and chin and neck
where they do no particular harm except to mar the appearance of the
host and to indicate that his skin has not had the care it should have.
Very recently certain investigators have found that the lepræ bacilli
are often closely associated with these face mites and believe that they
may possibly aid in the dissemination of leprosy. It is also thought
that they may sometimes be the cause of cancer, but as yet these
theories have not been proven by any conclusive experiment.

In dogs and cats these same or very similar parasites cause great
suffering. In bad cases the hair falls out and the skin becomes scabby.
Horses, cattle and sheep are also attacked. The disease caused by these
mites on domestic animals is not usually considered curable except in
its very early stages when salves or ointments may help some.

_Itch-mites._ "As slow as the seven-years' itch" is an expression, the
meaning of which many could appreciate from personal experience, for it
certainly seemed to take no end of time to get rid of the itch once it
was contracted. Just why seven years should have been set for the limit
of the disease is not clear, for if the little roundish mites that cause
the disease live for seven years on a host they are not going to move
out voluntarily even if their seven-year lease has expired.

[Illustration: FIG. 18--_Ornithodorus moubata_, the Tick that Transmits
Relapsing Fever. From Boyce's "Mosquito or Man."]

[Illustration: FIG. 19--The follicle mite (_Demodex folliculorum_).
(After Murray.)]

[Illustration: FIG. 20--Itch-mite (_Sarcoptes scabiei_). (After
Murray.)]

[Illustration: FIG. 21--Harvest-mites or "jiggers." (_Leptus irritaus_
and _L. americanus_.) (After Riley.)]

The minute whitish mites (Fig. 20) that cause this disgusting disease
are barely visible to the naked eye. They are usually very sluggish but
become more active when warmed. They live in burrows just beneath the
outer layer of skin, sometimes extending deeper and causing most intense
itching. As the female burrows, she lays her eggs from which come the
young mites that are to spread the infection. Various sulphur ointments
and washes are used as remedies. Cleanliness will prevent infection.

Closely related to the itch-mite of man (_Sarcoptes scabiei_) are
several kinds attacking domestic animals, causing mange, scab, etc. The
variety infesting horses burrows in the skin and produces sores and
scabs, and is a source of very great annoyance. These mites may also
migrate to man. Tobacco water and sulphur ointments are used as
remedies.

Horses and cattle are also infested by other mites (_Psoroptes
communis_) which cause the common mange. These do not burrow into the
skin but live outside in colonies, feeding on the skin and causing
crusts or scabs. The inflammation causes the animal to scratch and rub
constantly and often causes the loss of much of the hair.

_Harvest-mites._ A score or more of different varieties of mites cause
many other diseases of domestic animals, such as the scab of sheep and
hogs and chickens, various other manges of the horses and cattle and
dogs, etc. But we need to call attention to just one more example, that
of the harvest-mites or jiggers (Fig. 21). Professor Otto Lugger, from
whose report on the _Parasites of Man and Domestic Animals_ most of
these notes in regard to the mites are taken, thus feelingly refers to
this pest.

"About the very worst pests of man and domesticated animals are the
Harvest-bugs, Red-bugs or Jiggers.... Men and animals passing
through low herbage that harbors them are attacked by these pests,
which, whenever they succeed in finding a host, burrow in and under
the skin, causing intolerable itching and sores, the latter caused
by the feverish activity of the finger-nails of the host, if that
should be a man, whose energy in scratching, apparently, cannot be
controlled and who is bound forcibly to remove the intruders. The
writer has been there! Those who have ever passed through meadows
infested with red-bugs will remember the occasion."

Horses, cattle, dogs and cats and other animals suffer also. Again
sulphur ointments are the best remedies.

"The normal food of these mites must, apparently, consist of the
juices of plants, and the love of blood proves ruinous to those
individuals which get a chance to indulge it. For, unlike the true
chigoe, the female of which deposits eggs in the wound she makes,
these harvest-mites have no object of the kind, and when not killed
at the hands of those they torment they soon die victims to their
sanguinary appetite."

CHAPTER IV

HOW INSECTS CAUSE OR CARRY DISEASE

It has been estimated that there are about four thousand species or
kinds of Protozoans, about twenty-five thousand species of Mollusks,
about ten thousand species of birds, about three thousand five hundred
species of mammals, and from two hundred thousand to one million species
of insects, or from two to five times as many kinds of insects as all
other animals combined.

Not only do the insects preponderate in number of species, but the
number of individuals belonging to many of the species is absolutely
beyond our comprehension. Try to count the number of little green aphis
on a single infested rose-bush, or on a cabbage plant; guess at the
number of mosquitoes issuing each day from a good breeding-pond;
estimate the number of scale insects on a single square inch of a tree
badly infested with San José scale; then try to think how many more
bushes or trees or ponds may be breeding their millions just as these
and you will only begin to comprehend the meaning of this statement.

As long as these myriads of insects keep in what we are pleased to call
their proper place we care not for their numbers and think little of
them except as some student points out some wonderful thing about their
structure, life-history or adaptations. But since the dawn of history we
find accounts to show that insects have not always kept to their proper
sphere but have insisted at various times and in various ways in
interfering with man's plans and wishes, and on account of their
excessive numbers the results have often been most disastrous.

Insects cause an annual loss to the people of the United States of over
$1,000,000,000. Grain fields are devastated; orchards and gardens are
destroyed or seriously affected; forests are made waste places and in
scores of other ways these little pests which do not keep in their
proper places are exacting this tremendous tax from our people.

These things have been known and recognized for centuries, and scores of
volumes have been written about the insects and their ways and of
methods of combating them.

But it is only in recent years that we have begun to realize the really
important part that insects play in relation to the health of the people
with whom they are associated. Dr. Howard estimates that the annual
death rate in the United States from malaria is about twelve thousand,
entailing an annual monetary loss of about $100,000,000, to say nothing
of the suffering and misery endured by the afflicted. All this on
account of two or three species of insects belonging to the mosquito
genus _Anopheles_.

Yellow fever, while not so widespread, is more fatal and therefore more
terrorizing. Its presence and spread are due entirely to a single
species of mosquito. Flies, fleas, bedbugs, and many other insects have
been shown to be intimately connected with the spread of several other
most dreaded diseases, so it is no wonder that physicians, entomologists
and biologists are studying with utmost zeal many of these forms that
bear such a close relation not only to our welfare and comfort but to
our lives as well.

It would be out of place to try to give here even a brief outline of the
classification of insects, such as may be found in almost any of the
many books devoted to their study.

The most generally accepted classification divides the insects into
nineteen orders; as the Coleoptera, containing the beetles; the
Lepidoptera, containing the butterflies and moths; the Hymenoptera
containing the bees, ants and wasps, etc. Four or five of these orders
will be of more or less interest to us.

The order Diptera, or two-winged flies, is the most important because to
this belong the mosquitoes which transmit malaria and yellow fever, and
the house-fly that has come into prominence since it has been found to
be such an important factor in the distribution of typhoid and other
diseases.

FLIES

The order Diptera is divided into sixty or more families, many of which
contain species of considerable economic importance. For our present
consideration the flies may be divided into two groups or sections:
those with their mouth-parts fitted for piercing such as the mosquito
and horse-fly, and those with sucking mouth-parts such as the house-fly,
blow-fly and others.

Some of the species belonging to the first group are among the most
troublesome pests not only of man but of our domestic animals as well.
Next to the mosquitoes the horse-flies (Fig. 22) are perhaps the best
known of these. There are several species known under various names,
such as gad-fly, breeze-fly, etc. They are very serious pests of horses
and cattle, sometimes also attacking man. Their strong, sharp, piercing
stylets enable them to pierce through the toughest skin of animals and
through the thin clothing of man. The bite is very severe and
irritating, and as the flies sometimes occur in great numbers the
annoyance that they cause is often very great indeed. It has often been
claimed that these flies as well as the stable-fly and others carry the
anthrax bacillus on their proboscis from one animal to another, and
although this may not have been definitely proven the evidence is strong
enough to make a very good case against the accused. It is interesting
to note in this connection that anthrax, a very common disease among the
domestic animals and one which may attack man also, was the first
disease to be shown to be of bacterial origin. It was only about
thirty-five years ago that the investigations of Koch and Pasteur
demonstrated that the presence of this particular germ (_Bacillus
anthracis_) was the cause of the disease, and it was early recognized
that such biting flies may be important factors in the spread of the
disease.

[Illustration: FIG. 22--Horse-fly (_Tabanus punctifer_).]

[Illustration: FIG. 23--Stable-fly (_Stomoxys calcitrans_).]

[Illustration: FIG. 24--A Black-fly (_Simulium sp._). (From Kellogg's
Amer. Insects.)]

[Illustration: FIG. 25--Screw-worm fly (_Chrysomyia macellaria_).]

[Illustration: FIG. 26--Blow-fly (_Calliphora vomitoria_).]

The stable-fly (Fig. 23) (_Stomoxys calcitrans_) which looks very much
like the house-fly and, as will be noted later, frequently enters
houses, is often an important pest of horses and cattle. Its
blood-sucking habit makes it quite possible that it too may be concerned
in carrying anthrax and other diseases.

In a later chapter it will be shown how the tsetse-fly, which is
somewhat like the stable-fly, is responsible for the spread of the
disease known as the sleeping sickness. This disease is caused by a
Protozoan parasite, a trypanosome, which is transmitted from one host to
another by the tsetse-fly.

In Southern Asia and in parts of Africa there is a very serious disease
of horses known as surra which is caused by a similar parasite
(_Trypanosoma evansi_). This parasite attacks horses, mules, camels,
elephants, buffaloes and dogs, and has been recently imported into the
Philippines. It is supposed that flies belonging to the same genus as
the horse-fly (_Tabanus_ and others), and the stable-fly (_Stomoxys_)
and the horn-fly (_Hæmatobia_) are responsible for the spread of the
disease.

Nagana is one of the most serious diseases of domestic animals in
Central and Southern Africa. In some sections it is almost impossible to
keep any kind of imported animals on account of this disease which is
caused by a parasite (_Trypanosoma brucei_) similar to the one causing
surra. This parasite is to be found in several different kinds of
native animals which seem to be practically immune but are always a
source of danger when other animals are introduced. Two or three species
of tsetse-flies are responsible for the transmission of this disease.

Another group of flies much smaller but more numerous and much more
insistent are the black-flies or buffalo-gnats (Fig. 24). For more than
a century these little flies have been recognized as among the most
serious pests of stock, particularly in the south where, besides the
actual loss by death of many animals yearly, the annoyance is so great
as to sometimes make it impossible to work in the field. Human beings
are often attacked, and as the bite is poisonous and very painful great
suffering may result and cases of deaths from such bites have been
reported.

Belonging to another family, and smaller, but much like the buffalo-gnat
in habits, are the minute little "punkies" or "no-see-ums" which
sometimes occur in great swarms in certain regions where they make life
a burden to man and beast. While it has not been shown that either the
buffalo-gnats or the punkies are responsible for the transmission of any
disease, their habits of feeding on so many different kinds of wild and
domestic animals as well as on man makes it possible for them to act as
carriers of parasites that might under proper conditions become of
serious importance. Then, too, the irritation caused by the bites of
these insects usually causes scratching which may result in abrasions of
the skin that open the way for various harmful germs, particularly those
causing skin diseases.

Coming now to the group containing the house-flies and related forms we
find a number that are of interest on account of the suffering that they
may cause, particularly in their larval stages.

The screw-worm flies (_Chrysomyia macellaria_) are among the most common
and important of these (Fig. 25). These "gray flies," as they are
sometimes called, lay a mass of three or four hundred eggs on the
surface of wounds. The larvæ which in a few hours hatch from these make
their way directly into the wound where they feed on the surrounding
tissue until full grown when they wriggle out and drop to the ground
where they transform to the pupa and later to the adult fly. Of course
their presence in the wounds is very distressing to the infected animal,
and great suffering results. Slight scratches that might otherwise
quickly heal often become serious sores because of the presence of these
larvæ.

Many cases are recorded of these flies laying their eggs in the ears or
nose of children or of persons sleeping out of doors during the day.
Especially is this apt to occur if there are offensive discharges which
attract the fly. In such cases the larvæ burrow into the surrounding
tissues, devouring the mucous membranes, the muscles and even the bones,
causing terrible suffering and usually, death. The larvæ in such
situations may be killed with chloroform and, if the case is attended to
before they have destroyed too much of the tissues, recovery usually
occurs.

The blow-flies (Fig. 26) (_Calliphora vomitoria_) and the blue-bottle
flies (Fig. 27), (_Lucilia spp._) and the flesh-flies (Fig. 28)
(_Sarcophaga spp._) all have habits somewhat like the screw-worm fly.
Any of them may lay their eggs in wounds on man or animals with the same
serious results.

The flesh-fly instead of laying eggs deposits the living larvæ upon meat
wherever it is accessible, and as these develop with astonishing
rapidity they are able to consume large quantities of flesh in a
remarkably short time. In this way they may be of some importance as
scavengers, but it is better to get rid of the waste in other ways than
to leave it for a breeding-place for flies that are capable of causing
so much damage and suffering.

Not infrequently the larvæ of certain flies are to be found in the
alimentary canal where as a rule they do no particular damage.
Altogether the larvæ of over twenty different species of flies have been
found in or expelled from the human intestinal canal. In Europe, the
majority of these larvæ belong to a fly which looks very much like the
house-fly except that it is somewhat smaller and so is often known as
"the little house-fly" (Fig. 29) (_Homalomyia canicularis_). The same
species is very common in the United States, frequently occurring in
houses. Under certain conditions it may even be more abundant than the
house-fly. It is believed that the larvæ in the intestinal canal come
from eggs that have been deposited on the victim while using an outdoor
privy where the flies are often very abundant. Instances are also on
record where these larvæ have been discharged from the urethra.

Another fly (_Ochromyia anthropophaga_) occurring in the Congo region
has a blood-sucking larvæ which is known as the Congo floor-maggot. The
fly which is itself not troublesome deposits its eggs in the cracks and
crevices of the mud floors of the huts. The larvæ which hatch from these
crawl out at night and suck the blood of the victim that may be sleeping
on the floor or on a low bed.

BOT-FLIES

Another group of flies known as the bot-flies (Fig. 30) have their
mouth-parts rudimentary or entirely wanting so of course they themselves
cannot bite or pierce an animal. Nevertheless they are the source of an
endless amount of trouble to stockmen and sometimes even attack man.
Although these flies cannot bite, the presence of even a single
individual may be enough to annoy a horse almost to the end of
endurance. Horses seem to have an instinctive fear of them and will do
all in their power to get rid of the annoying pests.

The eggs of the house bot-fly are laid on the hair of the legs or some
other part of the body. The horse licks them off and they hatch and
develop in the alimentary canal of their host. Sometimes the walls of
the stomach may be almost covered with them thus of course seriously
interfering with the functions of this organ. When full grown the larvæ
pass from the host and complete their transformation in the ground.

[Illustration: FIG. 27--Blue-bottle fly (_Lucilia sericata_).]

[Illustration: FIG. 28--Flesh-fly (_Sarcophaga sp_).]

[Illustration: FIG. 29--"The little house-fly" (_Homalomyia
canicularis_).]

[Illustration: FIG. 30--Horse bot-fly (_Gastrophilus equi_).]

[Illustration: FIG. 31--Ox warble-fly (_Hypoderma lineata_).]

[Illustration: FIG. 32--Sheep bot-fly (_Gastrophilus nasalis_).]

The bot-flies of cattle or the oxwarbles (Fig. 31) gain an entrance
into the alimentary canal in the same way, that is, by the eggs being
licked from the hairs on the body where they have been laid by the adult
fly. But instead of passing on into the stomach they collect in the
esophagus and later make their way through the walls of this organ and
through the tissues of the body until they at last reach a place along
the back just under the skin. Here as they are completing their
development they make more or less serious sores on the backs of the
infested animals. The hides on such animals are rendered nearly
valueless by the holes made by the larvæ. When fully mature they drop to
the ground and complete their transformations.

The sheep bot-flies (Fig. 32) lay their eggs in the nostrils of sheep.
The larvæ pass up into the frontal sinuses where they feed on the mucus,
causing great suffering and loss. Many other species of animals are
infested with their own particular species of bots. Several instances
are recorded where the oxwarble has occurred in man, always causing much
suffering and sometimes death.

One or more species of bot-flies occurring in the tropical parts of
America frequently attack man. The early larval stage soon after it has
entered the skin is known as the _Ver macaque_. Later stages as _torcel_
or _Berne_. The presence of the larvæ produces very painful and
troublesome sores. It is supposed that the adult flies (one species of
which is _Dermatobia cyaniventris_) lay their eggs on the skin which
the larvæ penetrate as soon as they hatch. It has also been suggested
that they might reach the subcutaneous tissue by migrating from the
alimentary canal as do some of the other bot-flies. A very serious eye
disease, _Egyptian opthalmia_, is known to be spread by the house-flies
and others. These flies are often abundant about the eyes, especially of
children suffering from this disease. It is suspected that certain small
flies (Oscinidæ) in the southern part of the United States are
responsible for the spread of disease known as "sore eye."

FLEAS

The fleas used to be considered as degenerate Diptera and were placed
with that group but they are now classed as a separate order
(Siphonaptera). Within recent years these little pests have come into
special prominence on account of their importance in connection with the
spread of the plague. The fact that they are so abundant everywhere and
that they will so readily pass from one host to another makes the
possibility of their spreading infectious diseases very great. Besides
the kinds that are concerned in the transmission of plague, which are
discussed in another chapter, there are many other kinds infesting
various wild and domesticated animals and a few attacking birds.

One of the most important of these is the jigger-flea or chigoe
(_Dermatophilus penetrans_, Fig. 33). Various other names such as
chigger-flea, sand-flea, jigger, chigger are also applied to this insect
as well as to a minute red mite that burrows into the skin in much the
same way as the female of the flea. So although they are entirely
different creatures you can never tell from the common name, whether it
is the flea or the mite that is being referred to. Both the male and
female jigger-fleas feed on the host and hop on or off as do other
fleas, but when the female is ready to lay eggs (Fig. 34), she burrows
into the skin. Her presence there causes a swelling and usually an ulcer
which often becomes very serious, especially if the insect should be
crushed and the contents of the body escape into the surrounding tissue.

These little pests are found throughout tropical and subtropical America
and have been introduced into Africa and from there have spread to India
and elsewhere. They attack almost all kinds of animals as well as many
birds, being of course a source of great annoyance and no inconsiderable
loss. They are more apt to attack the feet of men, especially those who
go barefooted. Sometimes they occur in such numbers as to make great
masses of sores.

On account of being such general feeders they are difficult to control,
but some relief may be obtained by keeping the houses and barns as free
as possible from dirt and rubbish and by sprinkling the breeding-places
of the pest with pyrethrum powder or carbolic water. Those that gain an
entrance into the skin should be cut out, care being taken to remove the
insect entire.

BEDBUGS

In the order Hemiptera, or the true "bugs" in an entomological sense, we
find a few forms that may carry disease. The bedbug (Fig. 35) (_Cimex
lectularis_) has been accused of transmitting plague, relapsing fever
and other diseases. Very recent investigations show that the common
bedbug of India (_Cimex rotundatus_) harbors the parasite that causes
the disease known as _kala azar_, and there is no doubt that it
transmits the disease.

LICE

The sucking lice (Fig. 36) which also belong to this order are suspected
of carrying some of these same diseases. It is thought that the common
louse on rats (_Hæmatopinus spinulosus_) is responsible for the spread
from rat to rat of a certain parasite. (_Trypanosoma lewisi_), which,
however, does not produce any disease in the rats, but if they are
capable of acting as alternative hosts for such parasites, it is quite
possible that they may also carry disease-producing forms.

[Illustration: FIG. 33--Chigo or jigger-flea, male (_Dermatophilus
penetrans_). (After Karsten.)]

[Illustration: FIG. 34--Chigo, female distended with eggs. (After
Karsten.)]

[Illustration: FIG. 35--Bedbug (_Cimex lectularis_).]

[Illustration: FIG. 36--Body-louse (_Pediculus vestimenti_). (From
drawing by J.H. Paine.)]

HOW INSECTS MAY CARRY DISEASE GERMS

Insects may carry the germs or parasites which cause disease in a purely
mechanical or accidental way, that is, the insect may in the course of
its wanderings or its feeding get some of the germs on or in its body
and may by chance carry these to the food, or water, or directly to some
person who may become infected. Thus the house-fly may carry the typhoid
germs on its feet or in its body and distribute them in places where
they may enter the human body.

Several other flies as well as fleas, bedbugs, ticks, etc., may also
carry disease germs in this mechanical way. While this method of
transmission is just as dangerous as any other, and possibly more
dangerous because more common, another method in which the insect is
much more intimately concerned is more interesting from a biological
standpoint at least and will be discussed more fully in the chapters on
malaria, yellow fever and elephantiasis.

In these cases the insect is one of the necessary hosts of the parasite,
which cannot go on with its development or pass from one patient to
another unless it first enters the insect at a certain stage of its
life-history.

[Illustration: FIG. 37--One use for the house-fly.]

BABY-BYE.

1. Baby-Bye,
Here's a fly;
We will watch him, you and I.
How he crawls
Up the walls,
Yet he never falls!
I believe with six such legs
You and I could walk on eggs.
There he goes
On his toes,
Tickling Baby's nose.

CHAPTER V

HOUSE-FLIES OR TYPHOID-FLIES

The page shown in Fig. 37 was copied from one of our old second readers
and shows something of the spirit in which we used to regard the
house-fly. A few of them were nice things to have around to make things
seem "homelike." Of course they sometimes became too friendly during the
early morning hours when we were trying to take just one more little nap
or they were sometimes too insistent for their portion of the dinner
after it had been placed on the table, but a screen over the bed would
help us out a little in the morning and a long fly-brush cut from a tree
in the yard or made of strips of paper tacked to a stick or, still more
fancy, made of long peacock plumes, would help to drive them from the
table. Those that were knocked into the coffee or the cream could be
fished out; those that went into the soup or the hash were never missed!

Not only were the flies regarded as splendid things with which to amuse
the baby, but they were thought to be very useful as scavengers as they
were often seen feeding on all kinds of refuse in the yard. Then, too,
they seemed to be cleanly little things, for almost any time some of
them could be seen brushing their heads and bodies with their legs and
evidently having a good clean-up. More than that it never occurred to us
that it would be possible to get rid of them even should it be thought
advisable, for they came from "out doors," and who could kill all the
flies "out doors"?

Fortunately, or otherwise, these halcyon days have gone by and the
common, innocent, friendly little house-fly is now an outcast convicted
of many crimes and accused of a long list of others (Fig. 38).

Its former friends have become its sworn enemies. The foremost
entomologist of the land has suggested that we even change its name and
give it one that would be more suggestive of the abhorence with which we
now look upon it.

[Illustration: FIG. 38--The house-fly (_Musca domestica_).]

And all these changes have come about because science has turned the
microscope on the house-fly and men have studied its habits. We know now
that as the fly is "tickling baby's nose" it may be spreading there
where they may be inhaled or where they may be taken into the baby's
mouth thousands of germs some of which may cause some serious disease.
We know that as they are buzzing about our faces while we are trying to
sleep they may, unwittingly, be in the same nefarious business, and we
know that as they sip from our cups with us or bathe in our coffee or
our soup or walk daintily over our beefsteak or frosted cake they are
leaving behind a trail of filth and bacteria, and we know that some of
these germs may be and often are the cause of some of our common
diseases. As the typhoid germs are very often distributed in this way,
Dr. Howard has suggested that the house-fly shall be known in the future
as the typhoid-fly, not because it is solely responsible for the spread
of typhoid, but because it is such an important factor in it and is so
dangerous from every point of view. The names "manure fly" and "privy
fly" have also been suggested and would perhaps serve just as well, as
the only object in giving it another name would be to find a more
repulsive one to remind us constantly of the filthy and dangerous habits
of the fly.

