DIPT ERA. 



415 



larval skin. A few are enclosed in cocoons. When the 

 pupa state is passed within the last larval skin the body of 

 the pupa separates from the larval skin more or less com- 

 pletely ; but the larval skin is not broken till the adult fly 

 is ready to emerge. In this case the larval skin, which 

 serves as a cocoon, is termed a pupariuin (pu-paVi-um). In 

 some families the puparium retains the form of the larva ; 

 in others the body of the 

 larva shortens, assuming a 

 more or less barrel-shaped 

 form (Fig. 483, 2), before 

 the change to a pupa takes 

 place. 



This is a large order, 

 both in number of species 

 and individuals. The spe- 

 cies differ much in habits. 

 Some are very annoying to 

 man. Familiar examples 

 are the mosquito, which 

 attacks his person ; the 

 ilesh-flies, which infest his 

 food ; the bot-flies and gad- 

 flies that torment his cat- 

 tle ; and the gall-gnats that 

 destroy his crops. Other 

 species are very beneficial. 



TVinc;/- h^lnno-ino- fo thp Fig. 483.— The Apple-maggot; i. larva; 2, pupa- 

 1 nose OeiOnging rO tne j-ium; 3, aduU; la, head of larva from side, 



Syrphidse, and to the sub- 

 family Tachininse of the 

 Muscidae destroy many noxious insects; and very many 

 species, while in the larval state, feed upon decaying animal 

 and vegetable matter, thus acting as scavengers. 



Although the habits of these creatures, which revel in 

 all kinds of filth, are very disgusting, we cannot help admir- 

 ing that arrangement by which a mass of filth, instead of 



showing mouth-parts and cephalic spiracle ; 

 lb, head of larva from below ; ic, caudal 

 spiracle of larva. 



