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