DIPTEUA OR TUaE FLIES. 



217 



DIPTERA, 



OB Tbue Flies. 



The Diptera form the last order of insects with a complete 

 metamorphosis. They can easily be identified by the presence 

 of a single pair of wings — the posterior 

 or second pair being reduced to two 

 club-shaped processes called " balancers,'' 

 "halteres,'' or "poisers" (fig. 114). The 

 two anterior wings are iiat membranous 

 expansions, never completely covered with 

 scales, although they are present in mosquitoes (OuUcidcv) (fig. 

 115) and a few gall-flies [Gecidomyida'). Hairs are often present 

 on the wings, both upon and around them. Some Diptera, such 



Fig. 114.— Hai.tere of a 

 Fly. 



A Mosquito (Tlieohaldia annulala), S. 

 (Enlarged twic^) 



Head of a Mosquito (Piercing 



mouth). 



A, Hypopliarynx ; LI, lower lip ; Ul, 



upper lip ; Md, mandible ; Mx, maxilla ; 

 Lp, palpi. 



as the fleas (PuUcidce), fig. 148, and the Sheep-ticks (Melojyhaga), 

 fig. 146, are entirely apterous. In regard to the mouth parts of 

 Diptera there is also much variation : some have sucking mouths 



