Chapter I: 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.