196 



METAMORPHOSIS AND SUBDIVISION OF DIPTERA. 



F;G. 422. LARVA, PUPA, AND IMAGO OF STRATJOMYS 

 CHAJI.'ELEON. 



the larva is found, at first apparently but little altered, except 



that it has become de- 

 tached from the skin, to 

 the inside of which the 

 organs peculiar to the 

 larva, such as the parts 

 of the mouth, remain 

 adherent. Shortly after- 

 wards, the inclosed being 

 assumes the form of a 

 soft and gelatinous mass, 

 in which none of the 

 parts of the future insect 

 remain visible ; some 

 days afterwards, how- 

 ever, these organs become distinct, and the insect has then assumed 

 the real state of pupa, though without having yet thrown off its 

 larva skin. When ready to escape, it scales off the anterior end 

 of its case, like a cap. Many larvae, however, do throw off their 

 skins when assuming the perfect state, and some form a regular co- 

 coon. The duration of life in the perfect state is usually very short. 

 725. In subdividing this Order, we first separate from it a 

 small but remarkable group, which forms the transition to the 

 more aberrant orders of the class, especially the Aphaniptera. 

 Some of them are entirely destitute of wings ; and yet in their 

 general structure they correspond with the Diptera. They are 

 distinguished from all other insects by their curious mode of 

 reproduction. Not only are the eggs hatched within the body 

 of the parent, but the Larvae are retained there until they have 

 been transformed into Pupae, in which state they come forth to 

 the world. Hence this section has received the name of PUPI- 

 PARA. It contains two families, all the species of which are 

 parasitic. The HIPPOBOSCID^:, sometimes called Forest Flies, 

 are of small size, covered with bristles, and frequently destitute 

 of wings. They are known by the French under the name of 

 Spider-Flies. They reside upon quadrupeds and birds, running 

 with great agility, and often sideways, burying themselves 



