SOME of the most remarkable instances of protective resemblances amongst insects are exhibited by butterflies, one, perhaps, being the most perfect existing under nature; however, I only say perhaps. This is the world-renowned leaf butterfly of Sumatra, and elsewhere in the Malay Archipelago. Of the great purple emperor family, it is purple on the upper surface, and gleams like a meteor as it shoots about in the rich, sun-bathed atmosphere of the tropics, its conspicuousness being enhanced by a sort of miniature, sharp-pointed swallow-tail, in which the hinder pair of wings end, and a broad, orange bar, like a sash or scarf of honour, running right across the anterior wings. It flies boldly and strongly, and when it descends upon a bush or shrub it is as though a little purple torch had shot through the foliage; but all at once, even though you see it come down just in front of you, it has vanished utterly—the torch has gone out. You may look and look, but unless you know the trick, and have seen the settling, and never taken your eyes off the exact spot, you will never find the butterfly, or see anything more of it until, all at once, it gleams in the air again. For the under part of the leaf butterfly’s lovely purple wings is like the leaf indeed—“the sere, the yellow leaf”—with a midrib running down the centre veinings on either side, a curled tip at the top, a stalk at the bottom, and everything proper to leaves, but not as a rule to butterflies. All four wings join in this effect, for being thrown up in the usual way when the insect settles, the leaf-like shape is thus brought about, one-half of the under surface being seen on each side in clear profile, whilst the purple now lies hid within, like the pictures on a folded screen. As for the body of the butterfly, that is hidden inside the wings too; the legs are all but invisible, and the two little pointed swallow-tails, just touching the plant’s stem with their mutual tip, make the stalk of the leaf. Even on the wall of a room or a curtain it would seem as though a dead leaf were sticking there; how much more when, as is always the case, the butterfly flies into some bush or thicket crowded with dry, brown leaves, and settles all amongst them. It is not that you don’t see it there that makes you miss it, but that you see it and scores of brown leaves all about it, every one of which looks just the same as itself.
PROTECTIVE MIMICRY
The picture at the top shows birds pursuing butterflies, while in the one below the same birds have lost their prey, as the butterflies have alighted and show only the underside of their wings, which are practically indistinguishable from the neighbouring leaves.
To make the matter plainer, in case this is not a very accurate description, here is the account of an eye-witness: “This species,” says Dr. Wallace, “was not uncommon in dry woods and thickets, and I often endeavoured to capture it without success, for after flying a short distance it would enter a bush among dry or dead leaves, and however carefully I crept up to the spot, I could never discover it till it would suddenly dart out again, and then disappear in a similar place. At length I was fortunate enough to see the exact spot where the butterfly settled, and though I lost sight of it for some time, I at length discovered that it was close before my eyes, but that in its position of repose it so closely resembled a dead leaf attached to a twig as almost certainly to deceive the eye, even when gazing full upon it.” Then follows a minute explanation of the imposture. “The end of the upper wings terminates in a fine point, just as the leaves of many tropical shrubs and trees are pointed, while the lower wings are somewhat more obtuse, and are lengthened out into a short, thick tail. Between these two points there runs a dark, curved line exactly representing the midrib of a leaf, and from this radiate on each side a few oblique marks, which well imitate the lateral veins. The tint of the under surface varies much, but it is always some ashy brown or reddish colour, which matches with those of dead leaves. The habit of the species is always to rest on a twig and among dead or dry leaves, and in this position, with the wings closely pressed together, their outline is exactly that of a moderately-sized leaf, slightly curved or shrivelled. The tail of the hind wings forms a perfect stalk, and touches the stick, while the insect is supported by the middle pair of legs, which are not noticed among the twigs and fibres that surround it. The head and antennæ are drawn back between the wings, so as to be quite concealed, and there is a little notch hollowed out at the very base of the wings, which allows the head to be retracted sufficiently. All these varied details combine to produce a disguise that is so complete and marvellous as to astonish everyone who observes it, and the habits of the insect are such as to utilise all these peculiarities, and render them available in such a manner as to remove all doubt of the purpose of this singular case of mimicry, which is undoubtedly a protection to the insect. Its strong and swift flight is sufficient to save it from its enemies when on the wing, but if it were equally conspicuous when at rest it could not long escape extinction, owing to the attacks of the insectivorous birds and reptiles that abound in the tropical forests.”[[104]]
Dr. Wallace then speaks of another closely allied species which is common in India, on the under surface of whose wings there are sometimes, to the boot of all that has been described, in the way of disguise, “patches and spots formed of small black dots, so closely resembling the way in which minute fungi grow on leaves that it is almost impossible, at first, not to believe that fungi have grown on the butterflies themselves.”[[104]] The minuteness of a resemblance like this is really very surprising, for it seems as though the butterfly-hunting bird or insect—some powerful wasp may represent the latter—was capable of minutely examining the object in question, and saying to itself, as it were, “I don’t think that can be a leaf, because there are no black spots upon it,” or vice versâ. In reality, however, it is no doubt the general effect, to which every detail contributes, that tells. What such resemblances do seem to me to show—and this, I think, is a new idea—is the accuracy and precision of some insects’ sight. How insects see things has long been a question, and many, I suppose, think it quite uncertain whether a leaf, for instance, throws the same picture on their retina that it does on ours. But if, to deceive them, the copy must be such that it also deceives us, is it not clear that it does? Otherwise the effect of the original could probably be reproduced by a less accurate copy. How little, after all, does the finest painting really resemble nature! The effect alone does so, not the means by which it is arrived at. Surely, then, if an insect, looking at a leaf or any other object, received but a general impression of colour, with an outline more or less blurred, or ill-defined, these copies of nature by nature—made to deceive—would bear witness to the fact. A study of protective resemblances is perhaps the best way of forming an idea as to how creatures, other than ourselves, see the world. It is even possible that such resemblances exist, which we, because we see things differently, are totally incapable of detecting.
