372 



COMPARATIVE ZOOLOGY 



and lastly, the third left and second right together. 

 The front and hind pairs are, therefore, moved like 

 those of a quadruped. The insect has six legs, each 

 of five parts: the coxa ; trochanter ; femur; tibia, or 



shank ; and tarsus. 

 The last is sub- 

 divided usually into 

 five joints and a 

 pair of claws. 

 Such as can walk 

 upside down, as 

 the fly, have, in 

 addition, two or 

 three pads between 

 the claws. 138 These 

 pads bear hairs 

 which secrete a 

 sticky fluid by 

 means of which 

 the fly adheres to 

 the surface. While 

 the leg bones of 

 vertebrates are 

 covered by the muscles which move them, the limbs 

 of insects are hollow, and the muscles inside. The fore 

 legs are directed forward, and the two hinder pairs 

 backward. In motion, the fore and hind feet on one 

 side, and the middle one on the other, are moved simul- 

 taneously, and then the remaining three. 



The four-legged animals have essentially the same 

 apparatus and method of motion. The crocodile has 

 an awkward gait, owing to the fact that the limbs are 

 short, and placed far apart, so that the muscles act at 

 a mechanical disadvantage. The tortoise is proverbially 

 slow, for a similar reason. Both swim better than they 



FIG. 324. Feet of Insects, magnified: A, Bibiofebrilis; 

 B, House Fly (Musca domestic a) ; C, Water Beetle 

 (Dytiscus). 



