light of birds is, therefore, theoretically
superior to that of insects. As to the division of the muscular force
between the resistance of the air and the mass of the body of the bird,
we should compare the exertion made in walking on sand, for example, as
compared with walking on marble. This is easy to measure. When a fish
strikes the water with its tail to propel itself forward, it performs a
double task; one part consists in pushing backwards a certain mass of
water with a certain swiftness, and the other in pushing on the body in
spite of the resistance of the surrounding fluid. This last portion of
the task only is utilized. It would be greater if the tail of the fish
encountered a solid object. Almost all the propelling agencies employed
in navigation undergo this loss of labor, which depends on the mobility
of the _point d' appui_. The bird is placed among conditions especially
unfavorable.

_The Senses of Insects._ The eyes of insects are sometimes so large as
to envelop the head like an Elizabethan ruffle, and the creature's head,
as in the common house fly, seems all eyes. And this is almost literally
the case, as the two great staring eyes that almost meet on the top of
the head to form one, are made up of myriads of simple eyes. Each facet
or simple eye is provided with a nerve filament which branches off from
the main optic nerve, so that but one impression of the object perceived
is conveyed to the brain; though it is taught by some that objects
appear not only double but a thousand times multiplied. But we should
remember that with our two eyes we see double only when the brain is
diseased. Besides the large ordinary compound eyes, many insects possess
small, simple eyes, like those of the spider. The great German
anatomist, Johannes Mueller, believed that the compound eyes were adapted
for the perception of distant objects, while those nearer are seen by
the simple eyes. But it may be objected to this view that the spiders,
which have only simple eyes, apparently see both near and remote objects
as well as insects.

The sense of touch is diffused all over the body. As in the hairs of the
head and face of man, those of insects are delicate tactile organs; and
on the antennae and legs (insects depending on this sense rather than
that of sight) these appendages are covered with exquisitely fine
hairs. It is thought by some that the senses of hearing and smell are
lodged in the antennae, these organs thus