20 INSECTS. 



diversity of development in the parts, and great modifi- 

 cation of form to suit the needs of various modes of life. 

 But, while we may sometimes trace a curious resemblance 

 to certain of the vertebrata in the functions, and even in 

 the external forms of these members, their fundamental 

 differences are as great as ever, and the adherence of 

 each to the principles of the separate plan or system to 

 which it belongs only becomes the more evident when 

 the same end is attained under various systems by means 

 always in accordance with those systems. Thus the 

 burrowing mole-cricket has a flattened, hand-like fore-leg, 

 which forcibly reminds us of the mole ; the grasshopper 

 has the large and springing thigh of a frog ; the water 

 beetle has fin-like legs : but in each of these, we find 

 that it is but a change in the proportion of the parts 

 which makes the difference between the legs of the cricket, 

 the grasshopper, the beetle, even as we found before that 

 the limbs of the vertebrata have one series of parts 

 variously modified.* 



Besides these six legs, the perfect insect is furnished 

 with two pairs of wings. The wings of a bird are, as has 

 been said, composed of the same bones as those which 

 form the forelegs, or arms, of other vertebrata, only under 

 a different proportion and development of parts. 



The wings of an insect are, on the contrary, an ap- 

 pendage of the breathing apparatus ; with this they are 

 closely connected, and it maybe supposed that it receives 

 assistance from them in the performance of its functions. f 



* This will be illustrated by figures in the second chapter. 



+ That a communication exists between the lungs and the wing bones 

 in birds, and that the acts of respiration and of flight affect each other in 

 these also, is a farther proof of analogy in function under a different plan 

 in structure. 



