the development of the Euphorbia Hawk-moth. The
Sea Buckthorn Hawk-moth, for instance, even when full grown, is a
plain green, with only a trace of the line, and corresponds,
therefore, with a very early stage of the Euphorbia Hawk-moth; there
is another species found in South Russia, which has the line, and
represents the second stage of the Euphorbia Hawk-moth; another has
the line and the row of spots, and represents, therefore, the third
stage; lastly, there are some which have progressed further, and lost
the longitudinal line, but they never acquire the second row of spots
which characterizes the last stage of the Euphorbia Hawk-moth.

Thus, then, the individual life of certain caterpillars gives us a
clue to the history of the species in past ages.

For such inquiries as this, the larvae of Lepidoptera are particularly
suitable, because they live an exposed life; because the different
species, even of the same genus, often feed on different plants, and
are therefore exposed to different conditions; and last, not least,
because we know more about the larvae of the butterflies and moths than
about those of any other insects. The larvae of ants all live in the
dark; they are fed by the perfect ants, and being therefore all
subject to very similar conditions, are all very much alike. It would
puzzle even a good naturalist to determine the species of an ant
larva, while, as we all know, the caterpillars of butterflies and
moths are as easy to distinguish as the perfect insects; they differ
from one another as much as, sometimes more than, the butterflies and
moths themselves.

There are five principal types of coloring among caterpillars. Those
which live inside wood, or leaves, or underground, are generally of a
uniform pale hue; the small leaf-eating caterpillars are green, like
the leaves on which they feed. The other three types may, to compare
small things with great, be likened to the three types of coloring
among cats. There are the ground cats, such as the lion or puma, which
are brownish or sand color, like the open places they frequent. So
also caterpillars which conceal themselves by day at the roots of
their food-plant, tend, as we have seen, even if originally green, to
assume the color of earth. Nor must I omit to mention the
_Geometridae_, to which I have already referred, and which, from their
brown color, their peculiar attitudes, and the frequent presence of
warts or protuberances, closely mimic b