But though the prevailing color of caterpillars is green, there are numerous exceptions. In one great family of moths the prevailing color is brown. These caterpillars, however, escape observation by their great similarity to brown twigs—a resemblance which is heightened by their peculiar attitudes, and in many cases by the existence of warts or protuberances, which look like buds. Some, however, even of these caterpillars, when very young, are green. Again, some caterpillars are white. These feed on and burrow in wood. The Ringlet Butterfly also has whitish caterpillars, and this may at first sight appear to contradict the rule, since it feeds on grass. Its habit is, however, to keep at the roots by day, and feed only at night.
In various genera we find Black caterpillars, which are of course very conspicuous, and, so far as I know, not distasteful to birds. In such cases, however, it will be found that they are covered with hairs or spines, which protect them from most birds. In these species the bold dark color may be an advantage, by rendering the hair more conspicuous. Many caterpillars are black and hairy, but I do not know any large caterpillar which is black and smooth.
Brown caterpillars, also, are frequently protected by hairs or spines in the same way; but, unlike black ones, they are frequently naked. These fall into two principal categories: firstly, those which, like the Geometridæ, put themselves into peculiar and stiff attitudes, so that in form, color, and position they closely resemble bits of dry stick; and, secondly, those which feed on low plants, concealing themselves on the ground by day, and only coming out in the dark.
Yellow and yellowish-green caterpillars are abundant, and their color is a protection. Red and blue, on the contrary, are much less common colors, and are generally present as spots.
Moreover, caterpillars with red lines or spots are generally hairy, and this for the reason given above. Such species, therefore, would be avoided by birds. There are, no doubt, some apparent exceptions. The Swallow-tail Butterfly, for instance, has red spots and still is smooth; but as it emits a strongly-scented liquid when alarmed, it is probably distasteful to birds. I cannot recall any other case of a British caterpillar which has conspicuous red spots or lines, and yet is smooth.
Blue is, among caterpillars, even a rarer color than red. Indeed, among our larger larvæ, the only cases I can recall are the Lappets, which have two conspicuous blue bands, the Death’s-head Moth, which has broad diagonal bands, and two of the Hawk-moths, which have two bright blue oval patches on the third segment. The Lappets are protected by being hairy, but why they have the blue bands I have no idea. It is interesting, that both the other species frequent plants which have blue flowers. The peculiar hues of the Death’s-head caterpillar, which feeds on the potato, unite so beautifully the brown of the earth, the yellow and green of the leaves, and the blue of the flowers, that, in spite of its size, it can scarcely be perceived unless the eye be focussed exactly upon it.
The Oleander Hawk-moth is also an interesting case. Many of the Hawk-moth caterpillars have eye-like spots, to which I shall have to allude again presently. These are generally reddish or yellowish, but in this species, which feeds on the periwinkle, they are bright blue, and in form as well as color closely resemble the blue petals of that flower. One other species, the Sharp-winged Hawk-moth, also has two smaller blue spots, with reference to which I can make no suggestion. It is a very rare species, and I have never seen it. Possibly, in this case, the blue spots may be an inherited character, and have no reference to the present habits. They are, at any rate, quite small.
No one who looks at any representations of Hawk-moth caterpillars can fail to be struck by the peculiar coloring of those belonging to the Pine Moth, which differ in style of coloring from all other sphinx larvæ, having longitudinal bands of brown and green. Why is this? Their habitat is different. They feed on the leaves of the pinaster, and their peculiar coloring offers a general similarity to the brown twigs and narrow green leaves of a conifer. There are not many species of butterflies or moths which feed on the pine, but there are a few: and most, if not all of them, have a very analogous style of coloring to that of the Pine Moth, while the latter has also tufts of bluish-green hair which singularly mimic the leaves of the pine. It is still more remarkable that in a different order of insects we again find species—for instance one of the saw-flies—which live on the pine, and in which the same style of coloring is repeated.
Let us now take a single group, and see how far we can explain its various colors and markings, and what are the lessons which they teach us. For this purpose, I think I cannot do better than select the larvæ of the Hawk-moths, which have just been the subject of a masterly work by Dr. Weissmann, from which most of the following facts are taken.
The caterpillars of this group are very different in color—green, white, yellow, brown, sometimes even gaudy, varied with spots, patches, streaks, and lines. Now, are these differences merely casual and accidental, or have they a meaning and a purpose? In many, perhaps in most cases, the markings serve for the purpose of concealment. When, indeed, we see caterpillars represented on a white sheet of paper, or if we put them on a plain table, and focus the eye on them, the colors and markings would seem, if possible, to render them even more conspicuous; but amongst the intricate lines and varied colors of foliage and flowers, and if the insect be a little out of focus, the effect is very different.