I do not know if any other striking case of resemblance to an inanimate object (if plant life can be included under this term) is offered by the butterfly world, though there are several more of the same kind, but I cannot remember one just now. No doubt there are many which have not yet been discovered. We have, however, various instances of concealment even here in England, as, for instance, the peacock butterfly; but these, as well as special resemblances, are, for the most part, more marked in moths. The lappet moth, indeed, though it does not quite get the shape, looks very like a dead brown leaf, whilst in the buff-tip moth we are supposed to have a special resemblance to a piece of rotting wood, clothed with moss or lichen, and broken at each end. Personally, I have never received the impression of such a definite object, but only a general one of rot and decay. Even here, however, I do not believe I could ever be taken in, for the yellow head and tips of the wings, which are supposed to offer a perfect resemblance to the two broken ends of the piece of wood, are to me the tell-tale parts, and instantly cry out, “Moth!” In fact, soft as is the colouring of the buff-tip, it still seems to me a salient object, and I do not think very much of that bird’s eyesight who fails to detect it under anything like favourable circumstances.
Another moth that flies by day, and is not uncommon in the United States, bears, when sitting on a leaf, a much stronger resemblance to a bird-dropping, but in this not uncommon form of imitation moths, and all other insects, are outdone by spiders, who use it aggressively against them, and particularly, it would seem, against butterflies, as the following instances will show. Mr. Forbes once, whilst travelling in Java, saw a butterfly settled upon a bird-dropping. He watched it for some time, and then, wondering at its long stay, approached cautiously, and, slowly extending his hand, actually caught it by its wings, between his finger and thumb—no mean feat, as it seemed, yet there was nothing to boast of. As he lifted the butterfly only the wings came away, the rest of it staying with the supposed bird-dropping, which was now seen to be a spider, who, having caught the butterfly by means of this shameful imposture, was quietly occupied in eating it. The disguise in this case was of the most wonderful perfection. “Such excreta,” says Mr. Forbes, who discovered this one, “consist of a central and denser portion of a pure white, chalk-like colour, streaked here and there with black, and surrounded by a thin border of the dried-up, more fluid part.”[[105]] The appearance of each of these constituent parts was successfully counterfeited by the spider in question, who, in its own person, represented the more solid material, and spun the rest with its web.
As I know from early experience, when a naturalist makes a prize, all at once, of some interesting specimen, for some time afterwards he expects, or, rather, feels as if he would see some other on every leaf or twig; but time went by and no more of these “vain, delusive” spiders presented themselves. At length, years afterwards, the same naturalist found himself by the banks of the Moesi river, in Sumatra (which sounds much more interesting than the Thames, for instance), and this was his second experience. “I was,” he says, “rather dreamily looking on the shrubs before me, when I became conscious of my eyes resting on a bird-excreta-marked leaf. How strange, I thought, it is that I have never got another specimen of that curious spider I found in Java, which simulated a patch just like this! I plucked the leaf by the petiole while so cogitating and looked at it half-listlessly for some moments, mentally remarking how closely that other spider had copied nature, when, to my delighted surprise, I discovered that I had actually secured a second specimen, but the imitation was so exquisite that I really did not perceive how matters stood for several moments. The spider never moved while I was plucking or twirling the leaf, and it was only when I placed the tip of my little finger on it that I observed that it was a spider, when it, without any displacement of itself, flashed its falces into my flesh.”[[105]] (He means it bit him.)