LEAPING INSECTS. SWIMMING AND PLYING. 499 



executed by the Beetles of the family of Elateridce ; the larva 

 of one species of which devours the roots of wheat, and is 

 known under the name of the 

 "wire -worm;" whilst other 

 species inhabiting tropical cli- 

 mates, and having the power 

 of emitting light, are termed 

 "fire-flies" ( 397). The legs 

 of these insects are very short : 

 so that when they are laid on Fig " 239 - 



their backs they cannot by means of them recover their 

 natural position. This they are enabled to do, however, by 

 their power of jerking backwards the head and upper part 

 of the thorax, which causes the body to be projected verti- 

 cally into the air, whence it usually descends with the feet 

 towards the ground. The leap of the Crickets, Locusts, Frog- 

 hoppers, &c. is executed more in a horizontal direction ; and 

 it is assisted by the wings, which bear-up the body whilst it 

 is moving onwards through the air. In this manner a Locust 

 can traverse 200 times its length, and a Frog-hopper 250 

 times ; which is as if a Man were to take a quarter of a mile 

 at one leap. 



663. Swimming and Flying are movements which have 

 much resemblance to each other ; both being executed in a 

 fluid medium, which to a certain extent buoys-up the body, 

 which offers resistance to its progress, and which also offers 

 something resembling a fixed point against which the mem- 

 bers may act to propel it. The chief differences between 

 them depend upon the nature of the medium ; this being 

 liquid in the one case, and aeriform or gaseous in the other. 

 The liquid medium affords more support to the body, and a 

 firmer surface for the action of its propelling organs ; but at 

 the same time it offers more resistance to its progress. The 

 movement of a body through the atmosphere, as in flight, 

 requires a considerable expenditure of power to keep it up ; 

 and the yielding nature of the element prevents the propelling 

 organs from acting against a firm surface ; but the onward 

 movement, in consequence of the slight resistance, is easily 

 accomplished. 



664. When the feet of a Quadruped are to serve both as 

 walking and swimming organs, the end is accomplished by 



K K 2 



