;304 THE MECHANICAL FUNCTIONS. 



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 progression is impeded. The great prolongation 

 of the posterior legs is generally accompanied by 

 the power of jumping; imless, 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 sjmmaria 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 equiva- 

 lent to a man of ordinary stature vaulting through 

 the air the length of a quarter of a mile. When 

 the same insect is laid on glass, on which the spines 

 cannot fasten, it is unable to leap farther than six 



inches. t 



The insects belonging to the genus Elater are 

 provided with a peculiar mechanism for the special 

 purpose of accomplishing a singular mode of leap- 

 ing, independently of any action of the legs. The 

 legs of this insect are so short, that, when it is laid 

 on its back, it cannot turn itself, being unable to 



* This is particularly exemplified in the Sayra, the Chalcis, and 

 the Leucopsis, all which insects have the femora enormously deve- 

 loped and the tibiae much curved, and are thereby incapacitated 

 from jumping. 



t De Geer, iii. 178, quoted by Kirby and Spence. 



