Tropical Africa

by Henry Drummond

Mimicry is imposture in nature. Carlyle in his blackest visions of "shams and humbugs" among human kind never saw anything so finished in hypocrisy as the naturalist now finds in every tropical forest. There are to be seen creatures, not singly, but in tens of thousands, whose very appearance, down to the minutest spot and wrinkle, is an affront to truth, whose every attitude is a pose for a purpose, and whose whole life is a sustained lie. Before these masterpieces of deception the most ingenious of human impositions are vulgar and transparent. Fraud is not only the great rule of life in a tropical forest, but the one condition of it.

Although the extraordinary phenomena of mimicry are now pretty generally known to science, few workers have yet had the opportunity of studying them in nature. But no study in natural history depends more upon observation in the field; for while in the case of a few mimetic forms—the Heliconidæ, for example—the imitated form is also an insect, and the two specimens may be laid side by side in the cabinet at home, the great majority of mimetic insects are imitations of objects in the environment which cannot be brought into comparison with them in the drawers of a museum. Besides this, it is not only the form but the behavior of the mimetic insect, its whole habit and habitat, that have to be considered; so that mere museum contributions to mimicry are almost useless without the amplest supplement from the field naturalist. I make no further apology, therefore, for transcribing here a few notes bearing upon this subject from journals written during a recent survey of a region in the heart of Africa—the Nyassa-Tanganyika plateau—which has not yet been described or visited by any naturalist.

The preliminaries of the subject can be mastered in a moment even by the uninitiated, and I may therefore begin with a short preface on animal coloring in general. Mimicry depends on resemblances between an animal and some other object in its environment of which it is a practical gain to the creature to be a more or less accurate copy. The resemblance may be to any object, animate or inanimate. It may be restricted to color, or it may extend to form, and even to habit; but of these the first is by far the most important.

Apart from sexual selection, color in animals mainly serves two functions. It is either "protective" or "warning." The object of the first is to render the animal inconspicuous, the object of the second is the opposite—to make it conspicuous. Why it should be an object with some animals to be palpably exposed will be apparent from the following familiar instance of "warning" coloration. There are two great families of butterflies, the Danaidæ and Acraiedæ, which are inedible owing to the presence in their bodies of acrid and unwholesome juices. Now to swallow one of these creatures—and birds, monkeys, lizards, and spiders are very fond of butterflies—would be gratuitous. It would be disappointing to the eater, who would have to disgorge his prey immediately, and it would be an unnecessary sacrifice of the subject of the experiment. These butterflies, therefore, must have their disagreeableness in some way advertised, and so they dress up with exceptional eccentricity, distinguishing themselves by loud patterns and brilliant colorings, so that the bird, the monkey, and the rest can take in the situation at a glance. These animated danger-signals float serenely about the forests with the utmost coolness in the broadest daylight, leisureliness, defiance and self-complacency marking their every movement, while their duskier brethren have to hurry through the glades in terror of their lives. For the same reason, well-armed or stinging insects are always conspicuously ornamented with warning colors. The expense of eating a wasp, for instance, is too great to lead to a second investment in the same insect, and wasps therefore have been rendered as showy as possible so that they may be at once seen and as carefully avoided. The same law applies to bees, dragonflies, and other gaudy forms; and it may be taken as a rule that all gayly-colored insects belong to one or other of these two classes: that is, that they are either bad eating or bad-stingers. Now the remarkable fact is that all these brilliant and unwholesome creatures are closely imitated in outward apparel by other creatures not themselves protected by acrid juices, but which thus share the same immunity. That these are cases of mimicry is certain from many considerations, not the least striking of which is that frequently one of the sexes is protectively colored and not the other.

The brilliant coloring of poisonous snakes is sometimes set down by naturalists to "warning," but the details of coloring among reptiles have never been thoroughly worked out. The difficulty suggests itself that if the vivid yellows and oranges of some snakes are meant to warn off dangerous animals, the same conspicuousness would warn off the animals on which the venomous forms prey. Thus, while being hunted, a showy skin might be of advantage to the snake; in hunting it would be an equal disadvantage. But when one watches on the spot the manner in which snakes really do their hunting, it becomes probable that the coloring, vivid and peculiar as it is, in most cases is designed simply to aid concealment. One of the most beautiful and ornate of all the tropical reptiles is the puff-adder. This animal, the bite of which is certain death, is from three to five feet long, and disproportionately thick, being in some parts almost as thick as the lower part of the thigh. The whole body is ornamented with strange devices in green, yellow, and black, and lying in a museum its glittering coils certainly form a most striking object. But in nature the puff-adder has a very different background. It is essentially a forest animal, its true habitat being among the fallen leaves in the deep shade of the trees by the banks of streams. Now in such a position, at the distance of a foot or two, its appearance so exactly resembles the forest bed as to be almost indistinguishable from it. I was once just throwing myself down under a tree to rest when, stooping to clear the spot, I noticed a peculiar pattern among the leaves. I started back in horror to find a puff-adder of the largest size, its thick back only visible, and its fangs within a few inches of my face as I stooped. It was lying concealed among fallen leaves so like itself that, but for the exceptional caution which in African travel becomes a habit, I should certainly have sat down upon it, and to sit down upon a puff-adder is to sit down for the last time. I think this coloration in the puff-adder is more than that of warning, and that this semi-somnolent attitude is not always the mere attitude of repose. This reptile lay lengthwise, concealed all but a few inches, among the withered leaves. Now the peculiarity of the puff-adder is that it strikes backward. Lying on the ground, therefore, it commands as it were its whole rear, and the moment any part is touched, the head doubles backward with inconceivable swiftness, and the poison-fangs close upon their victim. The puff-adder in this way forms a sort of horrid trap set in the woods which may be altogether unperceived till it shuts with a sudden spring upon its prey.

