Some Protective Resemblances In Insects
( Originally Published Ealry 1900's )
The processes of eternal change, with the changes for the better surviving, and those for the worse dying out, is what is called natural selection, and if we understand it — we can understand this, that if any kind of creature is so strong and formidable that it would be an advantage for weaker creatures to be mistaken for it, then it is not at all unlikely that some of these weaker creatures will get more and more to resemble it, until at last they are so mistaken. For instance, the common wasps, who are armed with a formidable sting, and are very skilful in using it, are not attacked by any other insect, excepting hornets, which are not common. Any fly or moth, therefore, that resembles a wasp will be generally let alone, and the more so the more it resembles it. Accordingly we do find flies and moths that look very like wasps, and live safely in consequence. Still more would it be of advantage to look like a hornet, and there is a moth so like one that it is called the Hornetclear-wing.
On this same principle of being mistaken for something that is safe from attack or annoyance, all sorts of animals, and in a special degree insects, have come to look like various objects around them, and amidst which they live, such as stems of grass, pieces of moss or stick, leaves, flowers, and so on—some of the resemblances being more special and extraordinary. These are things which, though the eye may see, it does not as a rule dwell upon, because there are so many others round about. Who, for instance, would look at any particular blade of grass? So a bird that would pounce down upon an insect that it saw moving amongst the leaves of a tree, if there was no doubt that it was an insect, would not take any pains to examine what only looked like one leaf amongst many.
Thus, throughout nature we have these curious resemblances of certain creatures to certain other creatures, or to the plants or inanimate objects around them, but it is principally amongst insects that the phenomenon is met with, probably because they increase and multiply so quickly that there has been more time both for the laws of inheritance and for the great controlling one of natural selection to have come into play. Whatever is the reason, there is no doubt about the fact, which will be best illustrated by one or two salient instances. Ants, though they fall a prey to various animals larger than themselves—such as birds or ant-eaters—yet in their relations with other insects occupy a position of comparative safety, on account of their weapons and pugnacity, and, still more, of their numbers.
The river ants of equatorial Africa, and their South American representatives, the ecitons, are indeed, when they set out on their foraging expeditions, the terror, not only of insects, but of all animal life. " Wherever they move," says Bates, referring to the latter, " the whole animal world is set in commotion, and every creature tries to get out of their way." This, however, as they climb trees, and send out encircling columns which enclose a considerable extent of ground, is difficult, or rather, impossible, for all such as cannot fly some distance without alighting; for if an ant or two once seize upon them all is over. An insect, therefore, that cannot evade the onset of such an enemy is lucky if it has some such means of ensuring its safety as has been above referred to.
One at least thus specially favoured inhabits Nicaragua. " I was much surprised," says Mr. Belt, " with the behaviour of a green leaf-like locust. This insect stood immovably amongst a host of ants, many of which ran over its legs, without ever discovering there was food within their reach. So fixed was its instinctive knowledge that its safety depended on its immovability, that it allowed me to pick it up and replace it amongst the ants without making a single effort to escape. It might easily have escaped from the ants by using its wings, but it would only have fallen into as great a danger, for the numerous birds that accompany the army of ants are ever on the outlook for any insect that may fly up, and the heavy flying locust, grasshoppers, and cockroaches have no chance of escape."
This locust resembles a green leaf which, as we have seen, was a very protective resemblance indeed. It might, however, had it been a smaller insect, have resembled one of the ants themselves, and in that case would have run about with them, pretending or appearing to forage, also with perfect impunity. Whether the Eciton has such a double I know not, but various ants have. With some it is a spider that assumes their form. With others, as we have seen to a partial extent, a caterpillar, but the Sauba, or leaf-cutting ant—which is also the mushroom-growing one—is understudied, leaf and all, by an insect which, though the order to which it belongs has been determined, has not yet apparently received a name. "An example," says Professor Poulton, " of protective mimicry, which I believe to be more wonderful in its details and complexity than any which has been hitherto described, was observed and interpreted by my friend Mr. W. L. Sclater, in 1886, during his investigations in British Guiana. Mr. Sclater and his native servant had been collecting insects by shaking the branches of a tree over a sheet. The servant, although described as a very acute observer, saw an insect on the sheet which he mistook for one of the abundant Cooshie ants (perhaps the native name), carrying its little jagged segment of leaf over its back. Mr. Sclater looked more closely, and saw that it was an entirely different insect. Its length was about that of an ant carrying its leaf. The leaf was represented by the thin flattened body of the insect which along the back is no thicker than a leaf, and terminates in a sharp, jagged edge. The head and legs were brown, and suggested the appearance of that part of an ant which is uncovered by the piece of leaf. The jagged dorsal Iine, when seen in profile, evidently corresponds to the roughly gnawed edge of the fragment of leaf, for Mr. Sclater tells me that the contour of the latter is generally shaped by the mandibles of the ant rather than due to the natural margin."
