The Encyclopedia Americana (1920)/Imitation in Animals

From Wikisource
Jump to: navigation, search
The Encyclopedia Americana
Imitation in Animals
Edition of 1920. See also Mimicry on Wikipedia, and the disclaimer.

IMITATION IN ANIMALS. This is a somewhat technical phrase used in zoology to designate the fact that certain kinds of animals have acquired, by the gradual process of natural selection, a resemblance to other animals, which are said to be “imitated,” although, of course, no effort to become like them is consciously made. This is one of the two principal phases of what is called “protective resemblance” the other being an acquired and protective likeness to some vegetable or inanimate object that causes it to be overlooked by its enemies; this second phase is termed “mimicry,” and is described under that head. While the advantage gained by mimicry is that the animal is either overlooked altogether, or else mistaken by its enemy for something of no value to the latter, that gained by imitation is that the animal is avoided by predatory kinds because it is mistaken for something harmful or at least distasteful. The methods by which, according to the theory of Darwin and his followers, these conditions are matured are explained in the article Natural Selection; but it should be said that other, or at any rate modified, views of the method have been advanced by other and later writers.

The most widely quoted and typical case of apparently advantageous imitation is that of the South American heliconid butterflies which are evil-smelling creatures so distasteful to both monkeys and birds that they are very rarely if ever eaten, and hence fly about in the sunshine as carelessly as if they knew they were safe. Now, in the same situations are found butterflies of an entirely different family (Pieridæ) that have none of the inedible qualities of the heliconids, and they, too, fly about freely and are rarely if ever caught, or attempted to be caught by creatures that prey on butterflies, although both are rather clumsy fliers. The conclusion then seems logical that the pierids are getting benefit from their masquerade, however it may have come about. Similar cases of imitation among butterflies may be found in all parts of the world, in some groups a whole genus affecting a more or less close imitation of another genus or family, and elsewhere only a single species in a genus approximating some unrelated form; furthermore, cases occur where the female alone of a Species will imitate another species, whereas the male acquires no such resemblance; this seems to follow the rule, so widely illustrated in all classes of animals, that nature provides for the preservation of the female far more carefully than for the male, who in the inferior ranks of life at least appears to be of little or no importance after he has performed the duty of fertilization.

Many notable illustrations of our theme may be gathered among the insects of the United States. A conspicuous one is the case of one of our large butterflies of the genus Basilarchia. All the species of this genus, with one exception, are dark-colored, with a light border containing blue spots round the margins of the wings, as in the common “mourning cloak”; the single exception is Basilarchia hipparchus, which has a tawny brown color, diversified by black bands and marks. This brown basilarchia almost exactly copies our very common milkweed butterfly (Anosia plexippus). Here there seems no adequate motive for, or advantage gained by, the apparent imitation, and it suggests when it is closely studied various difficulties in respect to the whole subject

But not all cases of this adaptation are confined to resemblances between two insects or other creatures of the same class. A certain butterfly becomes an almost precise duplicate of a dragon-fly; spiders in both North and South America take the form of ants, and a well-known small black wasp of our own country is so ant-like that it is called the cow-ant or velvet-ant. Few town-bred persons go into the country in summer without mistaking for a hummingbird one or other of the big hawk-moths that poise on whirring wings before flowers very much after the manner of the bird, which is scarcely larger. These and similar cases are ruled out by some thinkers on the ground that they are not imitations but cases of parallel development. Many kinds of insects imitate the stinging, and therefore dreaded, tribes of bees and wasps, including flies, beetles and so forth. One common American fly (Volucella) “imitates the honey-bee so closely that one would hesitate to handle it even after being told that it is harmless”; and it profits by the deception to lay its eggs inside of hives. Other flies copy bumblebees so closely as to be remarkable, and they imitate their habits as well as their form and colors, “The drone-fly, for example,” says Metcalf, “which imitates a honey-bee, has the same kind of buzzing flight, and when standing occasionally teeters its abdomen up and down, as is characteristic of the bees and wasps. Some of these mimicking flies even protrude and withdraw the tip of the abdomen, as does an angry bee or wasp,” threatening to use a sting it does not possess.

Hints have already been given that scientific men are not agreed on the interpretation of all these phenomena, or the causes of them. An attempted discussion of the many questions involved is not possible here. Those who care to go more deeply into the matter should read the evidences and arguments in the books of such men as A. R. Wallace, H. W. Bates. Thomas Belt, Fritz Müller, S. H. Scudder, Lloyd Morgan, W. Bateson and others. A brief and excellent discussion of the matter will be found in ‘Animal Coloration,’ by F. E. Beddard (London 1892), where many good references to other sources are given.

Ernest Ingersoll.