^40 THE MECHANICAL FUNCTIONS. 



by the accumulated impulse, to a considerable height in the 

 air. The leaps of insects being generally forwards, all the 

 legs do not participate equally in the effect; for the fore legs 

 contribute much less to it than the hind legs, and are more 

 useful in modifying the direction of the leap, than in adding 

 to its force. The power of leaping is derived principally 

 from the great size and strength of the extensor muscles of 

 the legs, which, being contained within the femur, necessa- 

 rily swell that division of the limb to an 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, in- 

 deed, 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 an- 

 terior legs are much longer than the posterior, the power of 

 prehension may be increased, but that of progression is im- 

 peded. The great prolongation of the posterior legs is ge- 

 nerally accompanied by the power of jumping, unless, in- 

 deed, 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 w^hich they intend to spring, and which give 

 to the limb a steady fulcrum. The Cicada spitmaria 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 

 quarter of a mile. When the same insect is laid on glass, 

 on which the spines cannot fasten, it is unable to leap far- 

 ther than six inches.* 



The insects belonging to the genus Elciier are provided 



• De Geer, III. 178, quoted by Kii-by and Spence. 