STRUCTURE

In order that we may better understand why it is that the house-fly is
capable of so much mischief, let us consider briefly a few points in
regard to its structure, its methods of feeding and its life-history.

The large compound eyes are the most conspicuous part of the head (Fig.
39). In front, between the eyes, are the three-jointed antennæ, the last
joint bearing a short, feathery bristle. From the under side of the head
arises the long, fleshy proboscis (Fig. 40). When this is fully extended
it is somewhat longer than the head; when not distended and in use it is
doubled back in the cavity on the under side of the head. About half-way
between the base and the middle is a pair of unjointed mouth-feelers
(maxillary palpi). At the tip are two membranous lobes (Fig. 41) closely
united along their middle line. These are covered with many fine
corrugated ridges, which under the microscope look like fine spirals and
are known as pseudotracheæ. Thus it will be seen that the house-fly's
mouth-parts are fitted for sucking and not for biting. Its food must be
in a liquid or semi-liquid state before it can be sucked through the
tube leading from the lobes at the tip up through the proboscis and on
into the stomach. If the fly wishes to feed on any substance such as
sugar, that is not liquid, it first pours out some saliva on it and then
begins to rasp it with the rough terminal lobes of the proboscis, thus
reducing the food to a consistency that will enable the fly to suck it
up. Many people think that house-flies can bite and will tell you that
they have been bitten by them. But a careful examination of the
offender, in such instances, will show that it was not a house-fly but
probably a stable-fly, which does have mouth-parts fitted for piercing.

[Illustration: FIG. 39--Head of house-fly showing eyes, antennæ and
mouth-parts.]

[Illustration: FIG. 40--Proboscis of house-fly, side view.]

[Illustration: FIG. 41--Lobes at end of proboscis of house-fly showing
corrugated ridges.]

[Illustration: FIG. 42--Wing of house-fly.]

The thorax bears the two rather broad, membranous wings (Fig. 42) which
have characteristic venation. Three of these veins end rather close
together just before the tip of the wing, the posterior one of the group
being bent forward rather sharply a short distance from the tip. The
stable-fly has this vein slightly curved forward but not nearly so
conspicuously (Fig. 43).

Nearly all the other flies that are apt to be mistaken for the house-fly
do not have this vein curved forward. The wings, although apparently
bare, are covered with a fine microscopic pubescence. Among these fine
hairs on the wing as well as among similar fine ones and coarser ones
all over the body, particles of dust and dirt or filth (Fig. 44) or,
what interests us more just now, thousands of germs may find a temporary
lodgment and later be scattered through the air as the insect flies. Or
they may get on our food as the fly feeds or while it rests and combs
its body with the rows of coarse hairs on its legs.

The legs are rather thickly covered with coarse hairs or bristles and
with a mat of fine, short hairs. On some of the segments the larger
hairs are arranged in rows and are used as a sort of comb with which the
fly combs the dirt from the rest of its body. The last segment (Fig. 45)
of the leg bears at its tip a pair of large curved claws and a pair of
membranous pads known as the pulvillæ. On the under side of the pulvillæ
are innumerable minute secreting hairs (Fig. 46) by means of which the
fly is able to walk on the wall or ceiling or in any position on
highly-polished surfaces.

HOW THEY CARRY BACTERIA

These same little pads, with their covering of secreting hairs, are
perhaps the most dangerous part of the insect for they cannot help but
carry much of the filth over or through which the fly walks, and as this
may be well stocked with germs the danger is at once apparent.

As the result of a series of carefully planned experiments it has been
demonstrated that the number of bacteria on a single fly may range all
the way from 550 to 6,600,000 with an average for the lot experimented
with of about one and one-fourth million bacteria to each fly. Now where
do all these bacteria come from? Necessarily from the place where the
fly breeds or where it feeds.

[Illustration: FIG. 43--Wing of Stable-fly (_Stomoxys calcitrans_).]

[Illustration: FIG. 44--Wing of house-fly showing particles of dirt
adhering to it.]

[Illustration: FIG. 45--Last three segments of leg of house-fly showing
the claws, the pulvillæ and the hairs on the legs.]

[Illustration: FIG. 46--Foot of house-fly showing claws, hairs,
pulvillæ and the minute clinging hairs on the pulvillæ.]

[Illustration: FIG. 47--Larva of house-fly.]

LIFE-HISTORY AND HABITS

The eggs of the house-fly may be laid on almost any kind of decaying or
fermenting material. If this is kept moist and a proper temperature
maintained the larvæ or maggots (Fig. 47) that hatch from the eggs may
develop. As a rule, however, these requirements are found only under
certain conditions and are ordinarily found only in manure heaps or in
privy vaults or latrines. All observers agree that the female fly
prefers to deposit her eggs in horse manure when this can be found and
when this is piled in heaps in the barn-yard (Fig. 48) or in the field
the heat caused by the decay and fermentation makes ideal conditions for
the development of the larvæ. Cow manure may serve as a breeding-place
to a limited extent. The flies are immediately attracted to human
excrement and breed freely in it when opportunity offers. Decaying
vegetables or fruit, fermenting kitchen refuse and other materials
sometimes also serve as breeding-places.

In suitable places in warm weather the eggs will hatch in from eight to
twelve hours and the larvæ will become fully developed in from eight to
fourteen days. They then change to pupæ (Fig. 50) in which stage they
may remain for another eight to twenty days when the adult flies will
emerge. These figures must necessarily be indefinite because the weather
and other conditions always vary. Under the most favorable conditions of
moisture and temperature it is probably never less than eight days from
egg to adult fly and under unfavorable conditions it may be as long as
six weeks.

The larvæ thrive best when the manure is kept quite wet. I have often
found them in almost incredible numbers in stables that had not been
cleaned for some time. The horses standing there at night added fresh
material and kept it just wet enough to make conditions almost ideal
(Fig. 49).

The pupæ are usually found where the manure is a little dryer, but it
must not be too dry. When the flies issue from the pupæ they push their
way up to the surface where they remain for a short time and allow the
body to harden and the wings to dry before they fly away to other manure
or, as too often happens, to some near-by kitchen or restaurant or
market place.

[Illustration: FIG. 48--Barn-yard filled with manure. Millions of flies
were breeding here and infesting all the near-by houses.]

[Illustration: FIG. 49--Dirty stalls; the manure had not been removed
for some days and the floor was covered with maggots.]

Of course it is impossible for them to issue from this filth without
more or less of it clinging to their bodies. Now if these flies would
breed only in barn-yard manure and fly directly from the stable to the
house there would be comparatively little reason to complain, at least
from a sanitary standpoint, for the amount of barn-yard filth that they
carried to our food would be of little consequence. But when they breed
in privy vaults or similar places, or visit such places before coming
into the house or dairy or market place the results may be much more
serious.

FLIES AND TYPHOID

It has been abundantly demonstrated that the excrement or the urine of a
typhoid patient may contain virulent germs for some time before he is
aware that he has the disease, and it has been shown that the germs may
be present for weeks or months, and in some cases even years after the
patient has recovered. If a fly breeds in such infected material, or
feeds or walks on it, it is very apt to get some of the germs on its
body where they may retain their virulence for some time, and should it
visit our food while covered with these germs some of them would
probably be left there where they might produce serious results. More
than that. If the fly should feed on such infected material the typhoid
germs would go on developing in the intestine of the fly and would be
passed out with the feces in which they retain their virulence for some
days. In other words, the too familiar "fly-specks" are not only
disgusting, but may be a very grave source of danger. It will be seen
that in this way several members of a community might become infected
with the typhoid germs before anyone was aware that there was a case of
typhoid or a "bacillus carrier" in the neighborhood.

One more example out of the scores that might be cited to show how the
fly may carry typhoid germs. They may enter the sick chamber in the home
or in the hospital and there gain access to the typhoid germs. These
they may carry to other parts of the house or to near-by houses, or the
flies may light on passing carriages or cars and be carried perhaps for
miles before they enter another house and contaminate the food there.

These are hypothetical cases, but they illustrate what is taking place
hundreds of times every season all over the world wherever typhoid fever
and flies occur, and no country or race is known to be immune from
typhoid, and the fly is found "wherever man is found."

In the summer of 1898 a commission was appointed to investigate the
prevalence of typhoid fever in the United States Army Concentration
Camps. The following are some of the conclusions as reported by Dr.
Vaughan:

"FLIES UNDOUBTEDLY SERVED AS CARRIERS OF THE INFECTION

"My reasons for believing that flies were active in the
dissemination of typhoid may be stated as follows:

"_a._ Flies swarmed over infected fecal matter in the pits and then
visited and fed upon the food prepared for the soldiers at the mess
tents. In some instances where lime had recently been sprinkled
over the contents of the pits, flies with their feet whitened with
lime were seen walking over the food.

"_b._ Officers whose mess tents were protected by means of screens
suffered proportionately less from typhoid fever than did those
whose tents were not so protected.

"_c._ Typhoid fever gradually disappeared in the fall of 1898, with
the approach of cold weather, and the consequent disabling of the
fly.

"It is possible for the fly to carry the typhoid bacillus in two
ways. In the first place, fecal matter containing the typhoid germ
may adhere to the fly and be mechanically transported. In the
second place, it is possible that the typhoid bacillus may be
carried in the digestive organs of the fly and may be deposited
with its excrement."

In Dr. Daniel D. Jackson's report to the Merchants' Association of New
York on the "Pollution of New York Harbor as a Menace to the Health by
the Dissemination of Intestinal Diseases Through the Agency of the
Common House-fly," he shows graphically that the prevalence of typhoid
and other intestinal diseases is coincident with the prevalence of
flies, and that the greatest number of deaths from such diseases occurs
near the river front where the open or poorly constructed sewers scatter
the filth where the flies can feed on it, or along the wharves with
their inadequate accommodations and the resulting accumulation of filth.

FLIES AND OTHER DISEASES

Not only is the house-fly an important factor in the dissemination of
typhoid fever, but it has been definitely shown that it is capable of
transmitting several other serious diseases.

The evidence that flies carry and spread the deadly germs of cholera is
most conclusive. The germs may be carried on the body where they will
live but a short time, or they may be carried in the alimentary canal
where they will live for a much longer period and are finally deposited
in the fly-specks where they retain their virulence for some time. Flies
that had been allowed to contaminate themselves with cholera germs were
allowed access to milk and meat. In both cases hundreds of colonies of
the germs could later be recovered from the food. As with the typhoid
germs milk seems to be a particularly good medium for the development of
the cholera germs. In several of the experiments that have been made
along this line the milk has been readily infected by the flies visiting
it.

Of course an outbreak of cholera is of rare occurrence in our country,
but unfortunately this is not so in regard to some other intestinal
diseases such as diarrhea and enteritis which annually cause the death
of many children, especially bottle-fed babies. Those who have made
close studies of the way in which these diseases are disseminated are
convinced that the flies are one of the most important factors in their
spread.

It has long been observed that flies are particularly fond of sputum and
will feed on it on the sidewalk, in the gutter, the cuspidor or wherever
opportunity offers. It is well known, too, that the sputum of a
consumptive contains myriads of virulent tubercular germs. A fly feeding
and crawling over such material must necessarily get some of it on its
body, and as it dries and the insect flies about the germs will be
distributed through the air, possibly over our food. It has been shown
that the excretion from a fly that has fed on tubercular sputum contains
tubercular bacilli that may remain virulent for at least fifteen days.
Thus we see again the danger that may lurk in the too familiar
"fly-specks."

Although it is generally supposed that the flea is solely responsible
for the spread of the bubonic plague and no doubt is the principal
distributing agent, the fact must not be overlooked that the house-fly
may also be of considerable importance in this connection. Carefully
planned experiments have shown that flies that have become infected by
being fed on plague-infected material may carry the germs for several
days and that they may die of the disease. During plague epidemics flies
may become infected by visiting the sores on human or rat victims or by
feeding on dead rats or on the excreta of sick patients, and an infected
fly is always a menace should it visit our food or open wounds or sores.
Anthrax bacilli are carried about and deposited by flies showing the
possibility of the disease being spread in this way.

Some believe that leprosy, smallpox and many other diseases are carried
by the house-fly, so it is little wonder that it is fast losing its
standing as a household companion and that we are beginning to regard it
not only as a nuisance but as a source of danger which should no longer
be tolerated in any community.

Of course only a small per cent of the flies that visit our food in the
dairies or market places or kitchens actually carry dangerous diseases,
but they are all bred in filth and it is not possible without careful
experiments or laboratory analysis to determine whether any of the germs
among the millions that are on their bodies are dangerous or not. The
chances that they may be are too great. The only safe way is to banish
them all or to see that all of our food is protected from them.

FIGHTING FLIES

Screens and sticky fly-paper have their places and give some little
relief in a well-kept house. But of what use is it to protect your food
after it has entered your home if in the stores, in the market place, in
the dairy barn, or dairy wagon, in the grocers' and butchers' cart, it
has been exposed to contamination by hundreds of flies that have visited
it.

The problem is a larger one than keeping the house free from flies;
larger but not more difficult, for the remedy is simple, effective,
practicable and inexpensive. Destroy their breeding-places and you will
have no flies. As the flies breed principally in manure the first
remedial measure is to see that all manure is removed from the
barn-yard at least once a week and spread over the fields to dry, for
the flies cannot breed in the dry manure. If it is not practicable to
remove it this often the manure should be kept in a bin that is closed
so tight that no flies can get into it to lay their eggs. Sometimes the
manure may be treated with some substance such as kerosene, crude oil,
chlorid of lime, tobacco water or mixture of two or more of these and
thus rendered unsuitable for the flies to breed in, but in general
practice none of them has been found very satisfactory for the treatment
is either not thorough enough or is too expensive of time and material.

Outdoor privies and cesspools must be carefully attended to. The latter
can be easily covered so no flies can get in and if the filthy and in
every way dangerous pit under the privy be filled and the dry-earth
closet substituted one of the greatest sources of danger, especially in
the country and in towns with inadequate sewerage facilities, will be
done away with. After these things are done there remain only the
garbage cans and the rubbish heaps to look after.

Of course your neighbor must keep his place clean too, for his flies are
just as apt to come into your house as his, so the problem becomes one
for the whole community.

Almost all cities and many of the smaller towns have ordinances which
if enforced would afford adequate protection from flies, but they are
seldom if ever rigidly enforced and it yet remains for some enterprising
town to be able to advertise itself as a "speckless town" as well as a
"spotless town."

AN EXPERT'S OPINION

In a recent important bulletin issued by the Bureau of Entomology, Dr.
L.O. Howard discusses the economic importance of several of the insects
that carry disease. I wish to quote two or three paragraphs from the
pages in which he discusses the house-fly or typhoid fly to show the
opinion of this excellent authority in regard to this pest.

"Even if the typhoid or house fly were a creature difficult to
destroy, the general failure on the part of communities to make any
efforts whatever to reduce its numbers could properly be termed
criminal neglect; but since, as will be shown, it is comparatively
an easy matter to do away with the plague of flies, this neglect
becomes an evidence of ignorance or of a carelessness in regard to
disease-producing filth which to the informed mind constitutes a
serious blot on civilized methods of life."

On another page:

"We have thus shown that the typhoid or house fly is a general and
common carrier of pathogenic bacteria. It may carry typhoid fever,
Asiatic cholera, dysentery, cholera morbus, and other intestinal
diseases; it may carry the bacilli of tuberculosis and certain eye
diseases. It is the duty of every individual to guard so far as
possible against the occurrence of flies upon his premises. It is
the duty of every community, through its board of health, to spend
money in the warfare against this enemy of mankind. This duty is as
pronounced as though the community were attacked by bands of
ravenous wolves."

Again:

"A leading editorial in an afternoon paper of the city of
Washington, of October 20, 1908, bears the heading, 'Typhoid a
National Scourge,' arguing that it is to-day as great a scourge as
tuberculosis. The editorial writer might equally well have used the
heading 'Typhoid a National Reproach,' or perhaps even 'Typhoid a
National Crime,' since it is an absolutely preventable disease. And
as for the typhoid fly, that a creature born in indescribable filth
and absolutely swarming with disease germs should practically be
invited to multiply unchecked, even in great centers of population,
is surely nothing less than criminal."

The whole bulletin (No. 78, Bureau of Entomology) should be read and
studied by all who are interested in this subject.

OTHER FLIES

Occasionally other flies looking more or less like the house-fly are
seen in houses. Some of these have the same type of sucking mouth-parts
and have habits very similar to the house-fly, but as they are usually
much less common and as nearly all that has been said in regard to the
house-fly would apply equally well to them and as the same measures
should be adopted in fighting them they need not be discussed further
here.

I have already called attention to the fact that a fly which looks very
much like the house-fly is sometimes found in the house and will often
bite severely. It has quite a different style of beak, one that is
fitted for piercing so it may suck the blood of its victim (Fig. 51). As
these flies often seem to be more persistent before a rain the weather
prophet will tell you that "It is surely going to rain for the
house-flies are beginning to bite."

These stable-flies, as they are called, are great pests of cattle and
horses in some sections. It is thought that they are important factors
in the spread of some of the diseases of domestic animals, and their
habit of sometimes attacking human beings makes it possible for them to
carry certain disease germs from animals to man or from man to man.

CHAPTER VI

MOSQUITOES

Mosquitoes are no more abundant now than they have been in the past, but
when Linnæus in 1758 made his list of all the animals known to exist at
that time he catalogued only six species of mosquitoes. Only a few years
ago, 1901, Dr. Theobald of the British Museum published a book on the
mosquitoes of the world in which he listed three hundred and forty-three
kinds. Soon other volumes appeared, adding more species, and
systematists everywhere have been describing new ones until now the
total number of described species is probably over five hundred, more
than sixty of which occur in the United States.

This shows only one phase of the great interest that has been taken in
the mosquitoes since the discovery of their importance as carriers of
disease. Not only have they been studied from a systematic standpoint
but an endless amount of work has been done and is being done in
studying their development, habits, and structure until now, if one
could gather together all that has been written about mosquitoes in the
last ten or twelve years he would have a considerable library.

[Illustration: FIG. 50--Pupa of house-fly with the end broken to allow
the fly to issue.]

[Illustration: FIG. 51--Head of stable-fly showing sharp piercing beak.]

[Illustration: FIG. 52--Mass of mosquito eggs (_Theobaldia incidens_).]

[Illustration: FIG. 53--Mosquito eggs and larvæ (_Theobaldia
incidens_); two larvæ feeding on bottom, others at surface to breathe.]

[Illustration: FIG. 54--Mosquito larvæ (_T. incidens_), dorsal view.]

Those who are particularly interested in the group will find some of
these books and papers easily accessible, so there may be given here
only a brief summary of the more important facts in regard to the
structure and habits of the mosquitoes in order that we may more readily
understand the part that they play in the transmission of diseases and
see the reasonableness of the recommendations in regard to fighting
them.

THE EGGS

Mosquito eggs are laid in water or in places where water is apt to
accumulate, otherwise they will not hatch. Some species lay their eggs
in little masses (Fig. 52) that float on the surface of the water,
looking like small particles of soot. Others lay their eggs singly, some
floating about on the surface, others sinking to the bottom where they
remain until the young issue. Some of the eggs may remain over winter,
but usually those laid in the summer hatch in thirty-six to forty-eight
hours or longer according to the temperature.

THE LARVÆ

When the larvæ are ready to issue they burst open the lower end of the
eggs and the young wrigglers escape into the water. The larvæ are fitted
for aquatic life only, so mosquitoes cannot breed in moist or damp
places unless there is at least a small amount of standing water there.
A very little will do, but there must be enough to cover the larvæ or
they perish.

The head of the larvæ of most species is wide and flattened. The eyes
are situated at the sides, and just in front of them is a pair of short
antennæ which vary with the different species.

The mouth-parts too vary greatly according to the feeding habits. Some
mosquito larvæ are predaceous, feeding on the young of other species or
on other insects. These of course have their mouth-parts fitted for
seizing and holding their prey. Most of the wrigglers, however, feed on
algæ, diatoms, Protozoa and other minute plant or animal forms which are
swept into the mouth by curious little brush-like organs whose movements
keep a stream of water flowing toward the mouth.

Another group containing the _Anopheles_ are intermediate between these
two and have mouth-parts fitted for feeding on minute organisms as well
as for attacking and holding other larger things.

[Illustration: FIG. 55--Eggs, larvæ and pupæ of mosquitoes (_T.
incidens_).]

[Illustration: FIG. 56--Larva of mosquito (_T. incidens_).]

[Illustration: FIG. 57--Mosquito larvæ and pupæ (_T. incidens_) with
their breathing-tubes at the surface of the water.]

[Illustration: FIG. 58--Anopheles larvæ (_A. maculipennis_) resting at
the surface of the water.]

A few kinds feed habitually some distance below the surface, others on
the bottom, while still others feed always at the surface. With one or
two exceptions, the larvæ must all come to the surface to breathe (Figs.
53-57). Most species have on the eighth abdominal segment a rather long
breathing-tube the tip of which is thrust just above the surface of the
water when they come up for air. In this tube are two large vessels or
tracheæ which open just below the tip of the tube and extend forward
through the whole length of the body, giving off branches here and there
that divide into still smaller branches until every part of the body is
reached by some of the small divisions of this tracheal system that
carries the oxygen to all the tissues. The length of the breathing-tube
is correlated with the feeding-habits of the larvæ. _Anopheles_ larvæ
which feed at the surface have very short tubes (Fig. 58), others that
feed just below the surface have breathing-tubes as long or very much
longer than the ninth abdominal segment. The last segment has at its tip
four thin flat plates, the tracheal gills. These too are larger or
smaller according to the habits of the larvæ. Those species that feed
close to the surface and have the tip of the breathing-tube above the
surface most of the time have very small tracheal gills, while those
that feed mostly on the bottom have them well developed.

When first hatched the larvæ are of course very small. If the weather is
warm and the food is abundant they grow very rapidly. In a few days the
outer skin becomes rather firm and inelastic so it will not allow
further growth. Then a new skin forms underneath and the old skin is
cast off. This process of casting off the old skin is called molting,
and is repeated four times during the one, two, three or more weeks of
larval life.

PUPA

With the fourth molt the active feeding larva changes to the still
active but non-feeding pupa (Fig. 59). The head and thorax are closely
united and a close inspection will reveal the head, antennæ, wings and
legs of the adult mosquito folded away beneath the pupal skin. Instead
of the breathing-tube on the eighth segment of the abdomen as in the
larva, the pupa has two trumpet-shaped tubes on the back of the thorax
through which it now gets its air from above the surface. The pupal
stage lasts from two to five or six days or more. When the adult is
ready to issue the pupal skin splits along the back and the mosquito
gradually and slowly issues. It usually takes several minutes for the
adult to issue and for its wings to become hard enough so it can fly. In
the meantime, it is resting on the old pupal skin or on the surface of
the water, where it is entirely at the mercy of any of its enemies that
might happen along and is in constant danger of being tumbled over
should the water not be perfectly smooth.

[Illustration: FIG. 59--Mosquito pupæ (_T. incidens_) resting at the
surface of the water.]

[Illustration: FIG. 60--Mosquito pupa (_T. incidens_) with its
breathing-tubes in an air bubble below the surface of the water.]

[Illustration: FIG. 61--Mosquito larvæ and pupæ (_T. incidens_) resting
at the surface of the water.]

[Illustration: FIG. 62--A female mosquito (_T. incidens_); note the
thread-like antennæ.]

[Illustration: FIG. 63--A male mosquito (_T. incidens_); note the
feathery antennæ.]

THE ADULT

The adult mosquito is altogether too familiar an object to need
description, but it is necessary that we keep in mind certain particular
points in regard to its structure, in order that we may better
understand how it is that it is capable of transmitting disease.

