INSECTS 



pupae of the ancestors of such species lived in the same 

 medium as the larvae. , 



If our supposition is correct, we may see a reason for the 

 apparent exception in the flies to the general rule that the 

 pupa presents the adult structure and discards the pecu- 

 liarly larval characters. The pupae of some flies whose 



9' PS P 



•An 



Fig. 173. Larva (A) and pupa (B) of a horsefly, Tabanus puncli- 



fer (about 1 yi times natural sire) 

 An, anus; H, head; PSp, posterior spiracle; Sp t spiracle 



larvae live in the water, however, revert at once to the 

 adult system of lateral spiracles (Fig. 173 B, Sp). With 

 such species, the larva comes out of the water just before 

 pupation time and transforms in some place where 

 breathing is possible by the ordinary respiratory organs. 

 This is the general rule with other insects whose larvae 

 are aquatic. 



The order of the Diptera is a large one, and we might 

 go on indefinitely describing interesting things about flies 

 in general. Such a course, however, would soon fill a larger 

 book than this; hence, since we are already in the last 

 chapter, a more practical plan will be to select for special 

 consideration a few species that have become closely as- 

 sociated with the welfare of man or of his domesticated 

 animals. Such species include the mosquitoes, the house 

 fly, the blowfly, the stable fly, the tsetse fly, the flesh flies, 

 and related forms. 



[328] 



