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few of which are apparent except to a student of the Diptera — that 

 are directly or indirectly due to the presence of other forms. With 

 advance in a knowledge of the biology of the insects of this order 

 will come a realization that their injurious and beneficial effects are 

 practically balanced. 



ARRANGEMENT OF FAMILIES 



In agreement with the method generally used by systematists in 

 zoological work, the arrangement of the Diptera is in accordance with 

 the generally accepted theory of evolutionary development, and the 

 families are thus arranged as nearly as possible in their natural 

 sequence from lowest to highest, using as criteria the rather limited 

 data furnished by available life histories, and by a study of imaginal 

 characters ; but in endeavoring to trace affinities the reader must bear 

 in mind that the families included are but the tips of the evolved 

 branches, and not the entire genealogical tree. The hypothetical primi- 

 tive dipterous larva is assumed to have had a complete head with 

 horizontally moving mandibles, the head enclosing the first ganglion ; 

 three thoracic segments, the prothoracic with a pair of spiracles ; and 

 ten abdominal segments, the anterior seven, or more, with lateral 

 abdominal spiracles. No larva of this order has yet been discovered 

 which possesses true thoracic legs, but there are many species that 

 have pseudopods or sensory organs upon some of the thoracic seg- 

 ments. This anatomical feature is not accounted as pertaining to a 

 consideration of the phylogeny of the group, as pseudopods are gen- 

 erally regarded as of secondary importance, being developed, partially 

 developed, or absent, in species within the same family. 



The head is the best single unit available under all circumstances 

 for the purposes of classification, as it is wholly or in large part 

 chitinized and its component parts are accessible for examination eith- 

 er as a composite mass or after dissection, even in alcoholic material, 

 whereas the nerve ganglia and even the tracheal system of alcoholic 

 material are often indistinguishable, and in the case of exuvia entirely 

 lost. The modifications exhibited in the head of the different families 

 are remarkable, and though there are rather abrupt breaks in the chain 

 of ascent as we pass from the Nematocera, with their opposed mandi- 

 bles and complete or almost complete head-capsule, to the Brachycera, 

 with their vertical subparallel mandibles and much reduced head- 

 capsule, and, again, from the higher forms of this group to the 

 Cyclorrhapha, and particularly to the Muscidae, one can trace with 

 considerable probability the line of evolution up to the most highly 