If we examine closely the antennæ of a number of mosquitoes that are
bothering us with their too constant attentions we shall see that they
all look very much alike (Fig. 62), small cylindrical joints bearing
whorls of short fine hairs. But if we examine a number of mosquitoes
that have been bred from a jar or aquarium we will find two types of
antennæ, the one described above belonging to the female. The antennæ of
the male (Fig. 63) are much more conspicuous on account of the whorl of
dense, fine, long hairs on each segment. Another interesting difference
in the antennæ is to be noted in the size of the first joint. In both
sexes it is short and cup-shaped, but in the male it is somewhat larger.
This basal segment contains a highly complex auditory organ which
responds to the vibrations of the whorls of hairs on the other segments.
Interesting experiments have shown that these hairs vibrate best to the
pitch corresponding to middle C on the piano, the same pitch in which
the female "sings." Of course mosquitoes and other insects have no voice
as we ordinarily understand the word, but produce sound by the rapid
vibration of the wings or by the passage of air through the openings of
the tracheæ. The males and females are thus easily distinguished and, as
we shall see later, this is of some importance for only the females can
bite. The males and females differ in another way. Just below the
antennæ and at the sides of the proboscis or beak is a pair of three-to
five-jointed appendages, the maxillary palpi or mouth-feelers which in
the females of most species are very short (Fig. 64) while in the males
they are usually as long as the proboscis (Fig. 65). The females of
_Anopheles_ and related forms have palpi quite as long as the males, but
they are slender throughout while the male palpi are usually somewhat
enlarged toward the tip and bear more or less conspicuous patches of
rather long hairs or scales.

[Illustration: FIG. 64--Head and thorax of female mosquito
(_Ochlerotatus lativittatus_); the short maxillary palpi are just above
the proboscis and below the thread-like antennæ.]

[Illustration: FIG. 65--Head and thorax of male mosquito (_O.
lativittatus_); the maxillary palpi are as long as the proboscis.]

[Illustration: FIG. 66--Head of female mosquito (_Anopheles_), with
mouth-parts separated to show the needle-like parts: _a_, _a_ antennæ;
_b_, _b_, palpi; _c_, labrum; _d_, _d_, mandibles; _e_, hypopharynx;
_f_, _f_, maxillæ; _g_, labium; _h_, labella. (After Manson.)]

[Illustration: FIG. 67--Cross-section of proboscis of female (_a_) and
male (_b_) mosquito. _lxe_, labrum-epipharynx; _mn_, mandibles; _mx_,
maxillæ; _hp_, hypopharynx; _sal_, salivary duct; _li_, labium; _tr_,
trachea; _mus_, muscles. (After Nuttall and Shipley.)]

THE MOUTH-PARTS

The mouth-parts of the mosquito are of course of particular interest to
us. At first they appear to consist of a long slender beak or proboscis,
but by dissecting and examining with a microscope we find this beak to
be made up of several parts (Fig. 66). The labium, which is the largest
and most conspicuous, is apparently cylindrical but is grooved above
throughout its length. At the tip of the labium are the labellæ, two
little lobes which serve to guide the piercing organs. Lying in this
groove along the upper side of the labium are six very fine,
sharp-pointed needles. The uppermost of these, the labrum-epipharynx, or
labrum as we will call it, is the largest and is really a hollow tube
very slightly open on its under side. Just below this is the
hypopharynx, the lateral margins of which are very thin. Down through
the median line of the hypopharynx runs a minute duct (Fig. 67, sal)
which, though exceedingly small, is of very great importance, for
through it is poured the saliva which may carry the malaria germs into
the wound made when the mosquito bites. The other four needles consist
of a pair of mandibles which are lance-shaped at the tip and a heavier
pair of maxillæ, the tips of which are serrate on one edge.

HOW THE MOSQUITO BITES

When the female mosquito is feeding on man or any other animal the tip
of the labium is placed against the surface and the six needles are
thrust into the skin, the labellæ serving as guides. As they are thrust
deeper and deeper the labium is bowed back to allow them to enter. As
soon as the wound is made the insect pours out through the tube of the
hypopharynx some of the secretion from the salivary glands and then
begins to suck up the blood through the hollow labrum into the pharynx
and on into the stomach.

The mouth-parts of the male differ in some important respects from those
of the female. The hypopharynx is united to the labium, the mandibles
are wanting and the maxillæ are very much reduced so that the insect is
unable to pierce the tough skin of animals. The male feeds on the juices
of plants as do the females when they cannot get blood. It is not at all
necessary for mosquitoes to have the warm blood of man or other animals.
Comparatively few of them ever taste blood. They have been seen feeding
on blossoms, ripe fruit, watermelons, plant juices, etc. They are very
fond of ripe bananas and are fed on them in the laboratory when we wish
to keep mosquitoes for experimental purposes.

THE THORAX

The middle part of the body, called the thorax, is really a strong box
with heavy walls for the attachment of the powerful wing and leg
muscles. The three pairs of legs are covered with hairs and scales, and
their tips are provided with a pair of claws which vary somewhat in the
different species. The wings (Fig. 68) are long and narrow with a
characteristic venation. Along the veins and the margin of the wings are
the scales which readily enable one to distinguish mosquitoes from other
insects that may look much like them. In some species these scales are
long and narrow, almost hair-like, in others they are quite broad and
flat (Fig. 69). Just back of the wings is a pair of balancers, short
thread-like processes knobbed at the end. These probably represent the
second pair of wings with which most insects are provided, and seem to
serve as balancers or orienting organs when the insect is flying. On the
sides of the thorax are two small slit-like openings, the
breathing-pores. These are the openings into the tracheal or respiratory
system.

THE ABDOMEN

The long cylindrical abdomen is composed of eight segments. These are
rather strongly chitinized above and below, but a narrow strip along the
side is unchitinized. In this strip are situated the abdominal
breathing-pores. The tip of the abdomen is furnished with a pair of
movable organs, which in the male are variously modified and serve as
clasping organs at mating time.

THE DIGESTIVE SYSTEM

The mouth-parts of the mosquito have just been described. It will be
remembered that the labrum is provided with a groove. Through this the
blood or other food is sucked up by means of a strong-walled pumping
organ, the pharynx, situated in the head (Fig. 70). Just back of the
pharynx is the esophagus which leads to the beginning of the stomach.
Close to its posterior end the esophagus gives off three food
reservoirs, two above and a single larger one below. In dissections
these will often be seen to be filled with minute bubbles. The stomach
reaches from the middle of the thorax to beyond the middle of the
abdomen. At its posterior end are given off five long slender processes,
the Malpighian tubules which are organs of excretion, acting like the
kidneys of higher animals. The hindgut is that portion of the intestine
from the stomach to the end of the body.

[Illustration: FIG. 68--Wing of Mosquito (_O. lativittatus_).]

[Illustration: FIG. 69--End of mosquito wing highly magnified to show
the scales on the veins.]

[Illustration: FIG. 70--Diagram to show the alimentary canal and
salivary glands of a mosquito.]

[Illustration: FIG. 71--Salivary glands of _Culex_ at right. _Anopheles_
at left. (After Christophers.)]

THE SALIVARY GLANDS

Lying under the alimentary canal in the forward part of the thorax are
the salivary glands. There are two sets of these, each having three
lobes with a common duct which joins the duct from the other set a short
distance before they enter the base of the hypopharynx. Each of these
lobes is made up of a layer of secreting cells (Fig. 71) which produces
the saliva that is poured into the wound as soon as the insect pierces
the skin of the victim, and we shall see, too, that the malarial germs
also collect in these glands to be carried by the saliva to the new
host.

EFFECTS OF THE BITE

After a mosquito has bitten a person and withdrawn the stylets, a small
area about the puncture whitens, then soon becomes pink and begins to
swell, then to itch and burn. Some people suffer much more from the
bites of mosquitoes than do others. For some such bites mean little or
no inconvenience, indeed may pass wholly unnoticed, to others a single
bite may mean much annoyance, and several bites may cause much
suffering.

After an hour or so the itching usually ceases, but in some cases it
continues longer. In some instances little or no irritation is felt
until some hours, sometimes as much as a day, after the bite. In such
cases the effect of the bite is apt to be severe and to last for several
days. Sometimes a more or less serious sore will follow a bite, probably
due to infection of the wound by scratching. It is doubtless the saliva
that is poured into the wound that causes the irritation. It is
frequently asserted that if the mosquito is allowed to drink its fill
and withdraw its beak without being disturbed no evil results will
follow. Those who hold this theory say that the saliva that is poured
into the wound is all withdrawn again with the blood if the mosquito is
allowed to feed long enough. There may be some truth in this, but for
most of us a bite means a hurt anyway and few will be content to sit
perfectly still and watch the little pest gradually fill up on blood.

It is not known just what the action of the saliva is, its composition
or reaction on the tissues. It is generally supposed to prevent
coagulation of the blood that is to be drawn through the narrow tube of
the labrum. Others think that its presence causes a greater flow of
blood to the wound. But the sad part of it is, for us at least, that it
hurts and may cause malaria and possibly other diseases.

HOW MOSQUITOES BREATHE

Mosquitoes and other insects do not have any nostrils nor do they
breathe through any openings on the head. Along the sides of the thorax
and abdomen is a series of very minute openings known as the spiracles.
Through these the air passes into a system of air-tubes, the tracheæ.
There are two main trunks or divisions of the tracheæ just inside the
body-wall and a number of shorter connecting trunks. From these larger
vessels arise a great number of smaller ones which branch and subdivide
again and again until all the tissues are supplied by these minute
little air-tubes that carry the oxygen to all parts of the body and
carry off the waste carbon dioxid. These air-tubes are emptied and
filled by the contractions of the walls of the abdomen. When the
body-wall contracts the air is forced out of the thin-walled trachea
through the spiracles; when the pressure is removed they are refilled by
the fresh air rushing in.

THE BLOOD

After a mosquito has been feeding on a man or some other animal it is
often so distended that the blood shows rich and red through the thin
sides of the walls of the abdomen. This, however, is the blood of the
victim and not of the mosquito. The blood of insects is not red but pale
yellowish or greenish. It is not confined in definite vessels, but fills
all the space inside the body cavity that is not occupied by some of the
tissues or organs. It bathes the walls of the alimentary canal and
gathers there the nourishment which it carries to all parts of the body.
It does not carry oxygen or collect the carbon dioxid as does the blood
of higher animals. That work, as we have just seen, is done by the
air-tubes. Above the alimentary canal, extending almost the whole length
of the abdomen and thorax, is a thin-walled pulsating vessel, the heart.
This consists of a series of chambers each communicating with the one in
front of it by an opening which is guarded by a valve. When one of these
chambers contracts it forces the blood that is in it forward into the
next chamber which, in its turn, sends it on. As the walls relax the
valves at the sides are opened and the blood that is in the body-cavity
rushes in to fill the empty chamber. As these regular rythmical
pulsations recur the blood is forced forward through the heart into the
head where it bathes the organs there. We shall see in another chapter
that the malarial parasite escapes from the walls of the stomach of the
mosquito into the blood in the body-cavity and finally reaches the
salivary glands. As the heart is constantly driving blood to this part
of the body the parasites readily reach the glands from which they
finally escape into the new host.

[Illustration: FIG. 72--Heads of Culicinæ mosquitoes; _a_, male; _b_,
female. (After Manson.)]

[Illustration: FIG. 73--Heads of Anophelinæ mosquitoes; _c_, male; _d_,
female. (After Manson.)]

[Illustration: FIG. 74--Wing of _Anopheles maculipennis_.]

[Illustration: FIG. 75--Wing of _Theobaldia incidens_.]

[Illustration: FIG. 76--A non-malarial mosquito (_T. incidens_), male,
standing on the wall.]

[Illustration: FIG. 77--Female of same.]

[Illustration: FIG. 78--A malarial mosquito (_A. maculipennis_), male,
standing on the wall.]

[Illustration: FIG. 79--Female of same.]

CLASSIFICATION

For our purpose it will not be necessary to try to give a system of
classification of all the mosquitoes. Those interested in this phase of
the subject will find several books and papers devoted wholly to it. It
is quite important, however, that we know something about a few of the
more familiar groups and kinds, especially those concerned in the
transmission of diseases.

THE ANOPHELES

In pointing out the differences between male and female mosquitoes we
noted that in one group, the genus _Anopheles_, both sexes have long
maxillary palpi (Figs. 72, 73). This is the most important character
separating this genus from the other common forms and as the
_Anopheles_ are the malaria carriers it is important that this
difference be remembered. Most of the members of this group have spotted
wings (Fig. 74), but as some other common kinds also have spotted wings
(Fig. 75) this character will not always be reliable. When an
_Anopheles_ mosquito is at rest the head and proboscis are held in one
line with the body and the body rests at a considerable angle to the
surface on which it is standing. Other kinds rest with the body almost
or quite parallel to the surface on which they are standing. So if you
find a female mosquito with long mouth-palpi and spotted wings resting
at an angle to the surface on which it stands you may be reasonably sure
that it is an _Anopheles_ and therefore may be dangerous (Figs. 76, 77,
78, 79).

In the United States there are three species of
_Anopheles_--_maculipennis_, _punctipennis_ and _crucians_--which are
common in various localities, and one or two other species that so far
as known are local or rare.

The _Anopheles_ eggs are not laid in masses as are the eggs of many
other mosquitoes, but are deposited singly on the surface of the water
where they may be found often floating close together.

[Illustration: FIG. 80--Egg of Anopheles, side view. (After Nuttall and
Shipley.)]

[Illustration: FIG. 81--Egg of Anopheles, dorsal view. (After Nuttall
and Shipley.)]

[Illustration: FIG. 82--Anopheles larvæ, the one to the right feeding.]

[Illustration: FIG. 83--Anopheles larvæ, the one to the right feeding,
the other just coming to the surface.]

[Illustration: FIG. 84--Anopheles larva, dorsal view.]

[Illustration: FIG. 85--Anopheles pupæ resting at surface of water.]

The eggs (Figs. 80, 81) are elliptical in outline and are provided with
a characteristic membranous expansion near the middle.

The larvæ may be found at the proper season and in the localities where
they are abundant in almost any kind of standing water, in clear little
pools beside running streams, in the overflow from springs, in swamps
and marshy lands, in rain-barrels or any other places or vessels where
the water is quiet. They do not breed in brackish water. As they feed
largely on the algæ or green scum on the surface of the water they are
especially apt to be found where this is present. We have already noted
that their positions in the water differ from that assumed by other
species (Fig. 82).

As the breathing-tube is very short the larvæ must come close to the
surface to breathe, and when they are feeding we find them lying just
under and parallel to the surface of the water with their curious round
heads turned entirely upside down as they feed on the particles that are
floating on the surface (Figs. 83, 84).

The pupæ do not differ very much from the pupæ of other species although
the breathing-tubes on the thorax are usually shorter and the creature
usually rests with its abdomen closer to the surface, that is, it does
not hang down from the surface quite as straight as do other forms (Fig.
85).

The adults may be found out of doors or in houses, barns or other
outbuildings. They do not seem to like a draft and consequently will be
more apt to frequent rooms or places where there is little circulation
of air. Although they are usually supposed to fly and bite only in the
evening or at night, they may occasionally bite in the daytime. One
hungry female took two short meals from my arm while we were trying to
get her to pose for a photograph one warm afternoon.

The female passes the winter in the adult condition, hibernating in any
convenient place about old trees or logs, in cracks or crevices in doors
or out of doors. In the house they hide in the closets, behind the
bureau, behind the head of the bed, or underneath it, or in any place
where they are not apt to be disturbed. During a warm spell in the
winter or if the room is kept warm they may come out for a meal almost
any time.

THE YELLOW FEVER MOSQUITO

Ranking next in importance to _Anopheles_ as a disseminator of disease
and in fact solely responsible for a more dreaded scourge, is the
species of mosquito now known as _Stegomyia calopus_. While this species
is usually restricted to tropical or semi-tropical regions it sometimes
makes its appearance in places farther north, especially in summer
time, where it may thrive for a time. The adult mosquito (Fig. 104) is
black, conspicuously marked with white. The legs and abdomen are banded
with white and on the thorax is a series of white lines which in
well-preserved specimens distinctly resembles a lyre. These mosquitoes
are essentially domestic insects, for they are very rarely found except
in houses or in their immediate vicinity. Once they enter a room they
will scarcely leave it except to lay their eggs in a near-by cistern,
water-pot, or some other convenient place.

Their habit of biting in the daytime has gained for them the name of
"day mosquitoes" to distinguish them from the night feeders. But they
will bite at night as well as by day and many other species are not at
all adverse to a daylight meal, if the opportunity offers, so this habit
is not distinctive. The recognition of these facts has a distinct
bearing in the methods adopted to prevent the spread of yellow fever.
There are no striking characters or habits in the larval or pupal stages
that would enable us to distinguish without careful examination this
species from other similar forms with which it might be associated. For
some time it was claimed that this species would breed only in clean
water, but it has been found that it is not nearly so particular, some
even claiming that it prefers foul water. I have seen them breeding in
countless thousands in company with _Stegomyia scutellaris_ and _Culex
fatigans_ in the sewer drains in Tahiti in the streets of Papeete. As
the larvæ feed largely on bacteria one would expect to find them in
exactly such places where the bacteria are of course abundant.

The fact that they are able to live in any kind of water and in a very
small amount of it well adapts them to their habits of living about
dwellings.

So far as known the members of these two genera are the only two that
are concerned in the transmission of disease in the United States. In
other countries other species are suspected or proven disseminators of
certain diseases, but these will be discussed in connection with the
particular diseases in later chapters.

OTHER SPECIES

The many other species of mosquitoes that we have may be conveniently
divided as to their breeding-habits into the fresh-water and the
brackish-water forms. Among the fresh-water kinds some are found
principally associated with man and his dwelling places, others live in
the woods or other places and so are far less troublesome. Most of
these do not fly far. Several of the species that breed in brackish
water are great travelers and may fly inland for several miles. Thus the
towns situated from one to three or four miles inland from the lower
reaches of San Francisco Bay are often annoyed more by the mosquitoes
that breed only in the brackish water on the salt marshes than they are
by any of the fresh-water forms (Figs. 86, 87). The worst mosquito pest
along the coast of the eastern United States and for some distance
inland is a species that breeds in the salt marshes.

NATURAL ENEMIES OF MOSQUITOES

In combating noxious insects we learned long ago that often the most
efficient, the easiest and cheapest way is to depend on their natural
enemies to hold them in check. Under normal or rather natural conditions
we find that they are usually kept within reasonable bounds by their
natural enemies, but under the artificial conditions brought about by
the settling and developing of any district great changes come about. It
very often happens that these changes are favorable to the development
of the noxious insects and unfavorable to the development of their
enemies.

A striking example and one to the point is afforded in the introduction
of mosquitoes into Hawaii. Up to 1826 there were no mosquitoes on these
islands. It is supposed that they were introduced about that time by
some ships that were trading at the islands. Indeed it is claimed that
the very ship is known that brought them over from Mexico.

Once introduced they found conditions there very favorable to their
development, plenty of standing water and few natural enemies to prey on
them, so they increased very rapidly and gradually spread over all the
islands of the group. This was the so-called night mosquito, _Culex
pipiens_. Much later another species, _Stegomyia calopus_, just as
annoying and much more dangerous was introduced and has also become very
troublesome. We have a few species of top-minnows (Fig. 88) occurring in
sluggish streams in the southern part of the United States that are
important enemies of the mosquitoes of that region. A few years ago some
of these were taken over to Hawaii and liberated in suitable places to
see if they would not help solve the mosquito problem there. The fishes
seem to be doing well. Already they are destroying many mosquito larvæ,
and there are indications that they are going to do an important work,
but of course can be depended on only as an aid.

[Illustration: FIG. 86--Salt-marsh mosquito (_Ochlerotatus
lativittatus_); male.]

[Illustration: FIG. 87--Salt-marsh mosquito (_O. lativittatus_);
female.]

[Illustration: FIG. 88--Top-minnow (_Mollienisia latipinna_). (From
Bull., 47 U.S. Fish Com.)]

[Illustration: FIG. 89--Dragon-flies. (From Kellogg's Amer. Insects.)]

On account of the various habits of both the larvæ and adults it will
never be possible for any natural enemy or group of natural enemies
effectively to control the mosquitoes of any region, but as certain of
them are important as helpers they deserve to be mentioned.

ENEMIES OF THE ADULTS

Birds devour a few mosquitoes, the night-flying forms being particularly
serviceable, but the number thus destroyed is probably so small as to be
of little practical importance.

The dragon-flies (Figs. 89, 90, 91) or mosquito hawks have long been
known as great enemies of mosquitoes, and they certainly do destroy many
of them as they are hawking about places where mosquitoes abound. Dr.
J.B. Smith of New Jersey very much doubts their efficiency, but
observations made by other scientific men would seem to indicate that
they often devour large numbers of mosquitoes during the course of the
day and evening.

Spiders and toads destroy a few mosquitoes each night. Certain external
and internal parasites destroy a few more, but the sum total of all of
these agencies is probably not very considerable, for while the adults
may have several natural enemies they are not of sufficient importance
to have any appreciable effect on the number of mosquitoes in a badly
infested region.

ENEMIES OF THE LARVÆ AND PUPÆ

The larvæ and pupæ on the other hand have many important enemies. Indeed
under favorable conditions these may keep small ponds or lakes quite
free from the pests. The predaceous aquatic larvæ of many insects feed
freely on wrigglers. The larvæ of the diving beetles which are known as
water-tigers are particularly ferocious and will soon destroy all the
wrigglers in ponds where they are present (Fig. 92). Dragon-fly larvæ
also feed freely on mosquito larvæ. Whirligig beetles are said to be
particularly destructive to _Anopheles_ larvæ and many other insects
such as water-boatmen, back-swimmers, etc., feed on the larvæ of various
species. A few of these introduced into a breeding-jar with _Anopheles_
larvæ will soon destroy all of them, even the very young bugs attacking
larvæ much larger than themselves.

It is interesting to note that the larvæ of some mosquitoes are
themselves predaceous and feed freely on the other wrigglers that may
chance to be in the same locality.

[Illustration: FIG. 90--The young (nymph) of a dragon-fly. (From
Kellogg's Amer. Insects.)]

[Illustration: FIG. 91--The cast skin (exuvæ) of a dragon-fly nymph.]

[Illustration: FIG. 92--Diving-beetles and back-swimmers. (From
Kellogg's Amer. Insects.)]

Various species of fish are, however, the most important enemies of the
mosquitoes. Great schools of tide-water minnows (Fig. 93) are often
carried over the low salt-marshes by the extreme high-tides and left in
the hundreds of tide pools as the tide recedes. No mosquitoes can breed
in a pool thus stocked with these fish. In the fresh-water streams and
lakes there are several species of the top-minnows, sticklebacks (Fig.
94), etc., that feed voraciously on mosquito larvæ and unless the grass
or reeds prevent the fish from getting to all parts of the ponds or
lakes very few mosquitoes can breed in places where they are present.

Minute red mites such as attack the house-flies and other insects
sometimes attack adult mosquitoes, but they are rarely very abundant.
Parasitic roundworms attack certain species. Others suffer more or less
from the attacks of various Sporozoan parasites.

FIGHTING MOSQUITOES

When mosquitoes are bothering us we usually begin by trying to kill the
individual pests that are nearest to us. We try to crush them if they
bite us; we screen the doors and windows to keep them from the house. In
warmer countries the people are a little more hospitable and do not
screen the mosquitoes out of the house entirely, but screen the beds for
protection at night, and if the mosquitoes get too insistent during the
day the bed makes a safe and comfortable retreat. All the mosquitoes in
a room may be killed by fumigating with sulphur at the rate of two
pounds to the thousand cubic feet of air-space. Pyrethrum is also used
largely, but it only stupefies the mosquitoes temporarily instead of
killing them. While in that condition they may be swept up and
destroyed.

