PREFACE Vll 



familiar with the various organs by dissection of a few common types. 

 As far as possible those Diptera which are of practical importance 

 have been selected as examples for description and illustration, so 

 that, with the aid of the index, the chapter will also serve as a guide 

 to these forms. In the eleventh chapter practical laboratory methods, 

 which apply equally to any order of insects and to ticks, are dealt 

 with. It is not intended, of course, to provide a guide to laboratory 

 technique, but merely to indicate the deviations from the ordinary 

 methods which are necessary in dealing with insect tissues. It is not 

 claimed that the methods advocated are the ideal ones, but they are 

 those which have resulted from our own experience, and are, like most 

 practical methods, the outcome of mistakes and misfortunes. 



The external anatomy of the different groups, perhaps a very dry 

 subject, has been dealt with at some length. The reason for this is 

 that, rightly or wrongly, classification is founded almost entirely upon 

 external features, and unless the student has some familiarity with 

 these it is impossible for him to follow the systems of classification 

 or to use the keys for the identification of species. In most cases the 

 terminology used by authorities in systematic work has been retained, 

 even if it .is probably incorrect morphologically. For instance, the 

 terminology of the thorax of the mosquito as used by Theobald is not 

 altered either in the keys or descriptions, although, as was pointed 

 out some years ago by Nuttall and Shipley, it is not in accordance 

 with the anatomy of the parts ; similarly, the organ of the louse, 

 termed by Enderlein the pharynx, is referred to by that name in the 

 keys and descriptions taken from his work, although, as pointed out 

 in the account of the anatomy of that insect, it does not correspond 

 to the pharynx in other orders. It is not possible to ensure finality 

 in such matters, and the old terms are retained to avoid confusion. 



In classification the system of an authority in each group has 

 been followed ; where there are several to choose from, the selection 

 of one is to be taken as indicating merely a slight preference, and 

 does not imply a belief on our part that the alternative systems 

 are incorrect. For practical purposes one is often as good as another, 

 and only future research can show which will present the most natural 

 grouping. Such matters, we believe, can well be left to the specialist. 



Free use has been made of the keys to the genera and species 

 which have been compiled by specialists. Such keys, and partic- 

 ularly those which include only the species of a given locality, are of 

 the greatest service to the isolated worker who has no type collection 



