528 PHILOSOPHY OF ZOOLOGY. 



those which have long, slender limbs, as the crane-flies. 

 Those which walk upon walls and trees are enabled to 

 overcome the resistance of gravitation by their claws and 

 the cushions, or suckers of their feet. By means of the last 

 sort of organs, the common house fly securely walks upon 

 the vertical glass of the window, or alights upon the ceiling 

 of the room. In walking, they advance the fore and hind 

 legs of one side, and the middle leg of the other side at the 

 same time, alternately. Those that leap much, have usual- 

 ly the femoral joints of the limbs enlarged, and the hind legs 

 are the longest. Among insects, the skip-jack (Elater) is 

 able to leap to a considerable distance by the elasticity of 

 the process of the thoracic sternum suddenly thrust into 

 the cavity of the pectoral sternum, and again withdrawn. 

 The elastic process of the Podura is attached to the tail, 

 and, when at rest, is bent under the body. By suddenly 

 unbending it, the body is thrown to a considerable dis- 

 tance. The flying insects do not possess rapidity of flight 

 proportional to the number or size of their wings. In the 

 coleoptera, the body hangs down during flight, while in 

 the other classes it preserves nearly a horizontal position. 

 In the tribes which swim, the legs are either flattened like 

 the blade of an oar, or produced and ciliated on the edges. 

 Some swim upon their back, others upon their belly. Some 

 keep always floating upon the surface, others dive and per- 

 form their movements at various depths, regulated by the 

 condition of the organs of respiration. 



The nervous system, in the class of ^animals now under 

 consideration, exhibits a greater uniformity of structure 

 and disposition than any of the other great systems of or- 

 gans which they possess. The brain is situate in the 

 head, on the dorsal surface of the gullet. It consists of 

 two lobes, which exhibit slight variations of form, are fre- 

 quently so intimately united, that they appear as one. 