But that the main function of coloring is protection may be decided from the simplest observation of animal life in any part of the world. Even among the larger animals, which one might suppose independent of subterfuge and whose appearance anywhere but in their native haunts suggests a very opposite theory, the harmony of color with environment is always more or less striking. When we look, for instance, at the coat of a zebra with its thunder-and-lightning pattern of black and white stripes, we should think such a conspicuous object designed to court rather than to elude attention. But the effect in nature is just the opposite. The black and white somehow take away the sense of a solid body altogether; the two colors seem to blend into the most inconspicuous gray, and at close quarters the effect is as of bars of light seen through the branches of shrubs. I have found myself in the forest gazing at what I supposed to be a solitary zebra, its presence betrayed by some motion due to my approach, and suddenly realized that I was surrounded by an entire herd which were all invisible until they moved. The motionlessness of wild game in the field when danger is near is well known; and every hunter is aware of the difficulty of seeing even the largest animals though they are just standing in front of him. The tiger, whose stripes are obviously meant to imitate the reeds of the jungle in which he lurks, is nowhere found in Africa: but its beautiful cousin, the leopard, abounds in these forests, and its spotted pelt probably conveys the same sense of indistinctness as in the case of the zebra. The hippopotamus seems to find the deep water of the rivers—where it spends the greater portion of its time—a sufficient protection; but the crocodile is marvellously concealed by its knotted mud-colored hide, and it is often quite impossible to tell at a distance whether the objects lying along the river banks are alligators or fallen logs.

But by far the most wonderful examples of protective adjustments are found where the further disguise of form is added to that of color, and to this only is the term mimicry strictly applicable. The pitch of intricate perfection to which mimicry has attained in an undisturbed and unglaciated country like Central Africa is so marvellous and incredible, that one almost hesitates to utter what his eyes have seen. Before going to Africa I was of course familiar with the accounts of mimetic insects to be found in the works of Bates, Belt, Wallace, and other naturalists; but no description prepares one in the least for the surprise which awaits him when first he encounters these species in nature. My introduction to them occurred on the borders of Lake Shirwa—one of the smaller and less known of the great African lakes—and I shall record the incident exactly as I find it in my notes. I had stopped one day among some tall dry grass to mark a reading of the aneroid, when one of my men suddenly shouted "Chirombo!" "Chirombo" means an inedible beast of any kind, and I turned round to see where the animal was. The native pointed straight at myself. I could see nothing, but he approached, and pointing close to a wisp of hay which had fallen upon my coat, repeated "Chirombo!" Believing that it must be some insect among the hay, I took it in my fingers, looked over it, and told him pointedly there was no "Chirombo" there. He smiled, and pointing again to the hay, exclaimed "Moio!"—"It's alive!" The hay itself was the Chirombo. I do not exaggerate when I say that that wisp of hay was no more like an insect than my aneroid barometer. I had mentally resolved never to be taken in by any of these mimetic frauds; I was incredulous enough to suspect that the descriptions of Wallace and the others were somewhat highly colored; but I confess to have been completely stultified and beaten by the very first mimetic form I met. It was one of that very remarkable family the Phasmidæ, but surely nowhere else in nature could there be such another creature. Take two inches of dried yellow grass-stalk, such as one might pluck to run through the stem of a pipe; then take six other pieces nearly as long and a quarter as thick; bend each in the middle at any angle you like, stick them in three opposite pairs, and again at any angle you like, upon the first grass stalk, and you have my Chirombo. When you catch him, his limbs are twisted about at every angle, as if the whole were made of one long stalk of the most delicate grass, hinged in a dozen places, and then gently crushed up into a dishevelled heap. Having once assumed a position, by a wonderful instinct he never moves or varies one of his many angles by half a degree. The way this insect keeps up the delusion is indeed almost as wonderful as the mimicry itself; you may turn him about and over and over, but he is mere dried grass, and nothing will induce him to acknowledge the animal kingdom by the faintest suspicion of spontaneous movement. All the members of this family have this power of shamming death; but how such emaciated and juiceless skeletons should ever presume to be alive is the real mystery. These Phasmidæ look more like ghosts than living creatures, and so slim are they that, in trying to kill them for the collecting-box, the strongest squeeze between finger and thumb makes no more impression upon them than it would upon fine steel wire, and one has to half-guillotine them against some hard substance before any little life they have is sacrificed to science.

