DIPTEKA OK TRUE FLIES. 



195 



DIPTEKA, 



OR Teue Flibs. 



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 only — the pos- 

 terior or second pair being reduced to two !&■- -'^^^^ 

 club-shaped processes called " balancers," -^^ ^^ -if 

 "halteres," or "poisers" (fig. 90). The 



two anterior wings are flat membranous Fig. 90.— Haltebe of Ply. 

 expansions, never covered with scales, 



although they are sparsely present in gnats {Gulieidm) (fig. 91) 

 and a few gall-flies [Oecidomyidm). Hairs are often present on 



Fio. 91. 



A Ghat (Cidex antiulotus). 



Head or Gnat (Piercing mouth). 



A, Piercing lancet ; U, lower lip ; Ul, 

 upper lip ; Md, mi^ndible ; Mx, maxilUi ; 

 I^, labi^ palp. 



the wings, both upon and around them. S^me Diptera, such as 

 the Fleas (Pidicidm), fig. 117, and the Sheep-ticks (Melophaga), 

 fie. 116, are entirely apterous. In regard to the mouth parts of 



