302 THE MECHANICAL FUNCTIONS. 



§ 8. Progressive Motion of Insects on Land. 



The actions of the limbs of insects in walking are 

 quite different from what they are in sMimming, 

 and are very similar to those of the caterpillar, in 

 which we have seen that the motions of the ante- 

 rior 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 con- 

 sequence of their positions relatively to the trunk, 

 the anterior legs are advanced by the extension, 

 and the posterior legs by the flexion of the corre- 

 sponding joints. When the feet have fixed them- 

 selves 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. At other times 

 the order in which the fore and hind legs are 

 moved is similar to that which takes place in the 

 trot of quadrupeds : the left fore leg and the right 

 hind leg being advanced together : after which, the 

 right fore and left hind legs are advanced ; and last 

 of all, the middle legs, either simultaneously, or the 

 one after the other. 



The action of leaping is performed by the sudden 

 extension of all the joints of the limb, which are 

 previously folded as close as possible. The joints 



