Order DIPTERA. 



{Flics, Mosquitoes, Gnats, &c.) 



Char. Wings two, mesothoracic. Metathorax 

 furnished with a pair of knobbed threads, (halteres, 

 balancers, or poisers). 



Mouth-parts formed for sucking. 



Metamorphosis complete. 



In many species the wings are accompanied, at the base, by a small 

 wing-like membrane (alulet). The poisers are probably the homologues 

 of the hind wings of four-winged insects; and "are the most character- 

 istic organs of the order, being jjresent even when the wings themselves 

 are wanting." ^14, II., 499). 



The mouth-parts are typically composed of six setiform organs (lahjf A- 

 um, pair of mandibles, ))air of maxillae, and the lingua) partially enclosed 

 in a sheath (labium), and a pair of one or more jointed maxillary palpi. 

 Labial palpi wanting.* (14, II., 496, 498; I, 359). 



Larvae (maggots) fleshy, footless, cylindrical. In the majority of spe- 

 cies they have only two spiracles, which are situated at the posterior end 

 of the body. Pupae usually encased in the dried skin of the larvx, some 

 naked, others with a cocoon. The pupa; of the Culicida; are active. 



This is a large order, both in numl)er of species and individuals. The 

 species differ much in habits. Some are very annoying to man. Famil- 

 iar examples are the mosquito, which attacks his person; the flesh-flies, 

 which infest his food ; the bot-flies, and gad-flies, that torment his cattle; 

 and the gall-gnats, that destroy his crops. Other species are very benefi- 

 cial. Those belonging to the Syrphida;, and tlic genus Tachina, destroy 



* In the Pulicidx the maxilla: are small triangular scales, the labium is minute, and 

 furnished with three-jointed palpi. (14, II., 488). 



