A Book of Natural History / Young Folks' Library Volume XIV.

by Unknown

Fig. 8.—The Caterpillar of the Elephant Hawk-Moth (Chærocampa 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.

Fig. 9.—The Caterpillar of the Elephant Hawk-Moth (Chærocampa 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 concealed by day, it is by no means always the case that they are brown; some of them still retaining the green color. We may then conclude that the habit of concealing themselves by day came first, and that the brown color is a later adaptation. It is, moreover, interesting to note that while the caterpillars which live on low plants often go down to the ground and turn brown, those which feed on large trees or plants remain on the under side of the leaves, and retain their green color.

Thus, in the Eyed Hawk-moth, which feeds on the willow and sallow; the Poplar Hawk-moth, which feeds on the poplar; and the Lime Hawk-moth, which frequents the lime, the caterpillars all remain green; while in those which frequent low plants, such as the Convolvulus Hawk-moth, which frequents the convolvulus; the Oleander Hawk-moth, which feeds in this country on the periwinkle; and other species, most of the caterpillars turn brown. There are, indeed, some caterpillars which are brown, and still do not go down to the ground—as, for instance, those of the Geometridæ generally. These caterpillars, however, as already mentioned, place themselves in peculiar attitudes, which, combined with their brown color, make them look almost exactly like bits of stick or dead twigs.

The last of the five points to which I called your attention was the eye-spots. In some cases, spots may serve for concealment, by resembling the marks on dead leaves. In one species, which feeds on the hippophæ, or sea buckthorn, a gray-green plant, the caterpillar also is a similar gray-green, and has, when full grown, a single red spot on each side—which, as Weissmann suggests, at first sight much resembles in color and size one of the berries of the hippophæ. This might, at first, be supposed to constitute a danger, and therefore to be a disadvantage; but the seeds, though present, are not ripe, and consequently are not touched by birds. Again, in another caterpillar, there is an eye-spot on each segment, which mimics the flower of the plant on which it feeds. White spots, in some cases, also resemble the spots of light which penetrate foliage. In other instances, however, and at any rate in our Elephant Hawk-moth, the eye-spots certainly render the insect more conspicuous.

Now in some cases, this is an advantage, rather than a drawback. Suppose that from the nature of its food, from its being covered with hair, or from any other cause, a small green caterpillar were very bitter, or disagreeable or dangerous as food, still, in the number of small green caterpillars which birds love, it would be continually swallowed by mistake. If, on the other hand, it had a conspicuous and peculiar color, its evil taste would serve to protect it, because the birds would soon recognize and avoid it, as has been proved experimentally. I have already alluded to a case of this among the Hawk-moths, in a species which, feeding on euphorbia, with its bitter milky juice, is very distasteful to birds, and is thus actually protected by its bold and striking colors. The spots on our Elephant Hawk-moth caterpillar do not admit of this explanation, because the insect is quite good to eat—I mean, for birds. We must, therefore, if possible, account for these spots in some other way. There can, I think, be little doubt that Weissmann is right when he suggests that the eye-spots actually protect the caterpillar, by frightening its foes.