FLIES. 



403 



Oedbr VIII. — DIPTERA. 



If we take into account the number of individuals, with the number of species, 

 there is no order of insects so extensive as the Diptera, or two-winged flies. Among 

 them are found many that afEect man's economy or well-being very greatly. It is need- 

 less to point out some of these, such as the mosquito, house-fly, bot-fly, black-fly, etc., 

 but others, whose injury or benefit can hardly be estimated, are far less familiar to the 

 ordinary observer. Many, indeed most of the species, are small or inconspicuous, and 

 have but little, either in form or color, to attract the entomological collector. 



They may be distinguished almost invariably by the presence of a single pair of 

 wings, and never more. The second, or hind pair, corresponding to those in other 

 insects, are aborted into small organs, called the halteres or balancers. These have a 

 slender stem, terminating in a rounded head, and are constantly in vibration during 

 flight. Their function is unknown, though it has been thought that they are organs 

 of sense. The muscles for the front pair of wings are hence alone developed, and the 

 thorax is, in consequence, more globular, and composed chiefly of the mesothor^x. 

 The prothorax is confined to a ring or collar on the front part of the thorax, incon- 

 spicuous, or not visible from above. It may be most readily distinguished in the 

 Bibionidae and Tipulidae. The metathorax is yet more aborted and confined to the 

 lower and hind portions ; the so-called meta^ 

 notum has been shown by Hammond to really 

 belong to the mesothorax. The scutellum, cut 

 off by an impressed line, is an oval or semi- 

 oval portion on the hind part above. The 

 sides of the thorax, or pleurae, are seen to be 

 divided into irregular spaces by sutures, cor- 

 responding to the divisions in other insects, 

 each of which has received a name according 

 to its position. 



The legs, which generally are rather weak 

 or slender, but sometimes stout, are attached, 

 to the thorax, through the intervention of the 

 ring-like trochanters, by means of the conical, 

 sometimes elongated coxse. The femora, tibise, 

 and tarsi not infrequently show structural or 

 ornamental characters peculiar to one sex. 

 Such may consist of spurs, teeth, thickness, or 

 elongation, tufts, or discs of hairs, etc., and 

 are most usually confined to the male. The 

 tarsi are almost invariably five-jointed, the last of which terminates in two claws, 

 on whose under sides are usually two or three membranous appendages, called the 

 pulvilli, that serve as aids in climbing or grasping. 



The single pair of wings are rarely aborted or wholly wanting, as may be seen in 

 Chionea, a wingless genus of Tipulidas, or the sheep and bat ticks. More rarely they 

 are wanting in one sex only, while in the other they may be complete or rudimentary. 

 They are thin, membranous, and in some, as the common housefly, capable of extreme 

 rapidity of vibration, as many as three or four hundred a second. They are generally 



Fig. 508. — Sarcophaga camaria, common flesh-fly. 



