DIPTEKA OE TRUJS FLIES. 



195 



DIPTERA, 



OR Tbdb Flies. 



The Diptera form the last order of insects with a complete 

 metamorphosis. They can easily be identiiied 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,'' 

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

 two anterior wings are fiat membranous Fic- 'jo.— Halteee or Fly. 

 expansions, never covered with scales, 



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

 and a few gall-flies (Ccridumijidie). Hairs are often present on 



A Gnat {(.'ulex annnlaius). 



Head of Gnat (Pierc-ing mouth). 



A, Piercing lancet ; LI, lower lip : Ul, 

 uijper lip ; iVfd, mandible ; Mx, maxilla ; 

 Lp, labial palp. 



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

 the Fleas (Pulicidce), fig. 117, and the Sheep-ticks (Melophar/a), 

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



