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. Whenever,
therefore, nature is able to provide two separate instruments for
two separate uses, without the one hampering the other, she does so,
instead of acting like a coppersmith who for cheapness makes a spit
and lampholder in one. It is only when this is impossible that she
uses one organ for several functions.
The anterior legs are in some cases longer than the others, that
they may serve to wipe away any foreign matter that may lodge on the
insect's eyes and obstruct its sight, which already is not very
distinct owing to the eyes being made of a hard substance. Flies and
bees and the like may be constantly seen thus dressing themselves with
crossed forelegs. Of the other legs, the hinder are bigger than the
middle pair, both to aid in running and also that the insect, when
it takes flight, may spring more easily from the ground. This
difference is still more marked in such insects as leap, in locusts
for instance, and in the various kinds of fleas. For these first
bend and then extend the legs, and, by doing so, are necessarily
shot up from the ground. It is only the. hind legs of locusts, and not
the front ones, that resemble the steering oars of a ship. For this
requires that the joint shall be deflected inwards, and such is
never the case with the anterior limbs. The whole number of legs,
including those used in leaping, is six in all these insects.
7
In the Testacea the body consists of but few parts, the reason being
that these animals live a stationary life
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