When the caterpillar lives on a plant like a grass, the ribs or veins of
which run up and down longitudinally, he is usually striped or streaked
with darker lines in the same direction as those on his native foliage.
When, on the contrary, he lives upon broader leaves, provided with a
midrib and branching veins, his stripes and streaks (not to be out of
the fashion) run transversely and obliquely, at exactly the same angle
as those of his wonted food-plant. Very often, if you take a green
caterpillar of this sort away from his natural surroundings, you will be
surprised at the conspicuousness of his pale lilac or mauve markings;
surely, you will think to yourself, such very distinct variegation as
that must betray him instantly to his watchful enemies. But no; if you
replace him gently where you first found him, you will see that the
lines exactly harmonise with the joints and shading of his native leaf:
they are delicate representations of the soft shadow cast by a rib or
vein, and the local colour is precisely what a painter would have had to
use in order to produce the corresponding effect. The shadow of
yellowish green is, of course, always purplish or lilac. It may at first
sight seem surprising that a caterpillar should possess so much artistic
sense and dexterity; but then the penalty for bungling or inharmonious
work is so very severe as necessarily to stimulate his imitative genius.
Birds are for ever hunting him down among the green leaves, and only
those caterpillars which effectually deceive them by their admirable
imitations can ever hope to survive and become the butterflies who hand
on their larval peculiarities to after ages.
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