With this caterpillar, since naturalists know what to call it, and there is no chance of its handing down any of their names in Latin to posterity, it has been possible to make experiments, and on the whole perhaps they have been in favour of the protective resemblance theory. The most interesting one—that I have read, that is to say—was made by Professor Poulton with a full-sized green lizard, and is thus described by him: “The lizard was evidently suspicious, and yet afraid to attack the caterpillar, which maintained the terrifying attitude in the most complete manner throughout. The lizard kept boldly advancing, and then retreating in fright; but at each advance it approached rather nearer to the caterpillar. After this had taken place many times and nothing had happened, the lizard grew bolder and ventured to gently bite what appeared to be the head of the caterpillar; it then swiftly retired, but finding that there was no retaliation it again advanced and gave it a rather harder bite. After a few bites had been given in this cautious manner, the lizard appeared satisfied that the whole thing was a fraud, and devoured the caterpillar in the ordinary manner.”[[98]] Professor Poulton has no doubt as to the lizard having been alarmed at first by the appearance of the caterpillar, and adds that he has never seen one act in the same way on any other occasion; other large hawk-moth caterpillars being eaten at once with entire sang-froid. It may be observed, however, that if every lizard were to act in the way recorded, under natural conditions, the advantage to the caterpillar would be nil, since though a species may survive through not being eaten, it certainly will not through being eaten with hesitation. And why should a lizard be more timid in the open air than in a box or a fern-case? Unless we assume, therefore, that this particular one was bolder than most others would be, the result of the experiment was not for, but against, the theory it was designed to test; and since we have no business to assume, the only thing to do is to get more caterpillars, and give them to more lizards. Small birds, however—and this in a country like England is more to the point—seem really to fear these pseudo-snakes to the extent of flying away from them.[[99]] But would an ordinary large caterpillar of the Sphingidæ—say, of a privet hawk or death’s-head moth—frighten them in the same way? If so, then again we are nowhere.
Perhaps a still more extraordinary instance of protective resemblance than any of the foregoing is that of a caterpillar which pretends to be an ant—one provided with an efficient sting, and of an irritable disposition. Here, as in the snake cases, it is by one portion of the body only that the fraud is perpetrated, but this, instead of being the front, is the hind part, in which, perhaps, it offers a unique example of the sort. The colour of the caterpillar is exactly that of the ant, and whilst its extremity represents the latter’s head, two black spots which are there situated bear an equally close resemblance to the eyes. The jaws are represented by the last pair of false legs or claspers, which are of disproportionate size, and can upon occasion be stretched widely apart, whilst a number of thin, tentacle-like processes, attached in pairs to the segments of the body, have all the appearance of an ant’s legs and antennæ. Armed with these properties, which, however, in a state of quiescence are not very recognisable, the caterpillar waits, as one may say, to have its feelings ruffled, when, by flinging the hinder part of its body into the air, each separate appurtenance begins at once to act the part assigned it, and the whole becomes a startling make-up. The head, with eyes, is jerked from side to side, the jaws gape, the legs move, the antennæ quiver, and an angry, threatening ant starts, as by magic, into being. “When,” says Mr. Annandale, “the caterpillar is seen in an end-on position, or when the anterior two-thirds of the body are hidden, the resemblance is positively startling,” so that “it is difficult to imagine how a lizard or a frog with a previous experience of the ant could fail to be deterred.”[[100]]
In the light of the above cases, that of the cuckoo-bees does not seem so very wonderful, since both the species are bees, and all or most of the members of any group or family of animals as a rule bear some resemblance to one another, since they descend from a common and not very remote ancestor. Many flies, however, have almost as close a resemblance to various bees and wasps, whilst one of the latter is even the model for a species of cricket, which would otherwise fall a victim to it and others of its family. There is a beetle, too, so like a wasp, not only in its appearance, but in the way in which it runs about and moves its antennæ, that anyone almost would be taken in. Whether, under this disguise, it enters wasps’ nests and preys upon the larvæ, as the bee-like Volucella flies enter the nests of the humble-bees they imitate, I do not know, nor, I think, does anyone, but this might very well be the case. These flies, however, now I come to think of it, do not really injure the bees. It used to be the idea that they did, but lately it has been discovered that they are only scavengers, feeding on all the waste products of their hosts, and even on their dead bodies should such opportunities arise.[[101]] The humble-bees, on their part, seem to appreciate these services, though we are not entitled to say that they admit the flies into their nests on this account, since they probably do so owing to their likeness to themselves.
