se
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 poin