338 THE MECHANICAL FUNCTIONS. 



semblance to the oars of a boat ; and they act, 

 indeed, in the same manner, and on the same 

 principles. 



8. Progressive Motion of Insects on Land. 



THE actions of the limbs of insects in walking 

 are quite different from what they are in swim- 

 ming, and are very similar to those of the cater- 

 pillar, in which we have seen that the motions of 

 the anterior and posterior legs on one side are 

 combined with that of the middle one on the 

 other side ; and the two sets of legs are moved 

 alternately. In consequence of their relative 

 positions with the trunk, the anterior legs are 

 advanced by the extension, and the posterior 

 legs by the flexion of the corresponding joints. 

 When the feet have fixed themselves on the 

 ground, the contrary actions take place, and the 

 body is brought forwards. During this period 

 the legs which compose the other set are called 

 into play, and are advanced ; and the same 

 succession of actions takes place with these as 

 with the former. This can easily be seen when 

 the insect walks very leisurely ; but in a more 

 quickened pace, the succession of actions is too 

 rapid to be followed by the eye. 



The action of leaping is performed by the 



