CHAPTER XIII. 



THE DIPTERA — FLIES. 



There are only two large and well developed wings, which 

 are membranous and without scales, in the Diptera ; the second 

 pair are quite rudimentary, and form small projections of wing- 

 membrane, which are called balancers or poisers. In the common 

 gnat these poisers are like simple foot-stalks surmounted by a 

 round knob, and are attached one on each side to that segment 

 of the body which ought to sustain a large hind wing. This 

 is their form in the house fly. They are generally more or 

 less concealed beneath the winglets, which are little appendages 

 to the fore wings. The excessive size of that segment of the 

 body which bears the great proper wings of the Diptera, the front 

 pair, appears to render a diminution in the size of the segment 

 which ought to maintain the hind wings inevitable. 



The Diptera are sucking insects, but their suckers are formed 

 differently to those of the Heniiptera or th i ApJianiptera. More- 

 over these important organs of the mouth differ greatly in many 

 genera of the flies themselves. These insects undergo complete 

 metamorphoses ; the larvje are ordinarily vermiform and legless, 

 are known as maggots, and usually possess considerable activity, 

 and can crawl backwards and forwards. They do not require 

 maternal assistance, and are usually placed so that their food is 

 close at hand. The nymphs of the Diptera differ greatly; some 

 of them are active, although they take no nourishment ; some are 

 inactive, and quite as much so as the chrysalides of the Lepi- 

 doptera or the nymphs of Coleoptera, and others which are per- 

 fectly immobile, do not lose the skin belonging to the larvae ; these 



