d, and in some
specimens faint diagonal lines make their appearance. Some few assume
a brownish tint, but not many. A fourth moult takes place in seven or
eight days, and when the caterpillars are about an inch and a half in
length. Now, the difference shows itself still more between the two
varieties, some remaining green, while the majority become brown. The
eye-spots are more marked, and the pupil more distinct, the diagonal
lines plainer, while the white line is only indicated on the first
three, and on the eleventh segment. The last stage (Fig. 9) has been
already described.

[Illustration: FIG. 8.--THE CATERPILLAR OF THE ELEPHANT HAWK-MOTH
(_Chaerocampa elpenor_). Fourth Stage.]

Now, the principal points to which I wish to draw attention are (1)
the green color, (2) the longitudinal lines, (3) the diagonal lines,
(4) the brown color, and (5) the eye-spots.

[Illustration: FIG. 9.--THE CATERPILLAR OF THE ELEPHANT HAWK-MOTH
(_Chaerocampa elpenor_). Fifth Stage.]

As regards the first three, however, I think I need say no more. The
value of the green color to the young larva is obvious; nor is it much
less clear that when the insect is somewhat larger, the longitudinal
lines are a great advantage, while subsequently diagonal ones become
even more important.

The next point is the color of the mature caterpillars. We have seen
that some are green, and others brown. The green ones are obviously
merely those which have retained their original color. Now for the
brown color. This probably makes the caterpillar even more conspicuous
among the green leaves than would otherwise be the case. Let us see,
then, whether the habits of the insect will throw any light upon the
riddle. What would you do if you were a big caterpillar? Why, like
most other defenceless creatures, you would feed by night, and lie
concealed by day. So do these caterpillars. When the morning light
comes, they creep down the stem of the food plant, and lie concealed
among the thick herbage, and dry sticks and leaves, near the ground;
and it is obvious that under such circumstances the brown color really
becomes a protection. It might indeed be argued that the caterpillars,
having become brown, concealed themselves on the ground; and that we
were, in fact, reversing the state of things. But this is not so;
because, while we may say, as a general rule, that (with some
exceptions due to obvious causes) large caterpillars feed by night and
lie conceal