iv 



Preface 



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 atten- 

 tion 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 ex- 

 tended 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. 



