LEAPING INSECTS.—SWIMMING AND FLYING. 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-wormwhilst 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 
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 25 0 
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 
Fig. 239. —Podura. 
