ANATOMY AND PHYSIOLOGY 



53 



rt 



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 

 fewer than four hundred thousand species of insects are already known. 



Typically, there are two pairs of wings, at- 

 tached 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 Strepsiptera 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. 



M tiller and Packard have regarded the wings 

 as tergal outgrowths; Tower, however, has 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 left mid leg of a cock- 

 and tergum will originate, though the wings may 3~*;iSV: 



subsequently shift to a more dorsal position. adc < adductor of coxa; ef, 



-J./5 ,. f -re,- T* i extensor of tibia; et, ex- 



ModlficatlOnS 01 WingS. Being Commonly tensor of femur;//, flexor 



more or less triangular, a wing presents three mar- of tlbl f>f ta > flexor of 



sus; rl, retractor of 



gins: front (costal), outer (apical) and inner (anal) ; sus. Afte 

 and three angles : humeral (at the base of the costa) , 

 apical (at the apex of the wing) and anal (between outer and inner 

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

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

 Orthoptera, 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 hemelytron. Diptera have, in place of the hind wings, a 

 pair of clubbed threads, known as balancers, or halteres, and male 



FIG. 68. Muscles of 



tar- 



MIALL and 