Various substances are sometimes used as repellants by those who must be
in regions where the mosquitoes are abundant. With many of these,
however, "the cure is worse than the disease." Smudges are often built
to the windward of a house or barn-yard and the smoke from a good
smoldering fire will keep a considerable area quite free from
mosquitoes. The man who can keep himself enveloped in a cloud of tobacco
smoke will not be bothered by mosquitoes. Oil of pennyroyal, oil of tar
or a mixture of these with olive oil, and various other concoctions are
sometimes smeared over the face and hands. These will furnish protection
as long as they last. Dr. Smith says that he has found oil of citronella
quite effective and of course less objectionable than the other things
usually used. Care should be taken not to get it in the eyes. An
ointment made of cedar oil, one ounce; oil of citronella, two ounces;
spirits of camphor, two ounces, is said to make a good repellant and is
effective for a long time.

[Illustration: FIG. 93--Killifish (_Fundulus heteroliatus_). (From
Bull. 47, U.S. Fish Com.)]

[Illustration: FIG. 94--Stickleback (_Apeltes quadracus_). (From Bull.
47, U.S. Fish Com.)]

[Illustration: FIG. 95--An old watering trough, an excellent
breeding-place for mosquitoes.]

FIGHTING THE LARVÆ

All of the efforts directed against the adult mosquitoes are usually of
little avail in decreasing the number in any region. It is comparatively
easy, however, to fight them successfully in the larval stage. We have
seen that standing water is absolutely necessary for mosquitoes to breed
in. This makes the problem much simpler than if they could breed in any
moist places such as well-sprinkled lawns, a shady part of the garden,
etc. The whole problem of successful campaigns against the mosquitoes
resolves itself into the problem of finding and destroying or properly
treating their breeding-places. We have seen how certain kinds, such as
the yellow fever mosquito, are "domestic" species. They never go far
from their breeding-places. If a house is infected by one of these
species the immediate premises should be searched for the source.
Cisterns, rain-barrels, sewer-traps, cesspools, tubs or buckets of water
or old tin cans in out-of-the-way corners, are all suitable places for
them to breed in. Cisterns and rain-barrels should be thoroughly
screened so that no mosquitoes can get in or out, or the surface should
be covered with a film of kerosene which will kill all the larvæ in the
water when they come to the surface to breathe, and will also kill the
females when they come to deposit their eggs. The vent to open cesspools
should be thoroughly screened or the surface of the water kept well
covered with oil. Water standing in any vessels in the yards should be
emptied every week or ten days and the old tin cans destroyed or hauled
away. In fighting these domestic species you need be concerned only with
your own yard and that of your near-by neighbors. Other species, while
also rather local in their distribution, fly much farther than the
really domestic ones. In fighting these the region for a considerable
distance around must be taken into consideration. Watering-troughs (Fig.
95) that are left filled from week to week, the overflow from such
places, and the tracks made in the mud round about them (Fig. 96), small
sluggish streams, irrigating ditches, and small ponds or lakes not
supplied with fish are excellent breeding-places for several species of
mosquitoes including _Anopheles_ and others. The remedy at once suggests
itself. The watering-trough can be emptied and renewed every week during
the summer time, the overflow can be taken care of in a ditch that will
lead it away from the trough to where it will sink into the ground, the
banks of the streams or ponds or lakes can be cleared in such a way that
fish can get to all parts of the water; most of the small ponds can be
drained or their surface may be covered over with a thin film of
kerosene. This is best applied as a spray; one ounce to fifteen square
feet will suffice. If the oil is simply poured over the surface more
will be required.

The fighting of the species that breed on the extensive salt-marshes in
many regions is a larger and more difficult problem, but as it is a
matter that usually concerns large communities, sometimes whole states,
it can be dealt with on a larger scale. The very excellent results that
have been accomplished in New Jersey and on the San Francisco peninsula,
and in a smaller way in other places, show what may be done if the
community goes about the fight in an intelligent manner. In the fight in
New Jersey hundreds of acres of tide-lands have been drained so that
they no longer have tide pools standing where the mosquitoes may breed.
When it is impracticable to drain them the pools may be sprayed
occasionally with kerosene.

The value of the land that is reclaimed by a good system of draining is
often enough to pay many times over the cost of draining, thus the
mosquitoes are gotten rid of and the land enhanced in value by a single
operation.

CHAPTER VII

MOSQUITOES AND MALARIA

Ever since the beginning of history we have records of certain fevers
that have been called by different names according to the people that
were affected. As we study these names and the various writings
concerning the fevers we find that a great group of the most important
of them are what we to-day know as malarial fevers. Not only are these
ills as old as history but they have been observed over almost the
entire inhabited earth. There are certain regions in all countries where
malaria does not occur, but almost always it will be found that other
regions near by are infected and it very often happens that these
infected regions are the most profitable parts of the land, the places
where water is plentiful and vegetation is luxuriant. Indeed the
coincidence of these two things, low-lying lands with an abundance of
water, particularly standing water, and malaria has always been noted
and gave rise to the earliest theories in regard to the cause of the
disease.

For instance, we find some of the very early writers emphasizing the
point that swampy localities should be avoided for they produce animals
that give rise to disease, or that the air is poisoned by the breath of
the swamp-inhabiting animals.

These views of the origin of the fever prevailed until about the
beginning of the eighteenth century when the recently discovered
microscope began to reveal the various kinds of animalculæ to be found
in decaying material.

In 1718 Lancisi held that the myriads of insects, particularly gnats or
mosquitoes, that arose from such swampy regions might carry some of
these poisonous substances and by means of their proboscis introduce
them into the bodies of the people, and although he had made no
experiments to test the assumption he did not consider it impossible
that such insects might also introduce the smallest animalculæ into the
blood. It took almost two centuries of study and investigation before
this guess was proved to be right.

One reason why the mosquitoes were not earlier associated with these
diseases was that all who investigated the matter at all turned their
attention to the bad condition of the air in these swampy regions.
Malaria means bad air. We all know that we can see the mists arising
from such regions, particularly in the evening or at night, and as
exposure to these mists very often meant an attack of malaria they were
naturally supposed to be the cause of the disease. So for a long time
the whole attention of investigators was turned toward studying and
analyzing these vapors, and various experiments were made which seemed
to show conclusively that the malaria was caused only by these
emanations. The investigations even went so far that the exact germs
that were supposed to cause the fever were separated and experimented
with.

THE PARASITE THAT CAUSES MALARIA

The blood had been studied time and again and the characteristic
appearance of the blood of a malarial patient was well known. In 1880
Laveran, a French army surgeon in Algiers, began to study the blood of
such patients microscopically and soon was able to demonstrate the
parasite that caused the disease. His discoveries were not readily
accepted, but other investigations soon confirmed his observations and
the fact was gradually firmly established. Not until recently, however,
did this distinguished physician receive a full recognition of his work.
A few years ago he was awarded the Nobel prize for medicine, perhaps the
highest honor that can be bestowed on any physician. It is interesting,
too, to note in this connection that it was another French surgeon who
in 1840 discovered that sulphate of quinine is a specific for malaria.

[Illustration: FIG. 96--Horse and cattle tracks in mud filled with
water; good breeding-places for Anopheles.]

[Illustration: FIG. 97--A malarial mosquito (_Anopheles maculipennis_);
male.]

[Illustration: FIG. 98--A malarial mosquito (_A. maculipennis_);
female.]

The next important step was made in 1885 by Golgi, an Italian, who
studied the life-history of the parasite in the blood and distinguished
the three forms which cause the three most familiar kinds of malarial
fevers, the tertian, the quartan and the remittent types. From this time
on this parasite has been studied by physicians of many nationalities
and the whole course of its life-history worked out. In order that we
may understand how it was that mosquitoes were determined to be the
means of disseminating this parasite we will discuss first its
life-history in the human blood.

The parasites that cause the malarial fevers are Sporozoans and belong
to the genus _Plasmodium_. Other names such as _Hæmamoeba_ and
_Laverania_ have been used for them, but the term _Plasmodium_ is the
one now most commonly employed. The three most common species are
_vivax_, _malariæ_ and _falciparum_, causing respectively the tertian,
quartan and remittent fevers.

LIFE-HISTORY OF PARASITE

The life-history of all of these is very similar, the principal
difference being in the length of time it takes them to sporulate. Let
us begin with the parasite after it has been introduced into the blood
and trace its development there. At first it is slender and rod-like in
shape. It has some power of movement in the blood-plasm. Very soon it
attacks one of the red blood-corpuscles and gradually pierces its way
through the wall and into the corpuscle substance (Fig. 99); here it
becomes more amoeboid and continues to move about, feeding all the
time on the corpuscle substance, gradually destroying the whole cell. As
the parasite feeds and grows there is deposited within its body a
blackish or brownish pigment known as melanin.

During the time that the parasite is feeding and growing it is also
giving off waste products, as all living forms do in the process of
metabolism, but as the parasite is completely inclosed in the corpuscle
wall these waste products cannot escape until the wall bursts open.
After about forty hours if the parasite is _vivax_ or about sixty-five
hours if it is _malariæ_ it becomes immobile, the nucleus divides again
and again and the protoplasm collects around these nuclei, forming a
number of small cells or spores, as they are called. In about
forty-eight or seventy-two hours, depending on whether the parasite is
_vivax_ or _malariæ_ the wall of the corpuscle bursts and all these
spores with the black pigment and the waste products that have been
stored away within the cell are liberated into the blood-plasm.

[Illustration: FIG. 99--Diagram to illustrate the life-history of the
malarial parasite. 1 is a red blood-corpuscle, 2 to 7 shows the
development of the parasite in the corpuscle, _a_ _b_ _c_ _d_ and _a´_
_b´_ _c´_ and _e_ the development of the parasite in the stomach of the
mosquito, _f_ _g_ _h_ _i_ the development in the capsule on the outer
wall of the stomach of the mosquito, _k_ in the salivary gland.]

[Illustration: FIG. 100--Malarial mosquito (_A. maculipennis_) on the
wall.]

[Illustration: FIG. 101--Malarial mosquito (_A. maculipennis_) standing
on a table.]

These spores are round or somewhat amoeboid and are carried in the
blood for a short time. Very soon, however, each one attacks a new red
corpuscle and the process of feeding, growth and spore-formation
continues, taking exactly the same time for development as in the first
generation, so every forty-eight hours in the case of the _vivax_, and
every seventy-two hours in the case of the _malariæ_ a new lot of these
spores and the accompanying waste products are thrown out into the
blood. Thus in a very short time many generations of this parasite occur
and thousands or hundreds of thousands of the red-blood corpuscles are
destroyed, leaving the patient weak and anemic. It will be seen, too,
that the recurrence of the chills and fevers is simultaneous with the
escaping of the parasites from the blood-corpuscles, together with the
waste products of their metabolism.

These waste products are poisonous, and it is believed that this great
amount of poison poured into the blood at one time causes the regular
recurring crisis. Zoölogists well know that this process of asexual
reproduction, _i. e._, reproduction without any conjugation of two
different cells, cannot go on indefinitely, and those who were studying
the life-cycle of these parasites were at a loss to know where the
sexual stage took place. In the meantime studies of other parasites more
or less closely related to _Plasmodium_ showed that the sexual stage
occurred outside the vertebrate host. The remarkable work of Dr. Smith
on the life-history of the germ that causes the Texas fever of cattle
had a strong influence in directing the search for this other stage of
the malarial parasite. Another thing that indicated that this sexual
generation must take place outside the body of the vertebrate host was
the fact that the investigators found that the parasites in certain of
the cells did not sporulate as did the others. When these individuals
were drawn from the circulation and placed on a slide for study it was
found that they would swell up and free themselves from the inclosing
corpuscle and some of them would emit long filaments which would dart
away among the corpuscles.

Many men have worked on this problem, but perhaps the most credit for
its solution will always be given to Sir Patrick Manson, the foremost
authority on tropical diseases, and to Ronald Ross, a surgeon in the
English army. There is no more interesting and inspiring reading than
that which deals with the development of the hypothesis by Manson and
the persistent faith of Ross in the correctness of this theory, and his
continuous indefatigable labors in trying to demonstrate it. It was an
important piece of scientific work, and shows what a man can do even
when the obstacles seem insurmountable.

THE PARASITE IN THE MOSQUITO

Briefly stated again, the problem was this: We have here a parasite in
the blood which behaves as do many other forms of life. Some of these
parasites do not go on with their development until they are removed
from the circulation. Now, how are they thus removed from the
circulation under normal conditions? This must first be solved before
the still greater and more important problem of how the parasite gets
from one human host to another can be taken up. In studying this over
Manson reasoned that certain suctorial insects were the agencies through
which blood was most commonly removed from the circulation and he
ventured the guess that this change in the parasite that may be seen
taking place on the slide under the microscope, normally takes place in
the stomach of some insect that sucks man's blood. Ross was greatly
impressed with the theory and began his long and apparently hopeless
task of finding these parasites in the stomach of some insect. When we
remember that they are so minute that they can only be seen by the use
of the highest power of the microscope we can realize something of the
magnitude of the task. Ross, who was at that time stationed in India,
selected the mosquito as the most likely of the insects to be the host
that he was looking for. For over two and one-half years he worked with
entirely negative results, for after examining thoroughly many thousands
of mosquitoes he found no trace of the parasite.

Practically all his work was done on the most common mosquito of the
region, a species of _Culex_. But one day a friend sent him a different
mosquito, one with spotted wings, and in examining it he was interested
to note certain oval or round nodules on the outer walls of the stomach.
On closer examinations he found that each of these nodules contained a
few granules of the coal-black melanin of malarial fever. Further
studies and experiments showed that these particular cells could always
be found in the walls of the stomach of this particular species of
mosquito a few days after it had bitten a malarial patient. This
epoch-making discovery was made in 1898. Ross was detailed by the
English government to devote his whole time to the further solution of
the problem, and after two years more of careful experimentation and
study was able to give a complete life-history of this parasite. His
experiments have been repeated many times, and the conclusions he
arrived at are as undeniable as any of the known facts of science.

The whole life-history as we now know it can be summed up as follows:
The parasites develop within the circulation but certain of them seem to
wander about and do not go on with their development there. When these
particular parasites are taken into the stomach of most mosquitoes they
are digested with the rest of the blood. But when they are taken into
the stomach of a mosquito belonging to the genus _Anopheles_ or other
closely related genera they are not digested but go on with their
development, conjugation and fertilization taking place, resulting in a
more elongated form which makes its way through the walls of the stomach
on the outside of which are formed the little nodules discovered by Ross
on his mosquitoes. Within these nodules further division and development
takes place until finally the nodule is burst open and many thousand
minute rod-like organisms, sporozoites, are turned loose into the
body-cavity of the mosquito. Owing to some unknown cause these little
organisms are gathered together in the large vacuolated cells of the
salivary glands of the mosquito, and when the mosquito bites a man or
any other animal they pour down through the ducts with the secretion and
are thus again introduced in the circulation.

The nodules or cysts on the walls of the stomach of the mosquito may
contain as many as ten thousand sporozoites, and as many as five hundred
cysts may occur on a single stomach.

It takes ten, twelve or more days from the time the parasites are taken
into the stomach of the mosquito before they can go through their
transformations and reach the salivary gland, the time depending on the
temperature. So it is ten or twelve days or sometimes as much as
eighteen or twenty days from the time an _Anopheles_ bites a malarial
patient before it is dangerous or can spread the disease. On the other
hand, the sporozoites may lie in the salivary gland alive and virulent
for several weeks. It does not give up all the parasites at one time, so
that three or four or more people may be affected by a single mosquito.

It is well known that two parasites may often be seen in the same
corpuscle. This is often simply a case of multiple infection, but Dr.
Craig has very recently shown that under certain conditions two
individuals may enter the same corpuscle and conjugate and the resulting
individual will be resistant to quinine and may remain latent in the
spleen or bone marrow for a long time. Under favorable conditions it
may again begin the process of multiplication and the patient will
suffer a relapse.

SUMMARY

Now let us sum up some of the reasons why we believe that the malaria
fever can be transmitted only through the agency of mosquitoes. First,
we know the life-history of the parasite, it has been studied in both of
its hosts. Attempts have been made to rear it in other hosts but without
avail, and we know from the general relations of the parasite that it
must have this sexual as well as the asexual generations. Second, in
some regions which would seem to be malarial, that is, where the
miasmatic mists arise, no malaria occurs. Why? Usually it can be
definitely shown that no _Anopheles_ occur there. Other mosquitoes may
be there in abundance, but if no _Anopheles_, there is no malaria. In
certain regions this is well demonstrated. The west coast of Africa is
one of the worst pest-holes of malaria and _Anopheles_. The east coast
has no malaria and no _Anopheles_. In many islands the same condition
exists. On the other hand, the Fiji Islands have _Anopheles_ but no
malaria. No malaria has ever been introduced there to infect the
mosquitoes. In the same way _Stegomyia_ occurs in some of the South Sea
islands and yet there is no yellow fever there.

EXPERIMENTS

We may review, too, a few of the classic experiments that have served to
show that malaria can be contracted in no other way than through the
bite of the mosquito.

For many years Grassi, an Italian, devoted almost his whole time to the
study of malaria. In 1900 he received permission from the government to
experiment on the employees of a piece of railroad that was being built
through a malarial region. This was divided for the purpose of the
experiment into three sections, a protected zone in the middle and an
unprotected zone at each end.

Those working in the protected zone had their houses completely screened
and no one was allowed out of doors after sunset except they were
protected with veils and gloves. Early in the season they were all given
doses of quinine to prevent auto-infection. In the unprotected zone no
screens were used and every one was allowed to go without special
protection. The result for the summer was that there were no new cases
of fever in the protected zone. In the unprotected zones practically all
had the fever as usual.

[Illustration: FIG. 102--Salt-marsh mosquito (_O. lativittatus_)
standing on a table.]

[Illustration: FIG. 103--Anopheles hanging from the ceiling.]

In the same year two English physicians, Sambon and Low, went to Italy
where they built a cabin in one of the marshes noted as being a malaria
pest-hole. The house was thoroughly screened so that no mosquitoes could
enter, but the windows were always open so as to admit the air freely
day and night. Here they lived for three months, out of doors as much as
they pleased during the day but inside where they were protected from
the mosquitoes at night. No quinine was used and no fever developed,
although all about them other people were having the fever as usual.

Another English physician who had not been in malarial regions allowed
himself to be bitten by infected mosquitoes sent from a malarial
locality. In due time he developed the fever. Many other experiments
made in various places might be cited. The results have all been
practically the same. To-day the soldiers of many civilized nations are
required to protect themselves from mosquitoes because it has been found
that it pays. Disease has always been a worse terror than bullets in any
war, and we are fast learning that the great loss from diseases
heretofore considered unavoidable may be very largely eliminated by
proper sanitary arrangements and protection from noxious insects.

CHAPTER VIII

MOSQUITOES AND YELLOW FEVER

Yellow fever is a disease, principally of seaport towns, from which the
United States has suffered more than any other country. It is endemic
only in tropical regions but is often carried to subtropical, sometimes
even to temperate zones where, if the proper mosquitoes exist, it may
rage until frost.

Vera Cruz, Havana, Rio de Janeiro, and the west coast of Africa were
long regarded as permanent endemic foci, the disease appearing there in
epidemic form from time to time, often spreading to other ports in more
or less close communication with such places. In the United States the
Gulf states have been the greatest sufferers from the disease, although
it has spread as far as Baltimore, Philadelphia and Washington, where at
rare intervals it was most serious, abating its ravages only when frost
came.

The last severe outbreak occurred in New Orleans in 1905 when eight
thousand cases and nine hundred deaths occurred. At that time there was
waged one of the most remarkable warfares against death in its most
terrifying form that the world has ever known. And, thanks to the
achievements of science, particularly to the investigations of three
men, one of whom gave his life to the cause, the fight was successful
and this dreadful outbreak was checked just at the time when according
to all precedent it should have been at its height.

This result which at other times and under other conditions would have
been considered miraculous was achieved not by the usual custom of
isolation, quarantine, etc., but by a direct, we may almost say hand to
hand, conflict with mosquitoes: the mosquitoes belonging to a particular
genus and species, _Stegomyia calopus_ (_fasciata_).

Before taking up a discussion of this achievement in New Orleans let us
consider first the work of the men that made such results possible.

For many years the cause and methods of dissemination of this disease
had been a puzzle to physicians and scientists. Very early it was
believed that it might be transmitted through the air, and the fact that
infection usually occurred in the vicinity of the water and in the
tropics or in midsummer led to the belief that the disease was due to
fermentation. This theory received strong support in the fact that
serious outbreaks of the fever often followed the coming into port of
vessels from the tropics with the water in their holds in an offensive
condition. When it was discovered that bacteria were the cause of
fermentation and also of many diseases this theory was considered
abundantly proven. From time to time, announcements have been made that
the particular species of bacteria that causes the disease has been
isolated, but there has always been something lacking in the final
proof.

Yellow fever has always been regarded as a very highly contagious as
well as infectious disease, and the utmost precaution has been taken to
isolate the patients when possible and in recent years strict
quarantines have been established against infected localities and no
person or commerce or even the mails were allowed to come from such
places without thorough fumigations. But all these things proved
unsatisfactory. The disease could not ordinarily be checked by simply
isolating the patients. Many people became sick without ever having been
near a yellow fever patient, while others worked in direct daily contact
with the disease and did not suffer from it. Those who had once had it
and recovered became practically immune, rarely suffering from a second
attack. Negroes may suffer from the disease, but are usually regarded as
practically immune.

[Illustration: FIG. 104--Yellow-fever mosquito (_Stegomyia calopus_).
(R. Newstead, del.)]

It was early observed, too, that the danger zone might be quite well
defined and that outside this zone one would be safe. More than a
century ago the British troops and other inhabitants of Jamaica found
that by retreating to the mountains during the warm weather the
non-immunes could escape the fever. It was also observed that those who
slept on the first floor were more apt to take the disease than those on
the second floor.

THE YELLOW FEVER COMMISSION

In 1900, during the American occupation of Cuba, yellow fever became
very prevalent there. A board of medical officers was ordered to meet in
Havana for the purpose of studying the disease under the favorable
opportunities thus afforded. This board, which came to be known as the
Yellow Fever Commission, was composed of Drs. Walter Reed, James
Carroll, Jessie W. Lazear and Aristides Agramonte of the United States
Army. Agramonte was a Cuban and an immune, the others were non-immunes.
Dr. Manson in his lectures on Tropical Medicines says of them:

"I cannot pass on, however, to what I have to say in connection
with this work without a word of admiration for the insight, the
energy, the skill, the courage, and withal the modesty and
simplicity of the leader of that remarkable band of workers. If any
man deserved a monument to his memory, it was Reed. If any band of
men deserve recognition at the hands of their countrymen, it is
Reed's colleagues."

Their first work was to determine whether any of the germs that had been
claimed to be the cause of yellow fever were really responsible for the
disease. _Bacillus icteroides_ that for some time and by some
investigators had been named as the offender was particularly
investigated, but was proved to be a secondary invader only.

Dr. Charles Finlay of Havana had been claiming for some years that the
yellow fever was transmitted by means of the mosquito and possibly by
other insects also. He even claimed to have proved this theory
experimentally. We know now, however, that there must have been errors
in his experiments and that his patients became infected from sources
other than those he was dealing with.

The Yellow Fever Commission decided to put this theory to the test and
secured a number of volunteers for the experiments. The first thing was
to let an infected mosquito bite some non-immune person. How this was
done and the results, may be told in Dr. Carroll's own words.