The above-mentioned insect is a dweller in trees, and one might have supposed that a general resemblance to the leaves among which it moves would have been a sufficient protection for it. This probably was the beginning of the deception, which became more complex as time went on. In the leaf-like back of the insect we see probably the original disguise, but as the eyes of birds became more acute they began to pierce through it, more especially when the creature walked. Round about the would-be leaf, however, the leaf-cutting ants — distasteful to birds that so affected it—were constantly moving and walking. If only it could get to resemble one of these it might be as active as it pleased, and especially if its motions, as well as its appearance, became ant—or ant and leaf-like.
And this, indeed, was what gradually began to take place. Variation was always going on, and natural selections was always at hand to mould and shape its results. The two insects were, to begin with, of much the same size, and the general leaf-like appearance of the one was a good basis on which the more particular resemblance to the cut piece of leaf, carried by the other, might be founded. A few deeper washes of brown, some not very profound modifications of contour, and an ant-suggesting legs and body began to appear beneath it. Mean-while, however, hundreds of thousands —nay, millions--of bad or mediocre copies were swept away, the species became rarer and rarer—trembled, perhaps, on the verge of extinction; but just when it might have appeared to the birds, who were no longer able to obtain a once much-enjoyed morsel, that it really was extinct, it was saved; nature's object had been gained. A certain number of individuals were left, were close at hand even; even now, at that very moment, one might be crawling on the same twig where a despondent bird sat, only it was not to be distinguished from a leaf-cutting ant. Such are the ways of nature, such the slaughter that attends her victories.
In Borneo, and the Malay Archipelago generally, there is a pretty pink flower known as the " Straits Rhododendron." Once a gentleman was looking at one of these flowers and admiring it, when all at once it turned round and stared him in the face. It was not a flower, but a mantis ; its flattened legs—pink like them—made the petals; its abdomen, turned up over the back and held thus motionless, resembled an opening bud. " When I held the branch on which the insect had established itself in my hand I could not tell exactly where animal tissue commenced and where flower ended, so perfectly was the one assimilated to the other, both in colour and surface-texture." When once established on a flower this mantis would remain there quite motionless, if undisturbed, until it had occasion to leave it; and of course, in nature, had any insect settled on or near it, it would have instantly been seized.
The ways of the mantis are well known. " Under a most sanctimonious aspect," says Fabre, speaking of the little green one of Provence, " it hides the morals of a cannibal;" and, indeed, the female, which is larger and stronger than the male, will often turn upon the latter and devour it in the very midst of a love-passage. This it does, as in all other cases, by suddenly launching forward one or both of its fore-arms — which have been previously held in an attitude of prayer — and enclosing the body of the victim between their first and second segments, each of which is toothed along the edge like a saw. The double row of teeth meet in the body, which, held aloft, and writhing on either side of the trap, is devoured piecemeal by the mantis, who, with its sharp jaws, tears little mouthfuls out of it as long as it, or its appetite, lasts. This process, made more interesting by the way in which it was brought about, was witnessed in the case of the above-mentioned species.
Small flies frequently settled upon it as it sat motion-less, flower-like amidst flowers. " These it made no attempt either to drive off or to capture; its motions seeming rather to attract than repel them. After a short time a larger Dipteron, as big as a common house-fly, alighted on the inflorescence within reach of the predatory limbs. Then the mantis became active immediately; the fly was seized, torn in pieces, and devoured." Such are the real propensities of the seeming flower, and such, too, it may be observed, are those of some actual flowers — to wit, insectivorous ones.
