340 THE MECHANICAL FUNCTIONS. 



unusual thickness ; and in order to procure 

 sufficient velocity of action, both the femur and 

 tibia are much elongated. Thus the locust, which 

 is so constructed, leaps with ease to a distance 

 two hundred times the length of its own body. 

 We may in general, indeed, infer the particular 

 kind of progressive motion for which the insect 

 is intended by observing the comparative length 

 of the different pairs of legs. When they are 

 of equal size, the pace is uniform : — swiftest in 

 those that have the longest legs, — slowest when 

 they are short. When the anterior legs are 

 much longer than the posterior, the power of 

 prehension may be increased, but that of pro- 

 gression is impeded. The great prolongation of 

 the posterior legs is generally accompanied by 

 the power of jumping, unless, indeed, they are 

 at the same time much bent, for such curvature 

 disqualifies them from acting advantageously as 

 levers. 



Many insects have the extremity of the tibia 

 armed with a coronet of spines, which assist in 

 fixing this point against the plane from which 

 they intend to spring, and which give to the 

 limb a steady fulcrum. The Cicada spumaria 

 has been known to leap to a distance of five or 

 six feet ; which is two hundred and fifty times 

 its own length : this, if the same proportions 

 were observed, is equivalent to a man of ordinary 

 stature vaulting through the air the length of a 