EXPERIMENTS

"Two separate lines of work now presented: one, the study of the
bacterial flora of the intestine and anaërobic cultures from the
blood and various organs; the other, the theory of the transmission
of the disease by the mosquito, which had been advanced by Dr.
Carlos Finlay in 1881. After due consideration it was decided to
investigate the latter first. Then arose the question of the
tremendous responsibility involved in the use of human beings for
experimental purposes. It was concluded that the results
themselves, if positive, would be sufficient justification of the
undertaking. It was suggested that we subject ourselves to the same
risk and this suggestion was accepted by Dr. Reed and Dr. Lazear.
It became necessary for Dr. Reed to return to the United States and
the work was begun by Dr. Lazear, who applied infected mosquitoes
to a number of persons, himself included, without result. On the
afternoon of July 27, 1900, I submitted myself to the bite of an
infected mosquito applied by Dr. Lazear. The insect had been reared
and hatched in the laboratory, had been caused to feed upon four
cases of yellow fever, two of them severe, and two mild. The first
patient, a severe case, was bitten twelve days before; the second,
third and fourth patients had been bitten six, four and two days
previously, and were in character mild, severe and mild
respectively. In writing to Dr. Reed that night of the incident, I
remarked jokingly that if there was anything in the mosquito
theory, I should have a good dose. And so it happened. After having
slight premonitory symptoms for two days, I was taken sick on
August 31, and on September 1, I was carried to the yellow fever
camp. My life was in the balance for three days, and my chart shows
that on the fifth, sixth and seventh days my urine contained
eighth-tenths and nine-tenths of moist albumin. On the day I was
taken sick, August 31, 1900, Dr. Lazear applied the same mosquito,
with three others, to another individual who suffered a
comparatively mild attack and was well before I had left my bed. It
so happened that I was the first person in whom the mosquito was
proved to convey the disease.

"On the eighteenth of September, five days after I was permitted to
leave my bed, Dr. Lazear was stricken, and died in convulsions just
one week later, after several days of delirium with black vomit.
Such is yellow fever.

"He was bitten by a stray mosquito while applying the other insects
to a patient in one of the city hospitals. He did not recognize it
as a _Stegomyia_, and thought it was a _Culex_. It was permitted to
take its fill and he attached no importance to the bite until after
he was taken sick, when he related the incident to me. I shall
never forget the expression of alarm in his eyes when I last saw
him alive in the third or fourth day of his illness. The spasmodic
contractions of his diaphragm indicated that black vomit was
impending, and he fully appreciated their significance. The
dreaded vomit soon appeared. I was too weak to see him again in
that condition, and there was nothing that I could do to help him.

"Dr. Lazear left a wife and two young children, one of whom he had
never seen."

These experiments and many others like them conducted on soldiers and
Spanish immigrants proved that this particular mosquito would transmit
the disease under certain conditions.

1. The mosquito must bite the patient during the first three days of the
fever; after that a yellow fever patient cannot infect a mosquito.

2. A period of twelve days must elapse before the mosquito is able to
infect another person. After that she may infect anyone she may bite;
that is, the germs remain virulent during the rest of the mosquito's
life. The French Yellow Fever Commission working in Rio de Janeiro claim
that the first generation of offspring from such an infected mosquito is
capable of causing the disease after they are fourteen days in the adult
condition.

The next step was to ascertain whether the disease could be contracted
in any other way than by the bites of infected mosquitoes. A camp named
Camp Lazear was established and the following tests made: A
mosquito-proof building of one room was completely divided by a wire
screen from floor to ceiling. In one room fifteen mosquitoes that had
previously bitten yellow fever patients and had undergone the proper
period of incubation were liberated. In this room a non-immune exposed
himself so that he was bitten by several of the insects. A little later
the same day and again the next day the mosquitoes were allowed to feed
on him for a few minutes. Five days later, the usual incubation period,
he developed yellow fever.

At the same time that he entered the building two other non-immunes
entered the other compartment where they slept for eighteen nights
separated from the mosquitoes by the wire screen. They showed no signs
of taking the fever.

In another mosquito-proof house two soldiers and a surgeon, all
non-immunes, lived for twenty-one days. From time to time they were
supplied with soiled articles of bedding, clothing, etc., direct from
the yellow fever hospital in the city. These articles had been soiled by
the urine, fecal matter and black vomit obtained from fatal and other
cases of yellow fever. These articles were handled and shaken daily, but
no disease developed among the men and at the end of the twenty-one
days, two other non-immunes relieved them and handled a new supply of
clothing in the same way, sleeping between the same sheets that had
been used by a patient dying of yellow fever and exposing themselves in
every possible way to the soiled clothing. But no disease developed.
That these men were susceptible was shown later by inoculating some of
them, when they developed the disease.

In another experiment certain men in a camp allowed themselves to be
bitten by mosquitoes that had passed through the proper period of
incubation and every one of them and no others contracted the disease.
It was also shown that a mosquito was capable of communicating the
disease as long as fifty-seven days after it had bitten a yellow fever
patient. Another set of experiments showed that a subcutaneous injection
into a non-immune of a very small quantity of blood from the veins of a
yellow fever patient in the first two or three days of the disease would
produce the fever.

SUMMARY OF RESULTS

Since that time much other work has been done by independent workers as
well as by French and English Commissions both working at Rio de
Janeiro. The results of their investigation are practically the same and
may be summed up as follows:

1. The virus of the yellow fever is in the blood-plasma, not in the
corpuscles, for these may be removed and the plasma still be infective.

2. The virus is conveyed from one patient to another by the yellow fever
mosquito, _Stegomyia calopus_, and in no other way except by
experimental injections.

3. The patient is a source of infection only during the first three or
four days of the disease (this after the three to six days of
incubation).

4. The virus must undergo an incubation period of twelve to fourteen
days in the mosquito before she is capable of transmitting the disease.

5. The parasite, whatever it is, has never been seen. It is probably too
small to be seen by any of our present microscopes, even the recently
invented ultramicroscope. It is probably not a bacterial parasite but
very likely a Protozoan, and certain specialists have even shown by the
study of all the available data that it almost certainly belongs to the
Sporozoan genus _Spirocheta_.

Now what does all this mean? It means the saving of hundreds of human
lives annually. It means the banishing from many localities and possibly
very soon from the face of the earth of a disease that since the
earliest settlements on this continent has been a source of terror. It
means the making habitable of certain places which heretofore a white
man has entered only at the risk of his life. It means that quarantines
need no longer be established when yellow fever breaks out in a
district; quarantines which have inevitably caused the loss of millions
of dollars to the world of commerce.

RESULTS IN HAVANA

The first practical work based on these findings was done in Havana. The
Yellow Fever Commission made their recommendations in 1900. In 1901 and
1902 they were put into effect. The following table of the death rate
there during a period of ten years shows graphically the results:

DEATHS IN HAVANA FROM YELLOW FEVER

-----+------+------+------+------+------+------+------+------+------+------
| 1893 | 1894 | 1895 | 1896 | 1897 | 1898 | 1899 | 1900 | 1901 | 1902
-----+------+------+------+------+------+------+------+------+------+------
Jan. | 15 | 7 | 15 | 10 | 69 | 7 | 1 | 8 | 7 | 0
Feb. | 6 | 4 | 4 | 7 | 24 | 1 | 0 | 9 | 5 | 0
Mar. | 4 | 2 | 2 | 3 | 30 | 2 | 1 | 4 | 1 | 0
Apr. | 8 | 4 | 6 | 14 | 71 | 1 | 2 | 0 | 0 | 0
May | 23 | 16 | 10 | 27 | 88 | 4 | 0 | 2 | 0 | 0
June | 69 | 31 | 16 | 46 | 174 | 3 | 1 | 8 | 0 | 0
July | 118 | 77 | 88 | 116 | 168 | 16 | 2 | 30 | 1 | 0
Aug. | 100 | 73 | 120 | 262 | 102 | 16 | 13 | 49 | 2 | 0
Sep. | 68 | 76 | 135 | 166 | 56 | 34 | 18 | 52 | 2 | 0
Oct. | 46 | 40 | 102 | 240 | 42 | 26 | 25 | 74 | 0 | 0
Nov. | 28 | 23 | 35 | 244 | 26 | 13 | 18 | 54 | 0 | 0
Dec. | 11 | 29 | 20 | 147 | 8 | 13 | 22 | 20 | 0 | 0
-----+------+------+------+------+------+------+------+------+------+------

As long as the United States held control at Havana the yellow fever
was kept in check by fighting the mosquitoes, when this vigilance was
relaxed the fever began to appear again and the Cubans found that it was
necessary to keep up the fight against the mosquitoes if the island was
to be kept free from the disease.

THE FIGHT IN NEW ORLEANS

In the summer of 1905 came another opportunity to put the knowledge
gained during these experiments to a practical test. Samuel Hopkins
Adams in his article in _McClure's Magazine_, June, 1906, says of the
beginning of this fight:

"Eight years before, the mosquito-plague had infected the great,
busy, joyous metropolis of the south. Ignorant of the real
processes of the infection, New Orleans had fought it blindly,
frantically, in an agony of panic, and when at last the frost put
an end to the helpless city's plight, she lay spent and prostrate.
The yellow fever of 1905 came with a more formidable and unexpected
suddenness than that of 1897. It sprang into life like a secret and
armed uprising in the midst of the city, full-fledged and terrible.
But there arose against it the trained fighting line of scientific
knowledge. Accepting, with a fine courage of faith that most
important preventive discovery since vaccination, the mosquito
dogma, the Crescent City marshaled her defenses. This time there
was no panic, no mob-rule of terrified thousands, no mad rushing
from stunned inertia to wildly impractical action; but instead the
enlistment of the whole city in an army of sanitation. Every
citizen became a soldier of the public health. And when, long
before the plague-killing frost came, the battle was over, New
Orleans had triumphed not only in the most brilliant hygienic
victory ever achieved in America, but in a principle for which the
whole nation owes her a debt of gratitude."

For some time the authorities had been trying to keep secret the fact
that the disease was prevalent, but the rapidity with which it spread
made them realize that only united action on the part of all the
community would be of any avail. The Citizens Volunteer Ward
Organizations were organized for the purpose of fighting the mosquitoes
which were everywhere. To many the fight looked hopeless. The miles of
open gutters, the thousands of cisterns and little pools of standing
water everywhere furnished abundant breeding-places for the mosquitoes.
The ditches and ponds were drained or salted, the cisterns were
screened, infected houses were fumigated, yet the fever continued to
spread. Rains refilled the ditches, winds tore the screens from the
cisterns, the ignorant people of the French quarter refused to
coöperate. At last the city in desperation appealed to the President for
aid. Surgeon J.H. White and a number of officers and men of the United
States Public Health and Marine Hospital Service soon took charge of the
work. This was continued along the same lines as before with the same
object in view. But with the coming of the regulars the work was more
systematically and thoroughly done. Every case of fever was treated as
though it was yellow fever and every precaution taken to prevent
mosquitoes from biting such a patient. The houses in which the fever
occurred were thoroughly fumigated to kill any mosquitoes that might be
there, and the neighborhood was thoroughly searched to find any places
where the mosquitoes might be breeding. So confident were the
authorities that the mosquito was the sole cause of the disease
spreading, that besides fighting it no other work was undertaken save to
make the sick as comfortable as possible.

Finally the results began to be apparent. The number of cases gradually
diminished, until long before frost came the city was free from the
great pest. Yellow fever will doubtless appear from time to time in New
Orleans and other cities, but there is, at least there should be, small
danger of another great epidemic, for the people now know how the
disease is caused and the remedy.

Not long since I had occasion to write to a prominent entomologist in
Louisiana for some specimens of the yellow fever mosquito for laboratory
work. The following extract from his reply will show something of the
work that is still being done there.

"I am afraid we cannot furnish specimens of _Stegomyia_, in spite
of the fact that Louisiana is _supposed_ to be the most favorable
home of this species in the South. Since the light occurrence of
yellow fever in this State in 1905, a very vigorous war has been
kept up against _Stegomyia_, and the ordinances of all Louisiana
cities and principal towns require the draining of all breeding
places of this mosquito and the constant oiling or screening of all
cisterns or other water containers. The result is this species is
very rare. Here in Baton Rouge I only see one once in a great
while, and it would require perhaps a good many days' work at the
present season to get as good specimens and as many of them as you
require."

IN THE PANAMA CANAL ZONE

Yellow fever was one of the worst obstacles that confronted the French
when they were attempting to build the Panama Canal. The story of the
suffering and death from this dread disease there is most pathetic.
Ship-load after ship-load of laborers were sent over, as those who had
gone earlier succumbed to the fever. The contractors were responsible
for their men while they were sick and in order to avoid having to pay
hospital expenses the men were often discharged as soon as they showed
signs of sickness. Many of them died along the roadside while
endeavoring to reach some place where they could obtain aid. The
hospitals were usually filled with yellow fever patients, a very large
percentage of whom died.

Not only the day laborers suffered but many of the engineers, doctors,
nurses and others sickened and died of the disease. It is reported that
eighteen young French engineers came over on one vessel and in a month
after their arrival all but one had died of the yellow fever. Out of
thirty-six nurses brought over at one time, twenty-four died of the
fever, and during one month nine members of the medical staff of one of
the hospitals succumbed.

One of the first things that the United States Government did in
beginning work in the canal zone was to take up the fight against the
yellow fever mosquito. In Panama where the water for domestic purposes
was kept in cisterns and water-barrels, inspectors were appointed to see
that all such receptacles and other possible breeding-places for
mosquitoes were kept covered. After the first inspection, 4,000
breeding-places were reported. About six months later there were less
than 400. Similar work was done in all the towns and settlements along
the route of the canal. In addition to this fight against the yellow
fever mosquito considerable attention was paid to the breeding-places of
the malarial mosquito. The results have been remarkable. Cases of yellow
fever are now rare throughout this zone, and there has been a very great
reduction in the extent of the malarial districts. The last case of
yellow fever occurred in May, 1906. Before this work was done a man took
his life in his hands when he went into this region. Now it is regarded
as a perfectly safe place to live. Indeed it is a much safer place than
many sections of our own country where proper sanitary measures have not
been taken to protect the health of the community.

IN RIO DE JANEIRO

In Rio de Janeiro they have as yet been unable to get rid of the
mosquitoes, although thousands of dollars are spent annually in fighting
them. But the non-immunes there protect themselves by doing their
business in Rio during the day and going back at night to Petropolis,
twenty-five miles inland and twenty-five hundred feet higher, where
they are safe, for no _Stegomyia_ have ever been found there.

They claim there that the yellow fever mosquito does not bite during the
daytime after she has laid her eggs, and that she will not lay her eggs
until about three days after she has fed on blood, therefore a
_Stegomyia_ that bites during the day will not carry the yellow fever
because she is too young. This seems to explain why the fever cannot be
contracted by being bitten by a mosquito in the daytime. Certain other
experiments, however, have given different results so that as far as we
know it is not safe to be bitten at any time by such a mosquito in a
region where the disease is endemic or where it is epidemic.

In the main the work of the French Yellow Fever Commission working in
Rio de Janeiro has confirmed the findings of the American Commission.
One interesting special thing that the French Commission seems to have
established is that the female may transmit the infecting power to her
offspring, so that it would be possible for a mosquito that had never
bitten a yellow fever patient to be capable of infecting a non-immune
person. While all this is very probable in the light of what we know of
the disease and the way in which other diseases caused by similar
organisms may be transmitted by the parent to the offspring, yet the
most conservative investigators are waiting for further proof.

HABITS OF STEGOMYIA

The whole fight against yellow fever, then is directed, as we have seen,
against the mosquito, _Stegomyia calopus_. The habits of this species
are such as to make it easy in some respects to combat. It is seldom
found far away from human habitation. The adults will not fly far. Once
in a house they usually stay there except when they leave to deposit
their eggs.

On the other hand, some of these same habits make it all the more
dangerous. It will breed in almost any kind of water, no matter how
filthy, and a very small amount will suffice. Thus any leaks from
water-pipes or drains, cisterns, small cans of water or any such places
may become dangerous breeding-places. If conditions are unfavorable
there will often be developed small individuals which can easily make
their way through ordinary mosquito-netting.

Dr. Manson has pointed out an interesting possible result of the crusade
that is now being waged against the yellow fever mosquitoes. The
immunity of the people native to the endemic regions is supposed to be
due to their having had mild attacks of the fever during childhood, for
the children in these regions are subject to certain fevers which are
probably very mild forms of yellow fever.

Now if we kill practically all of the _Stegomyia_ so that these children
do not have this fever there will be developed, in due time, a
population most of whom are non-immune.

This freedom from the disease for some time will allow us to grow
careless in regard to fighting the mosquitoes. They will be allowed to
increase and by some chance the yellow fever will again be introduced
and there will then be very grave danger of most extensive and
destructive epidemics.

DANGER OF THE DISEASE IN THE PACIFIC ISLANDS

I have already referred once or twice to the conditions in many of the
Pacific tropical islands. In some of these various species of
_Stegomyia_ are abundant, and in some _Stegomyia calopus_ is the most
abundant and troublesome form. All the natives of these islands are
non-immune because there has never been any yellow fever there. Unless
extraordinary care is taken the disease will be introduced there sooner
or later and the results are sure to be most appalling. The climatic and
sanitary conditions and the habits of the people are ideal for the
development and spread of the disease, and what I have seen of the
conditions on some of these islands convinces me that it would be almost
impossible to control the disease before it had a chance to kill a large
percentage of the population.

With the opening of the Panama Canal these things become more possible.
Heretofore, the shipping to these regions has not been from ports where
yellow fever was endemic or even likely to be epidemic. But unless the
yellow fever is kept out of the canal zone, the danger will be many fold
what it is now.

The white man has already carried enough misery to these island peoples
in the way of loathsome diseases, and it is to be hoped that this,
another great curse, will not be carried to them with our civilization,
the beneficial results of which have been so often very justly
questioned.

What I have said in regard to these islands applies with equal force and
in some instances with even greater force to parts of Asia, the Eastern
Archipelago and other places.

CHAPTER IX

FLEAS AND PLAGUE

Plague has always been one of the most dreaded diseases, and when we
read of its ravages in the old world and the utter helplessness of the
people before it we do not wonder that the very word filled them with
horror. One of the greatest scourges ever known began in Egypt about
A.D. 542, and spread along the shores of the Mediterranean to Europe and
Asia. It lasted for sixty years, appearing again and again in the same
place and decimating whole communities.

Another great pandemic, beginning in 1364, spread over the whole of the
then known world and appeared in its most virulent form. On account of
diffuse subcutaneous hemorrhages it came to be known as the "black
death" and of course spread terror in all the communities where it
appeared. Whole villages and districts were depopulated. The death-rate
was very high, one authority placing the total mortality at twenty-five
million.

During this time new centers of infection were established, and since
then it has been carried by the commerce of the nations to all parts of
the world. It is not restricted, as many other epidemic diseases, to the
tropics or semi-tropics, although as a matter of fact we find it is more
prevalent in these regions on account of the sanitary conditions.

HOW PLAGUE WAS CONTROLLED IN SAN FRANCISCO

Attention is called to these things in order that we may compare past
conditions with present. During the last few years San Francisco has
been fighting an outbreak of plague that in other days would have been
nothing less than a national calamity. But with modern methods of
handling it, based on knowing what it is, what causes it and how it is
spread, the authorities there have been able not only to hold the
disease in check, but practically to stamp it out with the loss of
comparatively few lives.

Dr. Blue of the Public Health and Marine Hospital Service and his
co-workers directed their whole energy toward controlling the rats. A
small army of men were employed, catching rats in every quarter of the
city. Dr. Rucker reports that fully a million rats were slain in this
campaign. Their breeding-places were destroyed by making cellars,
woodsheds, warehouses, etc., rat-proof and removing all old rubbish.
Garbage cans were installed in all parts of the city, as it was required
that all garbage be stored where rats could not feed upon it, and
altogether every effort was made to make it as uncomfortable as possible
for the rats.

The marked success attending this work abundantly confirms the soundness
of the theory upon which it was based, and serves as another example of
the way in which science is teaching us how to prevent or control many
of our most serious diseases.

THE INDIAN PLAGUE COMMISSION

In 1896, what proved to be a very serious outbreak of plague, occurred
in Bombay and spread to other parts of India. In 1898, a commission was
appointed to inquire into the origin of the different outbreaks, the
manner in which the disease is communicated, etc. This was known as the
Indian Plague Commission, and its exhaustive report, together with the
minutes of the evidence presented to the committee, represents a
stupendous amount of work on this subject and is the basis for much of
the later investigation that has been undertaken.

After the consideration of the evidence from various sources the
commission decided that the principal mode of infection both for man and
rats was through some sort of an abrasion in the skin, although it
recognized also the possibility of infection through the nose and
throat, and possibly, very rarely, through the intestinal tract or other
places.

Considerable time was spent in considering Dr. Simond's claim, made in
1898, that fleas which have been parasitic on plague-infected rats
migrate on the death of their hosts and convey the infection to healthy
men and rats. Dr. Simond sought to establish the following:

"Firstly, that plague rats are eminently infective when infected
with fleas and that they cease to be infective when they have been
deserted by their parasites: Secondly, that living plague bacilli
are found in association with fleas which are taken from
plague-infected rats: Thirdly, that plague can pass from infected
rats to other animals which have not come directly in contact with
them or with their infected excretions: Fourthly, that fleas which
infest rats will transfer themselves as parasites to men."

After reviewing the experiments which had been made to establish these
claims the commission believed that sufficient precaution had not been
taken to prevent infection from other sources and that not enough
definite evidence was produced. Against this claim much negative
evidence was considered and the final conclusion was "that suctorial
insects do not come under consideration in connection with the spread of
plague."

In 1905 another body of men known as the Advisory Committee was
appointed to arrange for further studies in India and other places,
particularly in relation to the mode of dissemination of the disease.
They at once appointed a new working commission who immediately began
their studies and experiments. The preliminary reports of their work,
which are still known as the Reports of the Indian Plague Commission, as
well as the reports of contributing investigations that are being made
from time to time, have served to establish entirely Dr. Simond's claims
and have completely revolutionized the methods of fighting plague.

There are several different types of plague, seeming to depend largely
on the manner of infection. The most common type is that known as the
bubonic plague which is characterized by buboes or swellings in various
parts of the body. This form of infection is usually received through
the skin in some manner or other. Only rarely does direct man-to-man
infection occur though there is always the possibility of it. The
investigations have shown that the flea is the most common agent in
transferring the disease from rat to rat or from rat to man. This may be
accomplished by the flea transferring the bacilli directly from one host
to another on its proboscis, or they may be carried in the alimentary
canal of the flea and gain an entrance into the skin through an abrasion
of some kind when the flea is crushed as it is biting, or when some of
the bacilli are left on the skin in the excreta of the insect.

RESULTS OF VERJBITSKI'S EXPERIMENTS

A very important series of experiments bearing directly on this subject
was made in 1902 and 1903 by Dr. D.T. Verjbitski. The paper giving the
results of this work was not published in any scientific journal until
1908 when the Advisory Committee published it in one of their reports.
The experiments were so well planned and executed and the results so
definite that I think it is worth while to give in full his summary of
results. The bugs referred to are bedbugs.

"(1) All fleas and bugs which have sucked the blood of animals
dying from plague contain plague microbes.

"(2) Fleas and bugs which have sucked the blood of animals which
are suffering from plague only contain plague microbes when the
bites have been inflicted from 12 to 26 hours before the death of
the animals, that is, during that period of their illness when
their blood contains plague bacilli.

"(3) The vitality and virulence of the plague microbes are
preserved in these insects.

"(4) Plague bacilli may be found in fleas from four to six days
after they have sucked the blood of an animal dying with plague. In
bugs, not previously starved or starved only for a short time (one
to seven days), the plague microbes disappear on the third day; in
those that have been starved for four to four and one-half months,
after eight or nine days.

"(5) The numbers of plague microbes in the infected fleas and bugs
increase during the first few days.

"(6) The fæces of infected fleas and bugs contain virulent plague
microbes as long as they persist in the alimentary canal of these
insects.