Of the walking-stick insects, which are hardly to be distinguished, even with close attention, from the grass or twigs on which they cling, everyone has heard or read, and the caterpillars, common enough in England, which remain motionless, projecting like a twig from its stem, and looking just the same as one, are almost as good instances of unconscious deception. But neither these caterpillars, nor any of the other insects that have been mentioned, do anything, except through the attitudes they assume, to produce their wonderful disguises. They have nothing to do with the cutting out of the material. They do not dress up for the part themselves. That, however, is what some caterpillars do. There is one, for instance, in Borneo, that has a number of spines arranged in pairs down its back, and on each of these spines it fixes several little buds of the plant on which it is feeding, such buds, and not the leaves of the plant, being the actual food it eats. Consequently the caterpillar, which is quite a small one, looks like a spray of tiny buds itself, and can hardly, by possibility, be noticed amidst its flowery chaplet. The buds are not impaled on the spines, as might be supposed, but are attached to them with silk, which the caterpillar weaves for the purpose, and the whole process of the thing has been observed by the gentleman who gives the account, and who is no less competent a person than the curator of the Sarawak Museum. This is what he says: “A bud would be shorn off with the mandibles, then held in the two front pairs of legs, and covered all over with silk issuing from the mouth of the caterpillar. The caterpillar then twisted the front part of its body round, and attached with silk the bud to one of the spinous processes, and another bud would then be attached to this, and so on until a sufficiently long string—generally three or four buds—was made, when operations on another spine would be commenced. The caterpillar fed on these buds, scooping out the interior, and when not hurried, using the empty shells in preference to whole buds for its covering. When irritated it curled up, and remained thus for fifteen or twenty minutes. At other times it would sway about, looking like a branchlet blown by the breeze.”[[102]]
In time this caterpillar made “a silk cocoon covered with buds,” but it never turned into a butterfly, for ants attacked it, and its life was nipped in the bud. It appears to be a very rare caterpillar, and nobody knows what butterfly it belongs to, or what is its full Latin name. Since it is a Geometer, however, why not Geometer ignota under a sketch (as given in Nature, June 25th, 1903), in the cabinet—which would, in all cases, be the better plan?
I really do not know whether this or another caterpillar of South America be the more extraordinary, for if the one makes itself like something, the other makes something like itself. Anæa (sp?)—I give the name as I find it—is a little green caterpillar having a very funny nondescript sort of shape—as much like a little piece of gnawed-out leaf, left hanging to the midrib, as anything else. Such an object, however, is not one of the common ones of nature, and if it stood alone might be unrecognised or misinterpreted. The caterpillar, therefore, feeding along the midrib of the leaf, gnaws out a number of such little pieces, more or less like itself, and leaves them sticking upright along it, attached by a point or two. All the rest of the leaf at that part of the midrib, it apparently eats, or bites away, so that there remains only the slender, bare stalk, with several bits of leaf upon it, one of which is the caterpillar. To say which bit is he is now very difficult, and it looks as if none of them were. This caterpillar is, of course, green, like the leaf he feeds on, but he is not the same colour all over. He is light above and dark below, and this exactly suits—I have it on authority—the chiaroscuro of the situation, so that, both in light and shadow, he looks for all the world like a little elongated bit of green leaf attached to the midrib by a couple of stalks.[[103]] One would say, “Some caterpillars must have been eating that leaf”; but one would never think the caterpillar that had been eating it was still there.
CHAPTER XX
Butterfly resemblances—A living leaf—How spiders trap butterflies—Butterfly doubles—Suggested explanation—More evidence wanted—Warning coloration—A theory on trust—A straightforward test—Advice to naturalists—A strange omission.