VI PREFACE 



breeding the house fly which is suitable for use in tropical countries ; 

 the only account of the anatomy of the body louse is so rare that 

 we have not, after two years' negotiations with book-sellers, succeeded 

 in obtaining a copy. 



Our object in writing this textbook has been to supply the needs 

 of our fellow-workers in these respects as far as possible ; to compile, 

 in fact, a guide to the study of the relations between arthropods and 

 disease, rather than a textbook on entomology. The general plan 

 of the work has been suggested by the difficulties with which we 

 ourselves have had to contend, and by the questions which have been 

 asked us by visitors to our laboratory. In so ill-defined a subject 

 it has often been difficult to decide what to include and what to 

 omit ; a work of this nature cannot hope to be complete, and, although 

 the book has grown in the making to be considerably larger than 

 what was at first projected, it has been found necessary to exclude 

 many forms of interest. At the same time it has been recognized that 

 too close a limitation to forms of known importance would be dis- 

 advantageous. In this subject, as in any other, a certain breadth of 

 view is essential to progress, and one has to remember that advance 

 has depended in the past, and will assuredly depend in the future, 

 to a large extent upon the study of non-pathogenic organisms, and 

 the subsequent application of the results so obtained to the study 

 of the related pathogenic forms. Very many blood-sucking arthropods 

 which have, so far as we know at present, -no connection with disease 

 have been dealt with, and for a similar reason special prominence 

 has been given to forms which act as the invertebrate hosts of the 

 non-pathogenic ' natural ' parasites. 



In the arrangement of the matter a regular sequence has been observ- 

 ed as far as possible, the general features of the group, its relation to 

 disease and its natural parasites, external anatomy, classification and 

 description of species, bionomics and breeding habits, methods of breed- 

 ing and laboratory manipulation, internal anatomy and methods of dis- 

 section, being discussed in turn, concluding with a list of papers dealing 

 with the group. To this rule there are a few exceptions. The second 

 chapter, dealing with the anatomy of the Diptera, is intended to serve 

 as an introduction to insect morphology, and matters discussed at length 

 therein are not referred to again in the anatomical portions of the sub- 

 sequent chapters ; the reader who has no previous experience of this kind 

 of work is recommended, even if he proposes to experiment with insects 

 gf another order, to read this chapter first and to make himself 