"(7) Animals could not be infected by the bites of fleas and bugs
which had been infected by animals whose own infection had been
occasioned by a culture of small virulence, notwithstanding the
fact that the insects may be found to contain abundant plague
microbes.

"(8) Fleas and bugs that have fed upon animals which have been
infected by cultures of high virulence convey infection by means of
bites, and the more certainly so the more virulent the culture with
which the first animal was inoculated.

"(9) The local inflammatory reaction in animals which have died
from plague occasioned by the bites of infected insects is either
very slight or absent. In the latter case it is only by the
situation of the primary bubo that one can approximately identify
the area through which the plague infection entered the organism.

"(10) Infected fleas communicate the disease to healthy animals for
three days after infection. Infected bugs have the power of doing
so for five days.

"(11) It was not found possible for more than two animals to be
infected by the bites of the same bugs.

"(12) The crushing of infected bugs in situ during the process of
biting, occasioned in the majority of cases the infection of the
healthy animal with plague.

"(13) The injury to the skin occasioned by the bite of bugs or
fleas offers a channel through which the plague microbes can easily
enter the body and occasion death from plague.

"(14) Crushed infected bugs and fleas and their fæces, like other
plague material, can infect through the small punctures of the skin
caused by the bites of bugs and fleas, but only for a short time
after the infliction of these bites.

"(15) In the case of linen and other fabrics soiled by crushing
infected fleas and bugs on them, or by the fæces of these insects
the plague microbes can under favorable conditions remain alive and
virulent during more than five months.

"(16) Chemical disinfectants do not in the ordinary course of
application kill plague microbes in infected fleas and bugs.

"(17) The rat flea _Typhlopsylla musculi_ does not bite human
beings.

"(18) Human fleas do bite rats.

"(19) Fleas found on dogs and cats bite both human beings and rats.

"(20) Human fleas and fleas found on cats and dogs can live on rats
as casual parasites, and therefore can under certain conditions
play a part in the transmission of plague from rats to human
beings, and vice versa."

RESULTS OF VARIOUS INVESTIGATIONS

Various other plague commissions from other countries as well as many
individuals have investigated the same subject, and the results all
point conclusively to the fact that the rats and the fleas are at least
the most important factors in the spread of the disease. The evidence
from many sources and from many experiments may be briefly summed up as
follows: The disease is caused by the presence in the system of minute
bacteria, _Bacillus pestis_. It is probable that plague is primarily a
disease of rats and only secondarily and accidentally, as it were, a
disease of man.

Rats are subject to the plague and are often killed by it in great
numbers. An outbreak of plague among men is often preceded by a very
noticeable outbreak among rats.

Rats dying of the plague have their blood filled with the plague
bacillus. Fleas or other suctorial insects feeding on such rats take
myriads of these bacilli into their stomach and get many on their
proboscis.

The fleas usually leave a rat as soon as it dies and of course seek some
other source of food. When such infected fleas are permitted to bite
other rats or guinea-pigs these animals often develop the disease.
Several of the species of fleas that infest rats will bite man also, and
in the cases of many plague patients it can be definitely shown that
they had recently been bitten by fleas.

STRUCTURE AND HABITS OF FLEAS

A study of the structure and habits of fleas shows that in many respects
they are particularly adapted for spreading such a disease as bubonic
plague. The piercing proboscis consists of three long needle-like
organs, the epipharynx and mandibles, and a lower lip or labium. The
mandibles have the sides serrate like a two-edged saw. The labium is
divided close to its base so that it really consists of two slender
four-segmented organs which lie close together and form a groove in
which the piercing organs lie. When the flea is feeding, the epipharynx
and mandibles are thrust into the skin of the victim, the labium serving
as a guide. As the sharp cutting organs are thrust deeper and deeper the
labium doubles back like a bow and does not enter the skin. Saliva is
then poured into the wound through minute grooves in the mandibles, and
the blood is sucked up into the mouth by the sucking organ which lies in
the head at the base of the mouth-parts. Just above this piercing
proboscis is a pair of flat, obtuse, somewhat triangular pieces, the
maxillary blades or maxillæ. When the proboscis is fully inserted into
the skin the tips of these maxillæ may also be embedded in the tissue
and perhaps help to make the wound larger. Attached to these maxillæ is
a pair of rather stout, four-jointed appendages, the palpi. They
probably act as feelers.

If the flea chances to be feeding on a plague-infected rat or person
many of the plague bacilli will get on the mouth-parts and myriads of
them are of course sucked up into the stomach with the blood. Those on
the proboscis may be transferred directly to the next victim that it is
thrust into, and those in the stomach may be carried for some time and
finally liberated when the flea is feeding again or when it is crushed
by the annoyed host. The latter is probably the most common method of
infection, for the bacilli that are liberated when the flea is crushed
may readily be rubbed into the wound made by the flea bite or into
abrasions of the skin due to the scratching. Kill the flea, but don't
"rub it in."

[Illustration: FIG. 105--Rat-flea (_Læmopsylla cheopis_); male.]

[Illustration: FIG. 106--Rat-flea (_L. cheopis_); female.]

[Illustration: FIG. 107--Head of rat-flea showing mouth-parts.]

[Illustration: FIG. 108--Human-flea (_Pulex irritans_); male.]

During the recent outbreak in San Francisco many thousand fleas that
were infesting man, rats, mice, cats, and dogs, squirrels and other
animals have been studied and it has been found that while each flea
species has its particular host upon which it is principally found, few
if any of them will hesitate to leave this host when it is dead and
attack man or any other animal that may be convenient.

COMMON SPECIES OF FLEAS

Throughout India and in all the warm climates where plague frequently
occurs the most common flea found on rats has come to be known as the
plague flea (_Læmopsylla cheopus_) (Figs. 105, 106), and is doubtless
the principal species that is concerned in carrying the disease in those
climates. It now occurs quite commonly on the rats in the San Francisco
Bay region and is occasionally found there on man also. In the United
States, Great Britain and other temperate regions another larger
species, _Ceratophyllus fasciatus_ is by far the most common flea found
on rats, and is commonly known as the rat flea. It occurs on both the
brown and the black rats _Mus norvegicus_ and _M. rattus_, on the house
mouse and frequently on man. It has also been taken in California on
pocket gophers and on a skunk.

The common human flea (_Pulex irritans_) (Figs. 108, 109), is found in
all parts of the inhabited world. Although we regard it primarily as a
pest of human beings it often occurs very abundantly on cats, dogs, mice
and rats as well as on some wild mammals such as badgers, foxes and
others and has occasionally been found on birds.

Most entomologists regard the fleas commonly found on cats and dogs as
belonging to one species _Ctenocephalus canis_. Others believe them to
be distinct species and call the cat flea _Ctenocephalus felis_. So far
as our personal comfort and safety is concerned it makes but little
difference to us whether the flea that bites us is called _canis_ or
_felis_ for they both look very much alike, and act alike and the bite
of one hurts just as much as the bite of the other. Although cats and
dogs are their normal hosts they are very often troublesome household
pests, sometimes making a house almost uninhabitable. They are
frequently found on rats, and therefore may carry the plague bacillus
from rat to rat or from rat to man.

GROUND-SQUIRRELS AND PLAGUE

As early as 1903 Dr. Blue, in charge of the plague suppressive measures
in San Francisco, became impressed with the possibility of the common
California ground-squirrels (_Otospermophilus beecheyi_), acting as an
agent in the transmission of plague. It was rumored at that time that
some epidemic disease was killing the squirrels in some of the counties
surrounding San Francisco Bay, notably in Contra Costa County. None of
the squirrels were examined at that time, but since then many thousand
have been carefully studied and it has been definitely shown that many
of them are plague-infected. Just how the plague got started among them
will probably never be really known. There is little doubt, however, but
that it was transferred in some way from the rats to the squirrels. The
trains and the bay and river steamers running out from San Francisco
would afford abundant opportunity for the rats to go from the city to
the warehouses all along the shore. Once there they would use the same
runways as the squirrels about the warehouses and in the near-by fields.
In harvest time the rats migrate to the fields and make constant use of
the squirrel holes. The farmers in some sections report that they
frequently catch more rats than squirrels in traps set in squirrel holes
at that season of the year.

This close association of the rats and the squirrels affords a good
opportunity for the fleas infesting them to pass from one host to the
other.

So far only two species of fleas have been recorded from the
ground-squirrels. One, _Ceratophyllus acutus_, is very common, sometimes
literally swarming over the squirrels, particularly if a squirrel is
sick or weak from any cause. The other species, _Hoplopsyllus anomalus_,
is less abundant but still quite common. Both of these species infest
rats also, so the chain of evidence is practically complete. We have
only to assume that at sometime one or more of the plague-infected rats
found their way into the region where the squirrels were, and the fleas
passing from the rats to the squirrels would carry the plague with them.

The fact that the plague already has such a start among the squirrels
opens a new and very serious phase of the problem of suppressing the
disease. All who have hunted the ground-squirrels will testify to the
readiness with which the fleas from them will bite those who are
handling them. As it is the sick or weak squirrels that are most often
taken there is always a chance that plague may be transferred from them
to human beings. The records of the plague cases in California show at
least three cases in which there seems to be very little doubt that the
disease resulted from handling plague-infected squirrels.

[Illustration: FIG. 109--Human-flea (_P. irritans_); female.]

[Illustration: FIG. 110--Mouse-flea (_Ctenopsyllus musculi_); female.]

A still more serious thing is the possibility of the disease remaining
in a more or less virulent form among the squirrels for some time,
possibly for years, and then breaking out again in some locality where
the rats or men may become infected. As long as there is a trace of the
disease among the squirrels there is always the chance of it spreading,
so that new areas may become infested. Those in charge of the
plague-suppressive measures are fully aware of these dangers and are
making a careful study of the situation and will doubtless be able to
cope with it successfully. It may be that the squirrels will have to be
exterminated in the infected regions. This would be a long and difficult
task, but the success attending the fight against the rats in a great
city shows what can be done when the determination to do it is there.

REMEDIES FOR FLEAS

We have seen how a great city set to work to rid itself of the
plague-sick rats. As a matter of fact it was not the rats that they
were after primarily. If the rats had not harbored fleas the city would
have been glad to let the disease take its course and destroy as many
rats as possible. But it was found that the only way to get rid of the
fleas that might possibly be infected with the plague was to kill their
rat hosts.

General cleaning-up measures will of course very materially lessen the
number of fleas about the private dwellings, but there often remains a
number of fleas in the house that are a source of great annoyance even
if the danger is eliminated.

Particularly is this apt to be so in places where cats or dogs are
members of the household. These animals almost always harbor at least a
few fleas, and where there are a few there is always a possibility, even
a great probability, that there will be many more unless an effort is
made to get rid of them.

In some sections of the country it is the cat and dog flea that is the
most troublesome to man. The minute white eggs of the fleas are usually
laid about the sleeping-places of these animals and the slender active
larvæ that hatch from them feed upon any kind of organic matter that
they can find in the dust or in the cracks and crevices. About eight or
ten days after hatching the larvæ spin delicate brownish cocoons in
which they pass the pupal stage, issuing a few days later as the adult
fleas.

It will at once appear, then, that it is important to provide the cats
and dogs with sleeping-places that can be kept clean. If they have a mat
or blanket to sleep on this can be taken up and shaken frequently and
the dust swept up and burned. In this way many of the eggs or larvæ may
be destroyed. Very often the dust under a carpet that has not been taken
up and dusted for some time will be found to be harboring a multitude of
fleas or their larvæ. In such cases a thorough cleaning of the carpet
and the floors will bring relief. Houses that are unused for some time
during the summer months are often found to be overrun with fleas in the
fall, for the fleas have had an unmolested opportunity to breed and
multiply. Such rooms of course require a thorough cleaning or it is
sometimes possible to kill the fleas by a liberal use of pyrethrum
powder or benzine or to fumigate. In this connection, Dr. Skinner's note
in the _Journal of Economic Entomology_ is worth repeating.

"In the latter part of last May (1908) I moved into a house that
had not been previously occupied. No carpet was used and being
summer only a few rugs were placed on the floors. A part of the
household consisted of a collie dog and three Persian cats. Very
soon the fleas appeared, the dog and cat flea, _Ctenocephalus
canis_. I did not count them and I can't say whether they numbered
a million or only a hundred thousand. On arising in the morning and
stepping on the floor one would find from three to a dozen on the
ankles. The usual remedies for fleas are either drastic or somewhat
unsatisfactory. The drastic one is to send the animals to the
institutions, where they are asphyxiated, or take the other advice,
'Don't keep animals.'

"I tried mopping the floors with rather a strong solution of
creolin but it did little good. Previous experience with pyrethrum
was not very satisfactory. Knowing the volatility of naphthalene in
warm weather and the irritating character of its vapor led me to
try it. I took one room at a time, scattered on the floor five
pounds of flake naphthalene and closed it for twenty-four hours. On
entering such a room the naphthalene vapor will instantly bring
tears to the eyes and cause coughing and irritation of the air
passages. I mention this to show how it acts on the fleas. It
proved to be a perfect and effectual remedy and very inexpensive,
as the naphthalene could be swept up and transferred to other
rooms. So far as I am concerned the flea question is solved and if
I have further trouble I know the remedy. I intend to keep the dog
and the cats."

CHAPTER X

OTHER DISEASES, MOSTLY TROPICAL, KNOWN OR THOUGHT TO BE TRANSMITTED BY
INSECTS

SLEEPING SICKNESS

One of the worst scourges of Africa and one that is to-day attracting
world-wide attention is the disease known as trypanosomiasis, the
terminal phase of which is sleeping sickness, one of the most ghastly
diseases that we know.

Among the Protozoa referred to in one of the earlier chapters mention
was made of certain trypanosomes which inhabit the blood of man and
certain animals. Very little was known concerning these parasites
previous to the beginning of the present century, but since that time
several have been found to be of great economic importance. The group is
being studied extensively and every day our knowledge of them is
increasing so that we now know quite definitely the life-history of
several.

_Trypanosoma lewisi_, a parasite of rats, is perhaps the best known as
it is always common where-ever rats are found. Sometimes as many as 30%
or 40% of the rats of certain districts are infected. It is thought that
these are transmitted from rat to rat by the common rat-louse which
serves as an intermediate host. Fleas may also act as disseminating
agents.

A few other kinds cause serious disease of animals, but we are more
interested just now in the particular one that is causing so much
trouble in Africa. This parasite was discovered in 1902 and was named
_Trypanosoma gambiensi_ (Fig. 111). Since then it has been found to be
widely distributed. Although the natives have doubtless long been
subject to the disease caused by this parasite, the recent influx of
whites to these regions and the consequent movements of the natives have
caused a great spread of the disease so that whole regions are now made
desolate, the inhabitants dying or fleeing to escape the uncanny death.

The disease may run its course in a few months or it may take years. The
symptoms are various, but infection is usually soon followed by fevers,
sometimes mild, sometimes severe, which recur at irregular intervals.
Certain glands or other parts of the body may become swollen. More or
less extensive skin eruptions occur on all parts of the body and the
patient gradually becomes anemic and physically and intellectually
feeble. The nervous system seems to be affected by the parasite, either
directly or by the action of the toxins it produces. The patient becomes
more debilitated and morose with an increasing tendency to sleep, hence
the name sleeping sickness. As the stupor deepens the patient looses all
desire or power of exertion and as little food is taken he rapidly
wastes away and finally succumbs for after this final stage is reached
there is no relief.

It is definitely known that a species of tsetse-fly, _Glossina palpalis_
(Fig. 112), which somewhat resembles our stable-fly, is responsible for
the dissemination of the disease, and some recent investigators have
suggested that certain species of mosquitoes may also carry the parasite
from one host to another. There still remains some doubt as to the exact
manner in which the fly transmits the disease, but it seems altogether
likely that it is an alternative host and does not serve as a simple
mechanical carrier. In this respect it is like the mosquito which is one
of the necessary hosts of the malaria parasites, and unlike the
house-fly which carries the germs of various diseases in a purely
mechanical way without serving as a definite necessary host for the
parasite.

The tsetse-fly is found only in tropical Africa and is limited in its
distribution there to certain very definite, narrow, brushy areas along
the water's edge. If these places can be avoided there seems to be
little danger. Those who are fighting the disease have found that if the
brush in the vicinity of watering-places and ferry-landings is cleared
away such places become comparatively safe. These flies do not lay eggs
but produce full-grown larvæ which soon pupate in the ground.

ELEPHANTIASIS

In many tropical regions human blood as well as that of other animals is
the normal habitat of certain worm-like parasites (Nematodes). They are
not entirely confined to the tropics but may extend far up into the
subtropical regions. Five or six different species of these parasites
are known, only one of which, however, has been shown to be of any
pathological importance, as far as human beings are concerned.

[Illustration: FIG. 111--_Trypanosoma gambiense_; various forms from
blood and cerebrospinal fluid. (After Manson.)]

[Illustration: FIG. 112--Tsetse-fly. (After Manson.)]

This species, _Filaria bancrofti_, is not only very widely distributed,
but in regions such as some of the South Sea Islands a very large per
cent of the natives have the filariæ present in their blood. When these
parasites are withdrawn from the circulation and placed on a slide for
study they are seen to be minute transparent, colorless, snake-like
organisms inclosed in a very delicate sack or sheath. They are but a
little more than one-hundredth of an inch long and about as big around
as a red blood-corpuscle. These are the larval forms of the parasite and
have been called by Le Dantec the micro-filaria.

If blood of the patient drawn from the skin, is examined during the day
few if any of these parasites are found, but if it is examined between
five or six o'clock in the evening and eight or nine o'clock the next
morning they may be found in numbers. During the daytime they have
retired from the peripheral circulation to the larger arteries and to
the lungs, where they may be found in great numbers.

This night-swarming to the peripheral circulation has been found to be a
remarkable adaptation in the life-history of the parasite, for it has
been demonstrated that in order to go on with its development these
larval forms must be taken into the alimentary canal of the mosquito.
Most of the mosquitoes in which the development takes place are
night-feeders, so that the parasites are sucked up with the blood of the
victim. Once inside the stomach they soon free themselves from the
inclosing sheath and make their way through the walls of the stomach and
enter the muscular tissue, particularly the thoracic muscles. Here they
undergo a metamorphosis and increase enormously in size, some attaining
one-sixteenth of an inch in length.

After sixteen to twenty days they leave these muscles and make their way
to other parts of the body. A few may be found in different parts of the
abdomen, but most of them make their way forward into the head of the
mosquito and coil themselves up close to the base of the proboscis,
finally finding their way down into the proboscis inside the labium.
Here they lie until an opportunity offers for them to escape to the warm
blood of a vertebrate. They probably pass through the thin membrane
connecting the labella with the proboscis and there find their way into
the wound made by the puncture when the insect bites. Whether these
parasites can gain an entrance into the circulatory system in any other
way is not known. It has been suggested that the mosquitoes dying and
disintegrating on the surface of water may liberate the filariæ which
may later find their way into the system of the vertebrate host when the
water is used for drinking, but most of the investigations made so far
seem to indicate that they make their way directly from the proboscis
into the new host.

Soon after entering the circulatory system of the human host the
parasites make their way into the lymphatics where they attain sexual
maturity, and in due time new generations of the larval filariæ or
microfilariæ are poured into the lymph, and finally into the definite
blood-vessels, ready to be sucked up by the next mosquito that feeds on
the patient.

In most cases of infection the presence of these filariæ in the blood
seems to cause no inconvenience to the host. They are probably never
injurious in the larval stage, that is, in the stage in which they are
found in the peripheral circulation.

In many cases, however, the presence of the sexual forms in the
lymphatics may cause serious complications. The most common of these is
that hideous and loathsome disease known as elephantiasis in which
certain parts of the patient becomes greatly swollen and distorted. An
arm or a leg may become swollen to several times its natural size, or
other parts of the body may be seriously affected.

In some of the South Sea Islands 30% to 40% of the natives are afflicted
in this way, some only slightly others seriously. There is little or no
pain, but in severe cases the distorted parts often render the patient
entirely helpless.

The exact way in which the parasites cause such swelling is not very
definitely known. Manson, who has done more work on these diseases than
any one else, believes that the trouble arises from the clogging of the
lymphatic glands or trunks, thus cutting them off from the general
circulation, in which case the affected parts may become distorted. This
clogging of the passages is believed to be due to the presence of great
numbers of immature eggs which have been liberated by parasites injured
in some way before their eggs were entirely developed.

This interference with the lymphatic circulation brings about the
anomalous condition of a patient with a serious filarial disease with
fewer of the filarial parasites in his blood than one who is not so
seriously affected. This is supposed to be due to the fact that the
disease-producing parasites have died and that the lymphatics have
become so obstructed that any microfilariæ they may contain cannot make
their way into the general circulation. Such a patient then would not be
as likely to infect a mosquito as would one less seriously affected.

It has always been thought that little or nothing could be done in the
way of successfully treating this disease, but quite recently a French
physician, who has been conducting a long series of experiments in the
Society Islands, announced that he is able to cure many cases by certain
surgical operations on the affected parts.

DENGUE OR "BREAKBONE FEVER"

This is another disease of the tropics often occurring in widespread
epidemics. It is probably most frequently met with in the West Indies,
but may occur in any of the tropical countries or islands. Occasionally
it spreads into subtropical or even temperate regions. Several extensive
epidemics have occurred in the United States. Once introduced into a
community it spreads very rapidly and nothing seems to confer immunity.

The various names by which it has been called well describe its effect
on the patient; breakbone fever, dandy-fever, stiff-necked or
giraffe-fever, boquet (or "bucket") fever, _scarlatina rheumatica_,
polka-fever, etc. While the suffering is intense as long as the disease
lasts it seldom terminates fatally.

It has always been classed as a very contagious disease and it has not
yet been definitely shown that it is not. Recent observations, however,
have shown that it is probably caused by a certain Protozoan parasite
that is found in the blood of dengue patients and several experiments
have been conducted by Dr. Graham which seem to indicate that it is
transmitted by mosquitoes. In these experiments, _Culex fatigans_, a
common tropical or subtropical mosquito, was used. The same parasite
that is found in the human blood may be found in the stomach and blood
of the mosquitoes up to the fifth day after it has fed on a dengue
patient.

Sick and healthy individuals were allowed to remain in close contact in
a room from which the mosquitoes had been excluded, and the disease was
not spread. Mosquitoes that had bitten dengue patients were taken to a
higher region where dengue had never occurred and allowed to bite two
healthy persons. Both developed the disease and as they were protected
from other mosquitoes until they had recovered, the disease did not
spread to others of the community. These and other observations seem to
make a complete chain of evidence, and most medical men to-day accept
the theory as well proved and in their practice take every precaution to
prevent the spread of the disease by keeping the infected patient from
being bitten by the mosquitoes.

The yellow fever mosquito is also suspected of carrying this same
disease, and it is possible that other species are also concerned. If
it is true that the parasite can be carried by several different species
of mosquitoes this would account very largely for its rapid spread
wherever it is introduced into a community. Where it occurs outside the
tropics it is only in the warm summer months when mosquitoes are always
abundant.

MALTA OR MEDITERRANEAN FEVER

This is also a tropical and subtropical disease that occasionally gets
up into the temperate region, sometimes occurring in the United States.
The fever begins with a severe headache, and other symptoms follow. It
is usually of the remittent type and may continue for some months.

It is caused by minute bacteria (_Micrococcus melitensis_) and is a very
infectious but not usually contagious disease. The germ is readily
conveyed by inoculation, and several investigators have sought to show
that the mosquito often serves as the inoculating agent. The disease is
especially prevalent during the mosquito season, and has twice been
conveyed to monkeys by infected insects.