I examined after this many thousands of Phasmidæ, Mantidæ, and other mimetic forms, and there is certainly in nature no more curious or interesting study. These grass-stalk insects live exclusively among the long grass which occurs in patches all over the forests, and often reaches a height of eight or ten feet. During three-fourths of the year it is dried by the sun into a straw-yellow color, and all the insects are painted to match. Although yellow is the ground tone of these grasses, they are variegated, and especially towards the latter half of the year, in two ways. They are either tinged here and there with red and brown, like the autumn colors at home, or they are streaked and spotted with black mould or other markings, painted by the finger of decay. All these appearances are closely imitated by insects. To complete the deception, some have the antennæ developed to represent blades of grass, which are often from one to two inches in length, and stick out from the end of the body, one on either side, like blades of grass at the end of a stalk. The favorite attitude of these insects is to clasp a grass-stalk, as if they were climbing a pole; then the body is compressed against the stem and held in position by the two fore-limbs, which are extended in front so as to form one long line with the body, and so mixed up with the stalk as to be practically part of it. The four other legs stand out anyhow in rigid spikes, like forks from the grass, while the antennæ are erected at the top, like blades coming off from a node, which the button-like head so well resembles. When one of these insects springs to a new stalk of grass it will at once all but vanish before your eyes. It remains there perfectly rigid, a component part of the grass itself, its long legs crooked and branched exactly like dried hay, the same in color, the same in fineness, and quite defying detection. These blades, alike with limbs and body, are variously colored according to season and habitat. When the grasses are tinged with autumn tints they are the same; and the colors run through many shades, from the pure bright red, such as tips the fins of a perch, to the deeper claret colors or the tawny gold of port. But an even more singular fact remains to be noted. After the rainy season, when the new grasses spring up with their vivid color, these withered-grass insects seem all to disappear. Their color now would be no protection to them, and their places are taken by others colored as green as the new grass. Whether these are new insects or only the same in spring toilets I do not know; but I should think they are a different population altogether, the cycle of the former generation being, probably, complete with the end of summer.

Besides the insects which imitate grass, another large class imitate twigs, sticks, and the smaller branches of shrubs. The commonest of these is a walking twig, three or four inches long, covered with bark apparently, and spotted all over with mould like the genuine branch. The imitation of bark here is one of the most perfect delusions in nature; the delicate striation and the mould spots are reproduced exactly, while the segmentation of the body represents node-intervals with wonderful accuracy. On finding one of these insects I have often cut a small branch from an adjoining tree and laid the two side by side for comparison; and when both are partly concealed by the hands so as to show only the part of the insect's body which is free from limbs, it is impossible to tell the one from the other. The very joints of the legs in these forms are knobbed to represent nodes, and the characteristic attitudes of the insect are all such as to sustain the deception.

A still more elaborate set of forms are those which represent leaves. These belong mostly to the Mantis and Locust tribes, and they are found in all forms, sizes, and colors, mimicking foliage at every stage of growth, maturity and decay. Some have the leaf stamped on their broadened wing-cases in vivid green, with veins and midrib complete, and with curious expansions over the thorax and along all the limbs to imitate smaller leaves. I have again and again matched these forms in the forest, not only with the living leaf, but with crumpled, discolored, and shrivelled specimens, and indeed the imitations of the crumpled autumn-leaf are even more numerous and impressive than those of the living form. Lichens, mosses, and fungi are also constantly taken as models by insects, and there is probably nothing in the vegetal kingdom, no knot, wart, nut, mould, scale, bract, thorn, or bark, which has not its living counterpart in some animal form. Most of the moths, beetles, weevils, and especially the larval forms, are more or less protected mimetically; and in fact almost the entire population of the tropics is guilty of personation in ways known or unknown. The lichen-mimicking insects even go the length of imitating holes, by means of mirror-like pools of black irregularly disposed on the back, or interrupting the otherwise dangerous symmetry of the fringed sides. The philosophy of these coal-black markings greatly puzzled me for a time. The first I saw was on a specimen of the singular and rare Harpax ocellaria, which had been thrown on the camp fire clinging to a lichen-covered log, and so well carried out was the illusion that even the natives were deceived till the culprit betrayed its quality by erecting its gauzy wings.