ANATOMY AND PHYSIOLOGY 57 
The leg muscles of a cockroach are shown in Fig. 66. 
Leaping.—The hind legs, inserted nearest the center of 
eravity, 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 Coccidee, though in male Stylopidee 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. 
Miller 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. 