LEPROSY

This loathsome disease has long been known to be caused by a particular
bacillus (_Bacillus lepræ_), but the way in which this organism gains
an entrance into the system is still unknown. Many theories have been
propounded, but none of them has been well established. Within recent
years the possibility of insects carrying the germ and in one way or
another transmitting it to healthy individuals has been suggested and
much discussed. As the lepræ bacilli are present in the skin and ulcers
of leprous patients, insects sucking the blood or feeding on the sores
could not help taking some of them into their body or becoming
contaminated. These bacilli have been found at various times in the
stomach or intestine of mosquitoes, fleas and bedbugs. So it is believed
by some that these and other insects, such as lice and flies, may
sometimes transmit the disease. On a previous page we have referred to
the possibility of the face-mites acting as disseminators of leprosy.

Leprosy occurs most commonly among people where little attention is paid
to bodily cleanliness. Such people are usually freely infested with
various parasites that thrive well in the filth, so if the germs can be
transmitted in this way the carriers are there in abundance.

The fact that the sores usually occur on exposed parts of the body has
been pointed to as evidence that inoculation is due to such insects as
flies and mosquitoes. It has been noted that leprosy is frequently very
common in regions where elephantiasis occurs, suggesting the possibility
of the same carrier, the mosquito, for both diseases. So while there is
as yet very little evidence one way or the other, insects that are found
around leprous patients are to be regarded with suspicion, for until we
know more definitely just how the disease is communicated the insects
must be looked on as possible sources of contamination.

KALA-AZAR OR DUM-DUM FEVER

This is a very fatal infectious disease of many tropical and subtropical
regions, spreading terror among the natives wherever it occurs. It is
caused by the presence in the system of Protozoan parasites, the
so-called Leishman-Donovan bodies, that have recently been studied by
several observers.

Dr. W.S. Patton of the Indian Medical Service has been making some
extensive experiments with the common bedbug of India (_Cimex
rotundatus_) which seem to demonstrate fully that this insect is
responsible for the transmission of the parasite that causes the
disease. He has found the parasite in all stages of development in the
bedbug. This, taken with a number of other observations in regard to the
tendency of the disease to cling to particular houses, makes a strong
case against the bedbug. Manson, however, believes that the parasite may
be transmitted by other agents also, possibly by means of flies that
visit the sores or in other ways.

ORIENTAL SORE

This disease, once supposed to be confined to the Orient, is now found
to be rather widely distributed throughout the tropics, where it is
sometimes very prevalent. It is caused by the presence in the system of
a parasite very similar to or identical with the one causing _kala-azar_
and is regarded by some as a modified form of that disease. The patient
is affected with one or more serious sores or ulcers which usually occur
on exposed parts of the body.

The parasite that causes the disease is supposed to be carried by
insects either directly or indirectly.

In the latter case the insect may act as an intermediate host.

Dogs and camels are also attacked by this disease and may be sources of
infection.

BIBLIOGRAPHY

A complete list of books and articles dealing more or less directly with
the subjects discussed in this book would be too extended for use here.
For the past ten or twelve years many of the medical and biological
journals have contained articles in almost every issue, discussing these
subjects in some of their phases. I have selected only a few of the more
important of them, and these only the English ones, confining myself
mostly to those that I have personally consulted, and giving brief
annotations. Many of these will be found to include very full
bibliographies of the particular subject treated.

In order to avoid repetition, references are given under one head only
although many might properly be included in other sections as well.

PARASITES AND PARASITISM

BRAUN, MAX. Animal Parasites of Man. Translated by Pauline Falcke
and edited by L.W. Sambon and F.V. Theobald. Third edition, 1906. A
chapter on the general subject of parasitism and a description of
parasites of all classes. Bibliography.

LEUCKART, R. The Parasites of Man and the Diseases Induced by Them.
Eng. transl., London, 1886.

NEUMAN, THEO. Entoparasites and Hygiene. _Trans. Vassar Bros.
Institute_, VII, 1895. A general discussion of parasitism;
life-history of some common parasites that infest man.

NEUMANN, L.G. Treatise on the Parasites and Parasitic Diseases of
the Domesticated Animals. Eng. transl. by Fleming, 1892.

RANSOM, B.H. How Parasites Are Transmitted. _Year Book U.S. Dept.
Agric._, 1905, pp. 139-166 (pub. 1906). Discusses the ways in which
parasites of all classes are transmitted.

SAMBON, L. The Part Played by Metazoan Parasites in Tropical
Pathology. _Jour. Trop. Med. & Hyg._, Vol. XI, Jan. 15, 1908. A
comprehensive discussion of this subject.

SHIPLEY, A.E., AND FEARNSIDES, E.G. Effects of Metazoan Parasites
on Their Hosts. _Jour. Econom. Biology_, Vol. I, 1906, pp. 41-62.
Discusses injury due to mere presence of parasite in host; to the
migration of the parasite; loss to host by feeding of parasites;
injury by certain toxins.

STILES, C.W. Diseases Caused by Animal Parasites. _Osler's Mod.
Med._, Vol. I, 1907, p. 525. General discussion; Trematodes;
Cestodes; Roundworms; Acariasis; Parasitic Insects; Myiasis.

VAN BENEDEN, P.J. Animal Parasites and Messmates. 1889. Contains
much that is interesting.

WARD, HENRY B. Influence of Parasitism on the Host. _Proc. Amer.
Assn. for Advancement of Science_, Vol. 56, 1907. A comprehensive
statement of this subject. List of literature.

PROTOZOA

CALKINS, G.N. The Protozoa. _Osler's Mod. Med._, Vol. I, 1907, p.
353. General notes on the Protozoa; classification; reproduction;
life-cycle of various forms. Regards Protozoa as subkingdom and the
four great divisions as phyla.

CALKINS, G.N. Protozoölogy. N.Y., 1909. Chapters on parasitism,
pathogenic Protozoa, etc.

CLARKE, J.J. Protozoa and Disease. London, 1903, Pt. I. Discusses
the various protozoa that cause disease, and refers frequently to
those that are transferred from host to host by insects.

CLARKE, J.J. Protozoa and Disease. London, 1908. Part II,
comprising sections on the causation of smallpox, syphilis and
cancer. Notes on parasitic Protozoa, tropical diseases, ticks,
piroplasmosis, etc.

DANIELS, C.W. Persistence of the Tropical Diseases of Man Due to
Protozoa. _Jour. Trop. Med. & Hyg._, 12, Aug. 2, 1909, pp. 232-234.
Same in _Lancet_, II, 1909, p. 460. Good summary of present
knowledge of the subject.

MINCHIN, E.A. Protozoa. In Albutt and Rolleston's _System of
Medicine_, II, 1907, pp. 9-122. A comprehensive chapter on
Protozoa. Many parasitic forms are figured and described.
Bibliography.

MINCHIN, E.A. The Sporozoa. In Lankester's _Treatise on Zoöl._, Pt.
I, pp. 150-360, 1903. Best account of this group, list of Sporozoan
hosts. Bibliography.

BACTERIA

FLEXNER, SIMON. Relation of Bacteria and Sporozoa to Disease.
_Science_, N.S., Vol. 27, No. 682, pp. 133-136. On these pages
discusses relation of bacteria and Protozoa to human diseases.

JORDAN, EDWIN O. General Bacteriology. Philad., 1898. A good
general treatment of the subject.

LEVY, ERNST, AND KLEMPERER, FELIX. Elements of Clinical
Bacteriology for Physicians and Students (transl. by A.A. Eschner),
Philad., 1909. Morphology and biology of bacteria; infection;
immunity; specific diseases of bacterial origin, etc.

MUIR, ROBT., AND RITCHIE, JAS. Manual of Bacteriology. N.Y., 1903.
Contains chapter on the relation of bacteria to diseases and
discussion of several bacterial diseases.

STERNBERG, G.M. A Manual of Bacteriology. N.Y., 1893. Part III is
devoted to pathogenic bacteria.

INSECTS AND DISEASE

HERMS, W.B. Medical Entomology, Its Scope and Methods. _Jour. of
Eco. Ento._, Vol. 2, No. 4, 1909, pp. 265-268.

HOWARD, L.O. Insects as Carriers and Spreaders of Disease. _Year
Book U.S. Dept. Agric._, 1901, pp. 177-192. Good review of the
subject.

HOWARD, L.O. How Insects Affect Health in Rural Districts. _U.S.
Dept. Agric., Farmers' Bulletin, No. 155_, 1902. Discussion of city
and county conditions; protection from typhoid, malaria and yellow
fever.

HOWARD, L.O. Economic Loss to the People of U.S. Through Insects
That Cause Disease. _Bull. 78, U.S. Dept. Agric. Bur. of Ent._,
1909. A comprehensive discussion and summary of the subject.
Discusses mosquitoes, flies, the Panama Canal, epidemic diseases
and the progress of nations.

KELLOGG, V.L. Insects and Disease, Chap. XVIII, in _American
Insects_, pp. 615-654, 1905. Discusses Mosquitoes and malaria;
yellow fever and filariasis.

KING, H.H. Report on Economic Entomology of Khartoum, in _Third
Rept. of Wellcome Research Lab._, 1908. Discusses insects injurious
to man: mosquitoes, blood-sucking insects other than mosquitoes,
etc.

MASON, C.F. The Spread of Diseases by Insects, with Suggestions
Regarding Prophylaxis. _International Clinics_, Vol. II, 1904, pp.
1-21. A brief summary of the subject.

MCCRAE, JOHN. Recent Progress in Tropical Medicine. _International
Clinics_, Vol. II, 1904, pp. 22-36. Discusses several diseases,
some of which are transmitted by insects.

NUTTALL, G.H.F. On the Rôle of the Insects Arachnids and Myriapods
as Carriers in the Spread of Bacterial and Parasitic Diseases of
Man and Animals. A critical and historical study. _Johns Hopkins
Hospital Reports_, Vol. 8, 1899, pp. 1-154. A review of all the
literature up to this date. Important article.

NUTTALL, G.H.F. Insects as Carriers of Disease. Recent advances in
our knowledge of the part played by blood-sucking arthropods
(exclusive of mosquitoes and ticks) in the transmission of
infectious diseases. Bericht über den XIV. Intern. Kongress für
Hygiene und Dermogrophic. Berlin, 1907, pp. 195-206. Discusses
protozoan and bacterial diseases.

STILES, C.W. Insects as Disseminators of Disease. _Virginia Medical
Semi-monthly_, Vol. 6, No. 3, May 10, 1901, pp. 53-58. Good
statement of subject with list of recent workers.

WHERRY, W.B. Insects and Infection. _Cal. State Jour. of Med._,
Nov., 1907. Discusses the rôle of insects, ticks, etc., in the
transmission of infectious diseases.

Symposium on Yellow Fever and Other Insect-borne Diseases.
_Science_, N.S., Vol. 23, Nos. 584-585, 1906. The Protozoan
Life-cycle, G.N. Calkins. Filariasis and Trypanosome Diseases, H.B.
Ward. The Practical Results of Reed's Findings on Yellow Fever
Transmission, J.H. White. Difficulties of Recognition and
Prevention of Yellow Fever, Q. Kohnke. The Practical Side of
Mosquito Extermination, H.C. Weeks. Without Mosquitoes There Can Be
No Yellow Fever, Jas. Carroll. Estivo-autumnal Fever, Cause,
Diagnosis, Treatment and Destruction of Mosquitoes Which Spread the
Disease, H.A. Veazie.

MOSQUITOES--SYSTEMATIC AND GENERAL

BALFOUR, ANDREW, AND STAFF. _Second Report of the Wellcome Research
Laboratories at the Gordon Memorial College_, Khartoum, 1906.
Includes reports on work on mosquitoes and other noxious insects.

BOYCE, SIR ROBERT W. Mosquitoes or Man? The Conquest of the
Tropical World. N.Y., 1909. Reviews medical and sanitary work in
the tropics and discusses the relation of insects to various
tropical diseases.

BUSCH, AUGUST. Report on a Trip for the Purpose of Studying the
Mosquito Fauna of Panama. _Smith. Miscell. Coll._, Vol. 5, Pt. I,
1908, p. 49. Work that is being done in Panama to control the
mosquitoes. Annotated list of species.

FELT, E.P. Mosquitoes or Culicidæ of New York State. In _N.Y. State
Museum Bull. 79_, Entomology 22, 1904. Discusses distribution,
migration and life-history of various species of mosquitoes and
mosquito diseases. Bibliography.

GILES, GEO. M. A Handbook of Gnats or Mosquitoes, Giving the
Anatomy and Life-History of the Culicidæ. London, 1902. Whole
subject treated very fully.

GRUBBS, S.B. Vessels as Carriers of Mosquitoes. _Pub. Health and
Mar. Hospt. Ser. Bull._ II, Mar. 3, 1903. Believes that mosquitoes
may come aboard when the vessel is lying at anchor one-half mile
from shore, and that under favorable conditions they may come
aboard when the vessel is fifteen miles from shore.

HOWARD, L.O. Mosquitoes. _Osler's Mod. Med._, Vol. I, p. 370, 1907.
General notes on classification and habits particularly in relation
to diseases.

HOWARD, L.O. Notes on Mosquitoes of the United States. _U.S. Dept.
Agric., 1900. Div. of Ento. Bull. No. 25_, N.S. Account of the
structure; biology; remarks on remedies.

HOWARD, L.O. Concerning the Geographic Distribution of the Yellow
Fever Mosquito. _Public Health Rept., Pub. Health and Mar. Hospt.
Ser._, Nov. 13, 1903. The same revised to Sept. 10, 1905.

HOWARD, L.O. Mosquitoes: How They Live; How They Carry Disease;
How They Are Classified; How They May Be Destroyed. N.Y., 1901. One
of the best popular books on mosquitoes.

MCCRACKEN, I. _Anopheles_ in California, with a Description of a
New Species. _Entomological News_, Vol. 15, Jan., 1904. Records of
three species, their breeding-places, habits, etc.

MITCHELL, EVELYN G. Mosquito Life. N.Y., 1907. A good popular
account of the mosquitoes and their relation to disease. The
appendix treats of mosquitoes and their possible relation to
leprosy.

SMITH, J.B. Mosquitoes Occurring Within the State of New Jersey.
Report of the New Jersey State Agric. Exper. Station upon the
mosquitoes occurring within the State. Trenton, N.J., 1904. Habits,
development, relation to disease, checks and remedies; systematic.

SMITH, J.B. The General Economic Importance of Mosquitoes. _Popular
Science Monthly_, 70, 1907, pp. 325-329. Mosquitoes affect not only
the health and comfort of the people, but hinder development of
agriculture and thus affect land values.

SMITH, J.B. The New Jersey Salt-marsh and Its Improvement. _New
Jersey Agricultural Experiment Station Bulletin_, 207, 1907. Shows
that the increased value of the land drained in the antimosquito
crusade more than pays for the cost of the drainage.

THEOBALD, F.V. Monograph of _Culicidæ_ of the World. Four Vols. and
one Vol. of plates. London, 1901 to 1907. Vol. I contains 96 pages
on structure, life-history, habits, etc. Vol. II contains a
bibliography. Vol. Ill contains a list of species that carry
disease.

THEOBALD, F.V. Mosquitoes or _Culicidæ_. In Albutt and Rolleston's
_System of Medicine_, II, 1907, pp. 122-168. Structure,
life-history, habits, distribution and classification of
mosquitoes. Bibliography.

MOSQUITO ANATOMY

BERKELEY, WM. M. Laboratory Work with Mosquitoes. N.Y., 1902.
Chapters on development, anatomy, dissection, malarial parasites,
filarial disease, yellow fever.

DIMMOCK, GEO. Anatomy of the Mouth-parts and Suctorial Apparatus of
_Culex_. _Psyche_, 3, pp. 231-241, Sept., 1881. Good.

IMMS, A.D. On the Larval and Pupal Stages of _Anopheles
maculipennis_. _Journal Hygiene_, Vol. 7, No. 2, April, 1907.
Morphology.

IMMS, A.D. On the Larval and Pupal Stages of _Anopheles
maculipennis_. _Parasitology_, Vol. I, No. 2, June, 1908.
Continuation of article in _Jour. Hyg._, Vol. 7, No. 2. Continues
discussion of morphology.

NUTTALL, GEO. F., CORBETT, LOUIS, AND STRANGEWAYS-PIG, T. Studies
in Relation to Malaria. Pt. I, The Geographical Distribution of
_Anopheles_ in Relation to the Former Distribution of Ague in
England. _Jour. Hyg._, Vol. I, No. 1, Jan., 1901.

NUTTALL, GEO. F., AND SHIPLEY, ARTHUR E. Studies in Relation to
Malaria. Pt. II, Structure and Biology of _Anopheles_, _Jour Hyg._,
Vol. I, No. 1, Jan., 1901: The Egg and Larva; Bibliography. Pt. II,
cont, Vol. I, No. 2, April, 1901: The Pupa. Pt. II, cont., Vol. I,
No. 4, Oct., 1901: Adult External Anatomy. Pt. II, cont., Vol. 2,
No. 1, Jan., 1902: Ætiology of Adult. Pt. II, cont., Vol. Ill, No.
2, April, 1903: Anatomy of Adult.

THOMPSON, MILLETT T. Alimentary Canal of the Mosquito. _Proc. Bost.
Soc. Nat. Hist._, Vol. 32, No. 6, 1905, pp. 145-202. Good summary
of recent investigations.

WESCHE, W. The Mouth-parts of _Nemocera_ and Their Relation to the
Other Families of _Diptera_. _Royal Microscopic Soc. Jour._, 1904,
pp. 28-47. Discussion with illustrations of the mouth-parts of
various _Diptera_.

MOSQUITOES--LIFE-HISTORY AND HABITS

AYERS, E.A. The Secrets of the Mosquito. A guide to the
extermination of the prolific pest. _World's Work_, 1907, Vol. 14,
pp. 8902-8910. Notes on life-history and methods of control.

JORDAN, E.O., AND HEFFERAN, MARY. Observations on the Bionomics of
_Anopheles_. _Jour. Infec. Diseases_, II, 1905, pp. 56-69.
Occurrence, breeding-places, habits, etc.

MORGAN, H.A., AND DUPREE, J.W. Development and Hibernation of
Mosquitoes. _Bull. 40_, N.S., _Div. of Ento._, pp. 88-92, 1903.
Results of observation on five genera of mosquitoes in the vicinity
of Baton Rouge, La.

ROSS, E.H. The Influence of Certain Biological Factors on the
Question of the Migration of Mosquitoes. _Jour. Trop. Med. & Hyg._,
12, 1909, pp. 256-258, Sept. 1. Only fecundated females feed on
blood, and must be fertilized after each batch of eggs. This
determines largely the time and place of breeding.

SMITH, J.B. Concerning Migration of Mosquitoes. _Science_, 18, Dec.
11, 1903, pp. 761-764. Observations on the migrations of
mosquitoes, particularly _C. sollicitans_.

MOSQUITO FIGHTING

CELLI, ANGELO. The Campaign Against Malaria in Italy. Transl. by
J.J. Eyre. _Jour. Trop. Med. & Hyg._, XI, Apr. 1, 1908, pp.
101-108. Includes a good discussion of the effectiveness of
destroying the mosquitoes in controlling malaria.

FELT, E.P. Mosquito Control. In _Report of the N.Y. State
Entomologist for 1905_, pp. 109-116. Notes on importance and
methods of control of various species.

GOLDBERGER, JOS. Prevention and Destruction of Mosquitoes. _Public
Health Reports, Pub. Health and Mar. Hospt. Ser._, July 17, 1908.
Life-histories and methods of fighting larvæ, pupæ and adults.

LE PRINCE, J.A. Mosquito Destruction in the Tropics. _Jour. Amer.
Med. Assn._, LI, p. 26, Dec. 26, 1908. Occurrence and habits of
_Anopheles_, methods of destruction. Results of anti-malarial work
on the isthmus. Discussion by various doctors.

QUAYLE, H.J. Mosquito Control Work in California. _Bull. No. 178,
Calif. Agric. Ex. Sta._, pp. 1-55, 1906. Habits and life-history of
California species, with an account of experiments to control the
salt-marsh species.

ROSENAN, M.J. Disinfection Against Mosquitoes with Formaldehyde and
Sulphur Dioxid. _Hyg. Lab. Pub. Health and Mar. Hospt. Ser., Bull.
6_, 1901.

ROSS, RONALD. Mosquito Brigades and How to Organize Them. New York,
1902.

ROSS, RONALD. Logical Basis of the Sanitary Policy of Mosquito
Reduction. _Science_, N.S., Vol. 22, No. 750, Dec. 1, 1905, pp.
689-699. Important article dealing with the methods of control.

SMITH, J.B. Salt-marsh Mosquitoes. _New Jersey Agric. Exper. Stn.
Special Bulletin T_, 1902. Breeding-places and methods of control
of this species.

SMITH, J.B. Mosquitocides. _Bull. 40, New Series U.S. Dept. Agric.,
Div. of Ento._, pp. 96-108, 1903. Results of experiments with a
number of substances, several of which were found to be effective
and some cheap enough to permit of their use to a limited extent.

SMITH, J.B. The New Jersey Salt-marsh and Its Improvement. _Bull.
No. 207_, Nov. 14, 1907, _New Jersey Agric. Exper. Stn._ Results of
draining the marshes to get rid of mosquitoes.

SMITH, J.B. The House Mosquito: a City, Town and Village Problem.
_N.J. Agric. Ex. Stn. Bull. 216_, 1908. Work done on salt-marshes
since 1904 practically eliminated the migratory species, so that
_C. pipens_, the house mosquito, is now the problem. Life-history
and methods of combating.

UNDERWOOD, W.L. Mosquitoes and Suggestions for Their Extermination.
_Pop. Sci. Mo._, Vol. 63, 1903, pp. 453-466. Life-history, habits
and methods of control.

UNDERWOOD, W.L. The Mosquito Nuisance and How to Deal with It.
Boston, 1903.

First Antimosquito Convention, 1903. Pub., Brooklyn, 1904. Contains
articles on what railroads, government and laws should do toward
mosquito extermination; mosquito work in Havana; how state
appropriations should be used, etc.

National Mosquito Extermination Society. Bulletin No. 1, 1904.
Object of Society; brief sketches of Ross, Reed, and others.
Reprints of a few articles on mosquito extermination.

American Mosquito Extermination Society. _Year Book for 1904-05._
N.Y., 1906. Containing reports of meetings and discussions of
various problems. Several interesting papers, among them "Criminal
Indictment of the Mosquito," F.W. Moss. "Mosquito Work at Panama
Canal," W.C. Sorgas. "Diversities Among New York Mosquitoes," E.P.
Felt. "Mosquito Extermination in New Jersey," J.B. Smith. "The
Mosquito Question," Quitman Kohnke.

Antimalarial Work in the Panama Canal Zone. Editorial in _Jour.
Trop. Med. & Hyg._, XI, Aug. 15, 1908, p. 251. Notes on the success
of the measures adopted there.

MOSQUITOES AND DISEASE

DOTY, A.H. The Mosquito, Its Relation to Disease and Its
Extermination. _New York State Journal of Medicine_, May, 1908.

FINLAY, CHAS. Mosquitoes Considered as Transmitters of Yellow Fever
and Malaria. _Med. Record_, May 27, 1899, pp. 737-739. Review of
his theory in regard to mosquitoes and disease and the probable
necessary changes in view of recent discoveries.

HOWARD, L.O. Mosquitoes as Transmitters of Disease. _Review of
Reviews_, XXIV, 1901, pp. 192-195. A review of the work of various
investigators.

SMITH, J.B. Sanitary Aspect of the Mosquito Question. _Medical
News_, Mar. 7, 1903. Note on mosquitoes and their relation to
disease.

TAYLOR, J.B. Observations on the Mosquitoes of Havana, Cuba.
Reprint from _La Revista de Medicina_, June, 1903, p. 27.

MALARIA

BANKS, C.S. Experiments in Malarial Transmission by Means of
_Myzomyia ludlowii_. _Phil. Jour. Sci._, B. 2, 1907, pp. 513-535.
Breeding-places of mosquitoes, life-histories of the species;
mosquitoes and malaria.

