THE DIPTERA 303 



The thorax, though composed of three segments as usual, has these 

 very closely and firmly united. In the abdomen, the number of visible 

 segments varies from nine to five, or even four in some instances. The 

 legs, usually at least, are well developed, with a pair of claws at the tip 

 and a pulvillus at the base of each claw. Between the claws there is 

 often an membranous pad, similar to a pulvillus, or it may be a bristle. 

 In either case, this centrally placed structure is called an empodium. 



On the surface of the body, bristles are often present which have 

 definite positions and are of aid in identifying the species. 



Fly larvae are usually called maggots. Some have well developed 

 heads while in others no structure of this nature can be recognized. 

 True legs appear to be absent, though projections of the body which can 

 be utilized in moving about are common and often bear circlets of hooks. 

 These vary in their position in different species. The larvae breathe 

 through spiracles, but the location of these differs greatly. -In some they 

 are found along the sides of the body as usual; in others there is a pair 

 near each end of the body; in still others there is only one pair at the 

 hinder end, and these may occur at the tip of a very extensible tube 

 which, when fully stretched out, may be several inches long. Nourish- 

 ment is sometimes obtained by osmosis directly through the body wall 

 of the larva but it is generally taken into the mouth. The mouth parts 

 in the least modified forms are of the chewing type but in most members 

 of the order they are greatly modified. In some cases, a pair of claws or 

 hooks appear to be the only structures, while in others a chitinous "rake" 

 consisting of a cross-bar bearing a row of teeth and connected with a 

 single rod running backward, serves to rasp and break open the vegetable 

 cell walls and expose their fluid or semifluid contents of which the larva 

 avails itself. 



Some flies construct regular cocoons but the pupa is usually either 

 naked or located in a puparium which is the last larval skin. In this 

 case, the larva, when ready to pupate, shrinks away from its skin and 

 pupates within it, using this skin or puparium as a protection instead 

 of making a cocoon. Escape from the puparium may be either through 

 a T-shaped split on the back near the front end; a transverse split 

 between the eighth and ninth abdominal segments in a few cases; or 

 through a" circular opening in the front end. 



The chief distinctive characters of the Diptera are: 



Insects which when adult have, with a jew wingless exceptions, only two 

 wings, these attached to the mesothorax; the hind wings greatly modified, 

 each consisting of a small knob attached to the metathorax by a stalk, these 

 structures being called halteres; mouth parts for sucking, and sometimes 

 for -piercing also. The larvce are called maggots and are without true 

 legs. Metamorphosis complete. 



This is one of the large orders of insects and members of the group 



