The Encyclopedia Americana (1920)/Mimicry in Animals

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MIMICRY IN ANIMALS. One of the ways by which species and groups of animals are perpetuated. Its effect is to assure the safety of individuals by reason of their having such a likeness to something else that the eye of another animal seeking to do them harm overlooks them, by mistaking their real nature for the object mimicked, so that the enemy passes on. This is one of the phases of “protective resemblance,” of which the two principal are: (1) Mimicry, similarity in form to some inanimate object; (2) Imitation, similarity to another animal that for some reason is immune from attack by the enemies of the imitating species. In both cases color is likely to serve as an important element in the practical deception. See Coloration, Protective; Imitation.

These two phases, and other manifestation of deception for safety's sake, are regarded by the exponents of the Darwinian theory of organic evolution as results of the gradual and unconscious acquirement, through the process of natural selection, of changes advantageous to the animal in its struggle for existence. The present article considers such beneficial results as have been obtained by acquiring by certain animals a resemblance to some vegetable or inanimate article of no interest or value to the enemy.

It is plain that if a moth or a caterpillar sitting on a tree-trunk can be mistaken by a bird or lizard for a bit of lichen or a dried twig it may frequently be passed by, and thus allowed to live long enough, perchance, to perpetuate its species. This happens actually in nature. Mimicry is developed, however, almost exclusively in small creatures that are otherwise defenseless; and sometimes is restricted to the young of a species whose individuals when adult can either take care of themselves well or develop some special method of concealment or passive resistance. It also occurs in a few plants.

It is mainly exhibited by insects, but is manifest in some marine animals, for example sea-anemones, which when closed are to all appearance lumps of mud; and many hydroids have a most deceiving resemblance (in our eyes, at least) to the seaweeds on or amid which they grow; while a pipe-fish, standing on its head, among the eel-grass alongshore, as is its custom, is as effectually concealed as anything can be. A few large examples might be cited, as the mata-mata turtle of South American rivers, which is so tagged and fringed with outlying processes that it has the exact appearance of a weedy rock; but in this case, as in some others of mimicry, the advantage is more that the turtle's prey (fishes, etc.), will come near enough to be seized before they suspect its fatal presence, than that the turtle itself will be safer.

Mimicry is most prevalent, however, among insects and spiders. These are to be found in all countries with deceptive resemblance to withered, gnawed or moldy leaves; to bits of twig, particles of dung, cocoons whose contents are breaking out; or to any of various objects under which insects seek refuge or concealment, as flakes of bark or bits of stone. The records of observation in the books of Darwin, Wallace, Bates, Fritz Müller, Semper, Belt, Forbes and other naturalists abound in instances, many of which were discovered by the merest accident. The classic example is the amazing likeness of a kind of butterfly of the East Indies, when at rest, to a leaf which extends not only to the general color of the under surface of the closed wings and their markings, but to the attitude of the insect as it alights, for it places itself in a relation to the branch to which it clings precisely similar to the natural arrangement of the real leaves about it; and, of course, frequents only the one kind of tree whose leaf is mimicked. A tropical “walking-stick” insect has flat extensions of the skin along its legs and body which are so shaped, and so mottled in color, that Wallace found even the sharp-eyed Dyak boys of the Bornean forest deceived into thinking this insect “a stick grown over by a creeping moss.” But our own northern, greenish-brown walking-sticks are sufficiently deceptive in their twig-like aspect when, as is their custom, they sit on a bush-branch, holding their bodies stiffly out at an angle with their fore legs stretched straight in front of their heads; many must be overlooked for one that is detected. More familiar, perhaps, are the measuring-worms (hairless caterpillars of geometrid moths) which, clasping a branch firmly by their hind legs, will stand out rigidly from it, and maintain this attitude for hours. Their dull gray color gives them exactly the appearance of a broken twig, and as long as they remain motionless they are virtually safe. The whole race of mantids exhibit this character in a greater or lesser degree.

Another very striking example of mimicry is that first made known by H. O. Forbes who met with it in Java and in Sumatra. In the first instance, he noticed a certain butterfly perched, as often happens, on a white patch of bird's excrement dropped on a leaf. Approaching cautiously, he closed finger and thumb over the wings of the insect, which seemed glued to the sticky substance; “To my surprise, however,” Forbes relates, “part of the body remained behind, adhering, as I thought, to the excreta. . . . I looked closely at, and finally touched with the tip of my finger, the excreta to find if it were glutinous. To my delighted astonishment I found that my eyes had been most perfectly deceived, and that the excreta was a most artfully colored spider lying on its back. with its feet crossed over and closely adpressed to its body,” Forbes made the same mistake a second time, some months later in Sumatra; and speaks of this extraordinary spider as “a living bait so artfully contrived as to deceive a pair of human eyes even intently examining it.” Consult for the examples mentioned above Wallace, A. R., ‘Malay Archipelago’ (New York 1869); Forbes, ‘A Naturalist's Wanderings’ (New York 1885).

Ernest Ingersoll.