To the Malays, however, whose minds are not yet open to the doctrines of protective or aggressive resemblance, or to evolution generally, this mantis is a flower, they " know not seems." The blossoms of " the sendudok " have become alive, and perhaps some analogies suggested by their own life-experience tempers their surprise at such an apparent change of disposition. They say, too, that few men ever see more than one flower-mantis in the whole course of their lives, so rare a creature is it. In this, no doubt, they are right; yet it would be possible, perhaps, even for a Malay to see several without knowing anything about it. Native eyes are almost always sharper and better than those of the Europeans who come amongst them; but, on the other hand, no native goes about like a modern entomologist, with his eyes specially open in one direction and the possibilities of protective resemblance in his mind.
The same naturalist, during the same expedition, was singularly delighted to secure a larva, whose resemblance to a snake was " so startling accurate that I was, for a moment, completely deceived." A description follows which, as it is of that kind which deals lengthily and learnedly in details without producing any particular general effect, may be left out. It would seem, however, that this caterpillar, like many others, has the power of withdrawing its actual head into a fold or two of its skin, which is here so marked that it performs the office of a mask, obscuring and taking the place of the real head thus obliterated. The mask is furnished with two spots, which at once become the creature's eyes, and both in colour, shape, and general appearance bear a remarkable resemblance to those of a snake; whilst a wrinkled fold, running back on either side from what appears to be the snout, suggests the mouth, and the flattened head with its characteristic arrangement of broad, flattened scales is also indicated by certain markings and colours on the required part of the caterpillar's body. An apparent head like this, thrown suddenly up as though threatening to dart forward with a hiss and distended jaws, might alarm anyone, and such a mock demonstration is evidently required to give full effects to the disguise. Thus we are told that " when the larva was moving about with the anterior segments well expanded the resemblance to a snake was not so start-ling; but directly it was touched the terrifying attitude was assumed, the anterior segments being drawn in and the front of the body turned towards the aggressor. When, at the same time, the hinder part of the body was hidden by leaves the deception became complete, and if effective enough to deceive, even temporarily, a human being, it must surely be equally effective in deterring less highly organised and timid, foes."
For the " timid " certainly, but for the " less highly organised " the conclusion does not seem so plain. No sight is better than a bird's, and it is practice that makes perfect in any particular direction. Still, unless we sup-pose the disguise to be accidental merely—and this no one with a knowledge of the whole subject can do—the object of it seems clearly apparent, and we may, there-fore, assume that, on the whole, it is successful--to the extent, at any rate, of keeping the species in existence.
Another large snake-resembling caterpillar was. found by Bates in the forest of Brazil, and the likeness was sufficiently striking to alarm several people to whom he showed it. In England, according to Professor Poulton, there is an excellent example of this kind of protective resemblance. This is no other than the caterpillar of the elephant hawk-moth, which by withdrawing its head into its body — just as does the Bornean species — produces a similar false face, with a pair, or, indeed, two pairs of fierce-looking eyes. This caterpillar feeds on the great willow herb, and when at rest keeps amongst the dead brown leaves at the base of the stem. " As soon," says Professor Poulton, " as the leaves are rustled by an approaching enemy, the caterpillar swiftly draws its head and the three first body-rings into the two next rings, bearing the eye-like marks. These two rings are thus swollen, and look like the head of the animal, upon which four enormous, terrible-looking eyes are prominent. The effect is greatly heightened by the suddenness of the transformation, which endows an innocent-looking and inconspicuous animal with a terrifying and serpent-like appearance."
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 cater-pillar. 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."
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 color 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 antenna. 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 antennae 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."
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 antenna, that anyone almost would be taken in. Whether, under this disguise, it enters wasps' nests and preys upon the larvae as the bee-like Volucella flies enter the nests of the bumble-bees they imitate, I do not know, nor, I think, does anyone, but this might very well be the case. These flies, how-ever, 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 waste products of their hosts, and even on their dead bodies should such opportunities arise. The bumble-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 around, 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."
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.
I really do not know whether this or another caterpillar cf South America be the more extraordinary, for if the one makes itself like something, the other makes something like itself. 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 cater-pillar. 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. 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.
Some of the most remarkable instances of protective resemblances amongst insects are exhibited by butter-flies, one perhaps, being the most perfect existing under nature. 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 honor, 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.