CRAIG, C.F. Malarial Fevers. _Osler's Mod. Med._, Vol. I, p. 392,
1907. Historical; distribution; malarial parasites; classification;
development; malarial mosquitoes; pathology; treatment, etc.

CRAIG, C.F. Studies in the Morphology of Malarial Plasmodia after
the Administration of Quinine and in Intracorpuscular Conjugation.
_Jour. Infec. Diseases_, VII, No. 2, 1910. See also same, IV, 1907,
pp. 108-140. Gives the evidence upon which he bases his theory of
the meaning of intracorpuscular conjunction.

CRAIG, C.F. The Malarial Fevers, Hemoglobinuric Fever and the Blood
Protozoa of Man. N.Y., 1909. A thorough consideration of the
subject of malaria and good discussion of the other subjects noted
in title. Bibliography.

DEADERICK, W.H. Malaria. Philad., 1909. The chapter on ætiology
treats of the transmission by mosquitoes.

HARRIS, S. Prevention of Malaria. _Jour. Amer. Med. Assn._, 53,
Oct. 9, 1909, pp. 1162-67. Effects of malaria, transmission by
mosquitoes, etc. In the discussion of the paper J.H. White
summarizes the fight against yellow fever in New Orleans.

HERRICK, G.W. Relation of Malaria to Agriculture and Other
Industries of the South. Economic losses occasioned by malaria;
malaria responsible for more sickness among the white population
than any other disease; relation to mosquitoes. _Pop. Sci. Mo._,
Vol. 62, Apr., 1903, pp. 521-525.

JONES, ROSS, ELLETT. Malaria. London, 1907. Small book,
introduction by Ross. Malaria in Greece and Italy; shows how this
disease contributed to the downfall of great nations.

MANNABERG, JULIUS. Malaria. In Nothnagel's _Encyclopedia of
Practical Med._, Amer. Ed., 1905, pp. 17-494. A very comprehensive
discussion of the disease and the relation of mosquitoes to the
malarial parasite.

MANSON, PATRICK. The Mosquito and the Malaria Parasite. _Brit. Med.
Jour._, Vol. II for 1898, pp. 849-853. History of the parasite in
the human and insect host; observations of Ross and others and
their meaning.

MANSON, PATRICK. Experimental Demonstration of the
Mosquito-malarial Theory. _Brit. Med. Jour._, Vol. 2 for 1900, pp.
949-951, also _Lancet_, II, 1900, pp. 923-925. Infected mosquitoes
sent from Rome allowed to bite men in England who had not been in
malarial regions. Malarial fever followed.

MANSON, PATRICK. Malarial Fever. Appendix to Vol. IX of T.C.
Albutt's _System of Med._, 1900. Relation of the malarial parasite
to the disease and to mosquitoes.

ROBERTSON, E.W. Renaming of Malaria--Anophelesis. _Va. Medical
Semi-monthly_, Sept. 10, 1909. Considers malaria a misnomer and
gives reasons for suggesting new name.

ROSS, RONALD. On Some Peculiar Pigmented Cells Found in Two
Mosquitoes Fed on Malarial Blood. _Brit. Med. Jour._, 1897, Dec.
18, p. 1786. Records in his experiments in feeding mosquitoes on
blood of malarial patients. Records finding the parasites in some
of them. Important article.

ROSS, RONALD. Pigmented Cells in Mosquitoes. _Brit. Med. Jour._,
1898, Feb. 26, p. 550. Further notes on them.

ROSS, RONALD. The Mosquito Theory of Malaria. Report dated
Calcutta, Feb. 16, 1899. Reprinted in _Pop. Sci. Monthly_, Vol. 56,
Nov., 1899, pp. 42-46. Tells of his investigations in India and
their results.

ROSS, RONALD. The Relationship of Malaria and the Mosquito.
_Lancet_, II, 1900, July 7, p. 4880. Observation on the
transmission of malaria.

ROSS, RONALD. Malaria Fever, Its Cause, Prevention and Treatment.
London, 1902. Chapters on malaria, mosquitoes, prevention and
treatment.

ROSS, RONALD. Parasites of Mosquitoes. _Jour. of Hyg._, VI, No. 2,
Apr., 1906. Brief review of several of his earlier papers on this
subject with additional notes.

SIMPSON, W.J.R. Recent Discoveries Which Have Rendered Antimalarial
Sanitation More Precise and Less Costly. _Brit. Med. Jour._, 1907,
II, pp. 1044-46. Discussion of the various factors in mosquito
control.

STEPHENS, J.W.W., AND CHRISTOPHERS, S.R. The Practical Study of
Malaria and Other Blood Parasites. London, 1908. Chapters on
mosquitoes, flies and ticks and their relation to diseases.

STERNBERG, G.M. The Malarial Parasite and Other Pathogenic
Protozoa. _Pop. Sci. Mo._, Vol. 50, 1897, pp. 628-641. Account of
the discovery of the malarial parasite and more recent studies on
it.

STERNBERG, G.M. Malaria. _Smith. Rept._, 1900, pp. 645-656. Review
of the experimental evidence in support of the mosquito-malaria
theory.

Malarial Fever. _Jour. Trop. Med. & Hyg._, II, Mar. 16, 1908, pp.
96-98. A list of literature mostly for the years 1906 and 1907.

YELLOW FEVER

ADAMS, S.H. Yellow Fever, a Problem Solved. The battle of New
Orleans against the mosquito. _McClure's Magazine_, Vol. 27, June,
1906, p. 178. An interesting popular article.

CARROLL, JAMES. Yellow Fever. _Osler's Mod. Med._, Vol. II, 1907,
p. 736. History, ætiology, treatment. A good review of the work of
the Yellow Fever Com. and the results of their work.

CARROLL, JAMES. The Transmission of Yellow Fever. _Amer. Med.
Assn._, 40, 1905, pp. 1429-33. Shows the relation of the mosquito
to the disease.

CARROLL, JAMES. Yellow Fever. Lessons to be learned from the
present outbreak of yellow fever. _Jour. of Amer. Med. Assn._, Vol.
45, 1905, pp. 1079-81. Among other things recommends that
mosquitoes be kept from patients.

CHAILLE, S.E. The _Stegomyia_ and Fomites. _Amer. Med. Assn._, 40,
1903, pp. 1433-40. Concludes that the mosquito is the only proven
disseminator of yellow fever. Extended discussion by various
physicians.

DASTRE, A. The Fight Against Yellow Fever. _Smith. Rept._, 1905,
pp. 339-350. History of the yellow fever epidemics, its
geographical distribution, and the work that is being done to
control it.

DOTY, A.H. On the Mode of Transmission of the Infectious Agent in
Yellow Fever and Its Bearing upon the Quarantine Regulations. _Med.
Record_, Oct. 26, 1901, pp. 649-653. Review of older theories in
regard to the spread of yellow fever. Believes that the quarantines
are now unnecessary.

FINLEY, CHAS. The Mosquito Theory of the Transmission of Yellow
Fever and Its New Development. _Med. Record_, Jan. 19, 1901. Refers
to his early observations on the subject, giving extracts from some
of his earlier papers to show that he had long held the mosquito
responsible for the dissemination of yellow fever.

GOLDBERGER, JOS. Yellow Fever, Ætiology, Symptoms and Diagnosis.
_Yellow Fever Inst. Bull. 16, Pub. Health and Mar. Hospt. Ser._,
1907. Includes discussion of the relation of mosquitoes to the
disease.

GUITERAS, JOHN. Experimental Yellow Fever at the Inoculation
Station of the Sanitary Department of Havana. _Amer. Med._, Vol.
II, No. 21, 1901, pp. 809-817. Experiments show that all types of
the yellow fever from mild to severe may be produced by the bite of
the mosquito.

MCFARLAND, JOSEPH. Life and Work of James Carroll. Memoir read at
the fifth annual meeting of the Soc. of Tropical Med., 1908. Early
life of Carroll and his work with the Yellow Fever Com.

PARKER, H.B., BEYER, G.E., AND POTHIER, O.L. Rept. of Working Party
No. 1, Yellow Fever Institute. _Bull. 13, Pub. Health and Mar.
Hospt. Ser._, 1903. As a result of their studies they believe that
the disease is caused by a protozoan parasite which they name and
describe. Discuss the relation of mosquitoes to the disease.

REED, WALTER; CARROLL, JAMES; AND AGRAMONTE, A. Experimental Yellow
Fever. _Amer. Med._, July 6, 1901, pp. 15-23. Records of certain
experiments and their results.

REED, WALTER; CARROLL, JAMES; AND AGRAMONTE, C.A. The Ætiology of
Yellow Fever. A preliminary note presented at the Amer. Pub. Health
Assn. _Philad. Med. Jour._, Oct. 27, 1900, pp. 790-796. Also an
additional note in _Jour. Amer. Med. Assn._, 36, pp. 431-440, 1901.
Records of their experiments and a summing up of the data in regard
to yellow fever and the mosquito.

REED, WALTER, AND CARROLL, JAMES. The Prevention of Yellow Fever.
_Med. Record_, Oct. 26, 1901, pp. 441-449. History of the disease,
especially in U.S., results of the work of Yellow Fever Com.
description, life-history and habits of the mosquito, its relation
to yellow fever, methods of control. Important paper.

REED, WALTER. Recent Researches Concerning the Ætiology,
Propagation and Prevention of Yellow Fever by U.S. Army Com. _Jour.
Hyg._, 2, 1902, pp. 101-119. Review of work of the Yellow Fever
Com. and the importance of the results. Bibliography.

ROSENAN, M.J., PARKER, H.B., FRANCIS, E., AND BEYER, G.E. Rept. of
Working Party No. 2, Yellow Fever Institute. Experimental studies
in yellow fever and malaria at Vera Cruz, Mex. _U.S. Pub. Health
and Mar. Hospt. Ser._, May, 1904. Includes experiments and
observations on mosquitoes.

ROSENAN, M.J., AND GOLDBERGER, JOS. Report of Working Party No. 3,
Yellow Fever Institute. _Yellow Fever Inst. Bull. 15, Pub. Health
and Mar. Hospt. Ser._, 1906. Unsuccessful attempts to grow the
yellow fever parasite. Negative results in the experimental study
of the hereditary transmission of the yellow fever in the mosquito.
Appendix A gives a translation of Marchoux and Simonds' report in
which they report positive results in their experiments along the
same line.

STERNBERG, G.M. Transmission of Yellow Fever by Mosquitoes. _Smith.
Rept._, 1900, pp. 657-673. Review of the early theories in regard
to yellow fever and the work and findings of the yellow fever
commission.

WHITE, J.H. Yellow Fever and the Mosquito. _Jour. Amer. Med.
Assn._, LI, No. 26, Dec. 26, 1908. Considers both _S. calopus_ and
_C. pungens_. Results of early mistakes. Necessity of destroying
mosquito. Methods of destroying mosquito. Habits of mosquito.

Abstract of the Report of the French Yellow Fever Com. at Rio de
Janeiro, 1903. _Pub. Health Report, Pub. Health and Mar. Hospt.
Ser._, Vol. 19, Pt. I, p. 1019. A summary of their findings and
conclusions to the date of report.

DE YBARRA, A.M.F. Yellow Fever Again in Cuba. _Jour. Trop. Med. &
Hyg._, XI, Mar. 2, 1908, pp. 73-78. Cites a number of cases of
yellow fever within the last few years and uses them as evidence
to show that the disease may be transmitted in other ways than by
the mosquito. A strong summing up of the arguments against the
mosquito theory. Reprint of editorial in _Tex. Med. Jour._, Oct.,
1907, also follows this article.

The Extinction of Yellow Fever at Rio de Janeiro. _Lancet_, II,
1909, p. 404. A review of a French publication giving the results
of the work from 1903 to present time. In 1903 before work was
begun there were 584 deaths from yellow fever. In 1908 only 4, and
none so far in 1909. Success accredited to mosquito work and
general sanitation.

A Pioneer in Research on Yellow Fever. Editorial in _Brit. Med.
Jour._, May 30, 1908, p. 1306. Refers to the work of L.D.
Beauperthuy, who, in 1853, set forth the theory that yellow fever
was transmitted by mosquitoes.

DENGUE

ASHBURN, P.M., AND CRAIG, C.F. Experimental Investigations
Regarding the Ætiology of Dengue Fever. _Jour. Infec. Diseases_,
Vol. V, 1907, pp. 440-475. Conclude that the disease is spread only
by mosquitoes.

COLEMAN, THOMAS D. Dengue. _Osler's Mod. Med._, Vol. II, 1907, p.
489. Ætiology, pathology, etc.; possibility of _Culex fatigans_
disseminating the disease.

GRAHAM, H. "The Dengue"; a Study of Its Pathology and Mode of
Propagation. _Jour. of Trop. Med. & Hyg._, July 1, 1903, p. 209.
Experiments which seem to show that dengue is transmitted by _Culex
fatigans_.

LEICHTENSTERN, O. Dengue. In Nothnagel's _Encyclopedia of Practical
Med._, Amer. Ed., 1905, pp. 720-743. Consideration of the disease
and its transmission.

ROSS, E.H. The Prevention of Dengue Fever. _Amer. Trop. Med. &
Parasit._, Vol. II, No. 3, July 1, 1908, pp. 193-195. A successful
campaign against the mosquitoes in Port Said in 1906 stopped the
outbreaks of malaria and dengue.

Dengue and Sand-flies. _Jour. Trop. Med. & Hyg._, 12, 1909, pp.
172-173. A note on these pages refers to the work of Dr. R. Doerr,
who suspects that dengue may be carried by sand-flies,
_Phlobotomus_, as well as by mosquitoes.

FILARIAL DISEASES AND ELEPHANTIASIS

CHRISTOPHERS, S.R. What Is Really Known of the Cause of
Elephantiasis. _Ind. Med. Gaz._, Nov., 1907, p. 404. Questions
Manson's theory in regard to the disease being caused by filaria.

MANSON, PATRICK. Tropical Diseases. London, 1908, pp. 594-648. A
most comprehensive chapter on filariasis and elephantiasis.

PHALEN, J.M., AND NICHOLS, H.J. Filariasis and Elephantiasis in
Southern Luzon. _Phil. Jour. Sci._, Sept., 1908. _Culex
microannulatus_ regarded as the carrier of the filaria.

PROUT, W.T. On the Rôle of Filaria in the Production of Disease.
_Jour. Trop. Med. & Hyg._, Apr. 1, 1908, p. 109. Discussion of same
in same journal, June 1, 1908.

WHITE, DUNCAN. Filarial Periodicity and Its Association with
Eosinophilia. _Jour. Trop. Med. & Hyg._, 12, July 15, 1909, pp.
175-183. Among other things he discusses the relation of mosquitoes
to filarial diseases.

LEPROSY

BRINCKERHOFF, W.R. A Note upon the Possibility of the Mosquito
Acting in the Transmission of Leprosy. _Pub. Health and Mar. Hospt.
Ser._ (general publications), 1908. Suggests the possibilities of
such transmission, but concludes that the probabilities are against
it.

GOODHUE, E.S. The Bacillus Lepræ in the Gnat and Bedbug. _Ind.
Med. Gaz._, Vol. XLI, Aug., 1906, p. 342. Has found this bacillus
in mosquitoes and bedbugs, but believes the latter is more
concerned in transmitting the disease.

GOODHUE, E.S. Mosquitoes and Their Relation to Leprosy in Hawaii.
_Amer. Med._, N.S., 2, 1907, p. 593. Suggests that mosquitoes may
carry the disease, also warns against danger from flies and
bedbugs.

HUTCHINSON, J. Mosquitoes and Leprosy. _Brit. Med. Jour._, Dec. 22,
1906, Vol. II, p. 1841. Evidence against the insect theory of
transmission of leprosy.

MUGLISTON, T.C. On a Possible Mode of Communication of Leprosy.
_Jour. Trop. Med._, Vol. VIII, July 15, 1905, p. 209. Suggests that
the itch-mite may be the carrier of leprosy. Studies on 77 lepers
led him to this conclusion.

SMYTH, W.R. Leprosy. _Brit. Med. Jour._, Dec. 8, 1906, Vol. II, p.
1670. Believes that bedbugs or some similar wingless parasite
conveys the disease.

PLAGUE

BRANNERMAN, W.B. Spread of Plague in India. _Jour. of Hyg._, Vol.
6, No. 2, Apr., 1906, pp. 179-211. A digest of experiments made in
India. Discusses various ways in which the disease may be spread.
Review of the evidence that insects may be concerned. Bibliography.

CALVERT, W.J. Plague. _Osler's Mod. Med._, Vol. II, 1907, p. 760.
History; bacteriology; pathology; plague among animals;
transmission, etc.

HAM, B. BURNETT. Report on Plague in Queensland, 1900-1907. P. 153
discusses the rat-flea theory of dissemination of bubonic plague,
summing up the evidence of various observers, including the Indian
Advisory Com. and others. Considers the evidence conclusive that
_P. cheopis_ and possibly _C. fasciatus_ transmit plague. Other
pages discuss various rat fleas and their relation to plague in
rats.

HANKIN, E.H. On the Epidemiology of Plague. _Jour. Hyg._, 5, 1905,
pp. 48-83. A comprehensive discussion of the disease and its
spread, several pages devoted to rats and fleas; evidence for and
against the theory that rats and fleas are the principal carriers
of the disease.

HERZOG, MAX. The Plague, Bacteriology, Morbid Anatomy &
Histopathology, Including the Consideration of Insects as Plague
Carriers. Biological Laboratory Bureau of Govt. Laboratories,
Manila, Oct., 1904. Reviews the evidence regarding the possibility
of fleas carrying plague; describes a new rat flea (_Pulex
philippinensis_); records experiments with fleas and cites a case
of bubonic plague in a child in which the infection was possibly
carried by _Pediculi_.

MCCOY, G.W. Plague Bacilli in Ectoparasites of Squirrels. _Pub.
Health Reports, Pub. Health and Mar. Hospt. Ser._, Vol. XXIV, No.
16, Apr. 16, 1909. Experiments with fleas and lice from infected
squirrels demonstrating presence of plague bacilli.

MCCOY, G.W. The Susceptibility of Gophers, Field-mice and
Ground-squirrels to Plague Infection. _Jour. of Infec. Diseases_,
Vol. 6, 1909, No. 3, pp. 283-288. Gophers highly resistant,
field-mice moderately susceptible and ground-squirrels very
susceptible to plague.

MITZMAIN, M.B. Insect Transmission of Bubonic Plague: a Study of
the San Francisco Epidemic. _Ento. News_, 19, No. 8, 1908, pp.
353-359. Fleas obtained in examination of 1,800 rats. Attempt to
locate source of rat and flea introduction.

MORTON, F.M. Eradicating Plague from San Francisco. Report of the
Citizens' Health Com. and an account of its work. San Francisco,
1909. Discusses the epidemics, methods of transmission, methods of
fighting, etc.

RUCKER, W.C. Plague Among Ground-squirrels in Contra Costa Co.,
Cal. _Pub. Health Reports, Pub. Health and Mar. Hospt. Ser._, Aug.
27, 1909. Reports of human cases supposed to be connected with
plague among ground-squirrels. Plague among squirrels; habits,
methods of fighting, etc.

RUCKER, W.C. Fighting an Unseen Foe. _Sunset Mag._, XXII, No. 2,
Feb., 1909. Story of the fight against plague in San Francisco.

SHIPLEY, A.E. Rats and Their Animal Parasites. _Jour. Eco.
Biology_, Vol. III, No. 3, Oct. 28, 1908. List of species of ecto-
and endoparasites.

SIMPSON, W.J. A Treatise on Plague. Cambridge Univ. Press, London,
1906. Deals with historical, epidemiological, clinical, therapeutic
and preventive aspect of the disease.

THOMPSON, J.A. The Mode of Spread and Prevention of Plague in
Australia. _Lancet_, Oct. 19, 1907, p. 1104. Rat fleas the
essential factor in transmitting plague, and preventive methods
should be directed against the rats.

THOMPSON, J.A. On the Epidemiology of Plague. _Jour. Hyg._, Vol.
VI, No. 5, Oct., 1906. Methods of infection, spread, relation of
rats to the disease and a review of the rat-flea theory.
Bibliography.

VERJBITSKI, D.T. The Part Played by Insects in the Epidemiology of
Plague. _Jour. Hyg._, 8, 1908, No. 2, pp. 162-208. Record of
extensive experiments with fleas. Fleas communicated plague for
three days, bedbugs for five days. Interrelation of fleas, rats,
dogs, cats, and man. An important article translated from Russian.

WHERRY, W.B. Further Notes on the Rat Leprosy and on the Fate of
the Human and Rat Leper Bacillus in Flies. _Jour. Infec. Diseases_,
Vol. 5, No. 5, 1908. Discussion and references, experiments with
flies, summary, etc. More than 1,115 lepra-like bacilli were
counted in a single fly-speck.

WHERRY, W.B. Plague Among the Ground-squirrels of California.
_Jour. Infec. Diseases_, Vol. 5, No. 5, 1908, pp. 485-533. How the
plague was first discovered among rats, records of cases and a
discussion of the possible relation of this to human plague cases.

Eradicating Plague in San Francisco; Report of the Citizens' Health
Committee, 1909. An account of the recent outbreaks and the methods
of fighting them.

Report of the Indian Plague Commission, Vol. V, pp. 75-77, 1901. In
these pages the Commission considers the question of the
transference of plague by suctorial insects. It considers Simonds'
claims and others and believes that "suctorial insects do not come
under consideration with the spread of plague."

Reports on Plague Investigations in India Issued by the Advisory
Committee Appointed by the Sec. of State for India, the Royal
Society and the Lister Institute. The reports include the reports
of the Working Commission appointed by the Advisory Committee and
reports on various contributory investigations. They are published
in the _Jour. of Hygiene_ as "Extra Plague Numbers." All these
reports deal very largely with the relation of the rat and flea to
plague, and are commonly referred to as "Reports of Indian Plague
Commission." The first number, Vol. VI, Sept., 1906, contains
articles on "Experiments upon the Transmission of Plague by Fleas."
"Note on the Species of Fleas Found on Rats, _Mus rattus_ and _Mus
decumanus_ in Different Parts of the World." "The Physiological
Anatomy of the Mouth-parts and Alimentary Canal of the Indian Rat
Flea, _Pulex cheopis_," and other papers on the relation of rats to
plague. The second number, Vol. VII, July, 1907, contains articles
on "On the Significance of the Locality of the Primary Bubo in
Animals Infected with Plague in Nature," "Further Observations on
the Transmission of Plague by Fleas with Special Reference to the
Fate of Plague Bacillus in the Body of the Rat Flea,"
"Experimental Production of Plague Epidemics Among Animals,"
"Experiments in Plague Houses in Bombay," "On the External Anatomy
of the Indian Rat Flea and Its Differentiation from Some Other
Common Fleas," "A Note on Man as a Host of the Indian Rat Flea,"
and others on the relation of rats to plague. The third number,
Vol. VII, Dec., 1907, contains articles on "Digest of Recent
Observations on the Epidemiology of Plague" (Bibliography),
"Epidemiological Observations in Bombay City," "Epidemiological
Observations in the Villages of Wadhala, Parel, Worli in the
Neighborhood of Bombay Village," "General Considerations Regarding
the Spread of Infection, Infectivity of Houses, etc., in Bombay
City and Island," "Epidemiological Observations in the villages of
Dhand and Kasel (Punjab)." The fourth number, Vol. VIII, May, 1908,
contains articles on "The Part Played by Insects in the
Epidemiology of Plague" (see also ref. under D.T. Verjbitski),
"Observations on the Bionomics of Fleas with Special Reference to
_P. cheopis_," "The Mechanism by Means of Which the Flea Cleans
Itself of Plague Bacilli," "On the Seasonal Prevalence of Plague in
India."