ANATOMY AND PHYSIOLOGY 57 



The leg muscles of a cockroach are shown in Fig. 66. 



Leaping. The hind legs, inserted nearest the center of 

 gravity, are the ones employed in leaping, and they act to- 

 gether. A grasshopper prepares to jump by bending the 

 femur back against the tibia; to make the jump, the tibia is 

 jerked back against the ground, into which the tibial spurs are 

 driven, and the straightening of the leg by means of the pow- 

 erful 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 movements 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 dif- 

 ferentiation, 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 insects are already 

 known. 



Typically, there are two pairs of wings, attached respec- 

 tively 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 Coccidse, though in male Stylopidae it is the posterior 

 pair, the fore wings being rudimentary. 



In bird lice, fleas and most other parasitic insects, the wings 

 have degenerated through disuse. In Thysanura and Collem- 

 bola 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 out- 

 growths; Tower, however, has recently shown that the wings 

 of Coleoptera, Orthoptera and Lepidoptera are pleural in ori- 

 gin, 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. 



