diate use.

Actual experimentation has taught that these animals can descry a man at
the distance of twenty or twenty-five feet. When approaching a crayfish
"town" for the purpose of making observations, I use the utmost caution;
otherwise, each inhabitant will retreat into its burrow before I can
come close enough to observe them, even with my field-glasses.

The gyrinus, or "whirligig beetle," whose dwelling-place during the
greater portion of its life is, like that of the crayfish, in ponds and
streams, has remarkably acute vision. This insect is a true cosmopolite,
however, and is as much at home on dry land as it is in the water. All
seasons seem to be alike to it, just so the sun shines; for, during the
hottest days of summer and the coldest days of winter (that is, if there
is sunlight and no ice on the water), it may be seen on the surface of
ponds and streams, gyrating hither and thither in a seemingly mad and
purposeless manner.

Several of these creatures will be seen at one moment floating on the
water, still and motionless; the next moment they will be darting here
and there over the surface of the water, their black and burnished backs
shining in the sunlight like brilliant gems. Suddenly, it is "heels up
and heads down," and they disappear beneath the surface, each of them
carrying a bubble of air caught beneath the wing-tips; or, as the late
William Hamilton Gibson expresses it, "they carry a brilliant lantern
that goes gleaming like a silver streak down into the depths, for a
bubble of air is caught beneath their black wing-covers, and a diamond
of pure sunlight accompanies their course down among the weeds until
they once more ascend to the surface."[14] This little beetle is well
provided with eyes, for it has a large pair beneath its head, with which
it sees all that is going on in the water below, and another pair on the
sides of its head, with which it keeps a bright lookout above. That it
has remarkably keen vision with the latter pair, any one who has tried
to steal upon them unawares can testify.[15]

[14] William Hamilton Gibson, _Sharp Eyes_, p. 307.

[15] I have a distinct purpose in introducing these and other
queer-eyed individuals while discussing the sense of sight. I wish to
demonstrate through one or more of them the correlation of morphology,
physiology, and psychology, as formulated in the first chapter of this
work. This is one of the most important facts in the doctr