This process 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—as there are few now who do not—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, our 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 left 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 Hornet-clear-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 driver 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.”[[91]] 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 locusts, grasshoppers, and cockroaches have no chance of escape.”[[92]] This locust resembled 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 could 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 detail 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 belonging to the order Homoptera. Its length was about that of an ant carrying its leaf. The leaf was represented by the thin flattened body of the insect which in its dorsal part is so compressed laterally that it is no thicker than a leaf” (or as we would say, 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 line, 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.”[[93]]
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 the 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 selection 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. Meanwhile, 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.”[[94]] 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”;[[95]] 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 motionless, 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.”[[96]] 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 temper 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 startlingly accurate that I was, for a moment, completely deceived.”[[96]] A description follows which, as it is of that kind which deals longly 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 effect 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 startling; 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 suppose 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, therefore, assume that, on the whole, it is successful—to the extent, at any rate, of keeping the species in existence. In such matters, however, there is nothing like practical experiments, if one has the chance of making them, as the finder of the caterpillar in question must have had, since he says, “Unfortunately I was unable to test the efficacy of the disguise, for fear of losing the larva, which I was anxious to rear for the purpose of identification.”[[96]] To me this appears a false judgment. Such a test would have been much more interesting, surely, especially if resulting in the way anticipated, than a dry pinned specimen and a Latin name.
Another large snake-resembling caterpillar was found by Bates in the forests of Brazil, and the likeness was sufficiently striking to alarm several people to whom he showed it. But it is not necessary to go so far afield, for here in England, according to Professor Poulton, we have 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.[[97]] 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.”[[98]]