AX ATOMY AND PHYSIOLOGY 



45 



spurs are driven, and the straightening of the leg by means of the power- 

 ful extensors throws the insect into the air. At the distal end of the femur 

 are two lobes, one on each side of the tibia, which prevent wobbling move- 

 ments of the tibia. 



Wings. The success of insects as a class is to be attributed largely 

 to their possession of wings. These and the mouth parts, surpassing all 

 the other organs as regards range of differentiation, have furnished the 

 best criteria for the purposes of classification. The 

 wings of insects present such countless differences 

 that an expert can usually refer a detached wing 

 to its proper genus and often to its species, though 

 no less than three hundred thousand species of in- 

 sects are already known. 



Typically, there are two pairs of wings, attached 

 respectively to the mesothorax and the metathorax, 

 the prothorax never bearing wings, as was said. 

 When only one pair is present it is almost invariably 

 the anterior pair, as in Diptera and male Coccidae, 

 though in male Stylopidae it is the posterior pair, 

 the fore wings being rudimentary. 



In bird lice, fleas and most other parasitic in- 

 sects, the wings have degenerated through disuse. 

 In Thysanura and Collembola there are no traces 

 of wings even in the embryo; whence it is inferred 

 that wings originated later than these orders of 

 insects. 



Miiller and Packard have regarded the wings as 

 tergal outgrowths; Tower, however, has recently 

 shown that the wings of Coleoptera, Orthoptera 

 and Lepidoptera are pleural in origin, arising just 

 below the line where later the suture between the 

 pleuron and tergum will originate, though the wings 

 may subsequently shift to a more dorsal position. 



Modifications of Wings. Being commonly more or less triangular, 

 a w r ing presents three margins: front (costal), outer (apical) and inner 

 (anal). Various modifications occur in the front wings, which are in 

 many orders more useful for protection than for flight. Thus, in Orthop- 

 tera, they are leathery, and are known as tegmina; in Coleoptera they are 

 usually horny, and are termed elytra; in Heteroptera, the base of the front 

 wing is thickened and the apex remains membranous, forming a hemely- 



FIG. 66. Muscles of 

 left mid leg of a cock- 

 roach, posterior aspect. 

 abc, abductor of coxa; 

 adc, adductor of coxa; 

 ef, extensor of femur; 

 el, extensor of tibia; Jf, 

 flexor of femur; //, 

 flexor of tibia; fta, 

 flexor of tarsus; rt, 

 retractor of tarsus. 

 After MIALL and 

 DENNY. 



