1920] Metcalf: Diptera 181 



The Diptera all belong to the group variously known as 

 Endopterygota or Holometabola or insects with a complete 

 metamorphosis. Egg, larva, pupa and adult are very distinct 

 from each other and usually well separated both structurally 

 and ecologically. In fact, I have not been able to think of a 

 single case in which the two active stages, larvae and adults, 

 occupy the same habitat and utilize the same kind of food. 

 The nearest approach to it appears to be in some scavenger 

 forms in which the adults and larvas may both partake of the 

 decaying material — in somewhat different condition, however; 

 and the habitats of the two stages are very different. The 

 blood-sucking habit is common for the adults, but the larvae 

 of these species have different habits. Even in the case of the 

 obligate, parasitic, blood-sucking Hippoboscidae, the larvas are 

 nourished, not by the host directly, but within the parent fly. 

 And in the few cases where the larvae are blood-sucking (Auch- 

 meromyia) the adults appear not to do so. 



If one were to point to a single factor which has had most 

 influence upon the life cycle of the Diptera it would seem to 

 be the habits of the larvae. This, as we shall see, determines 

 to a large extent the habitat of the egg and it has likewise a 

 profound effect upon the specializations and adaptations of 

 the pupa stage and to a lesser degree upon the habits of the 

 adult, particularly upon oviposition. 



Certain prominent lines of specialization are familiar to 

 all. There are the aquatic species with their host of specializa- 

 tions and adaptations in egg-laying, for the maintenance of 

 the" egg in this precarious environment, for the locomotion, 

 defense and respiration of the larva; for the maintenance and 

 preservation of the pupa and for the emergence of the adult. 



There are the numerous parasitic species with the perfection 

 of the instincts for locating prey, many unique contrivances 

 for safeguarding the eggs, interesting adaptations which enable 

 the larva to reach its feeding grounds and to maintain itself 

 variously as an external, a subcutaneous, a gastric, an intestinal, 

 a nasal, an auricular or a vaginal parasite. 



There are scavengers of all degrees of specialization; there 

 are fruit flies and leaf miners and gall-making species and 

 borers in root and stem; each showing peculiarities not encoun- 

 tered in the other, groups and each, indeed, far removed from 

 what must have been the original structure and habit of the 

 ancestral forms. 



