Fig. 163.—The dead-leaf butterfly, Kallima sp., a remarkable case of special protective resemblance. (From specimen.)

Warning colors, terrifying appearances, and mimicry.—While many animals are so colored as to harmonize with their habitual or usual environment, others on the contrary are very brightly colored and marked in such bizarre and striking pattern as to be conspicuous. There is no attempt at concealment; it is obvious that conspicuousness is the object sought or at least produced by the coloration. Animals like these, we shall find, are in almost all cases specially protected by special weapons of defence such as stings or poison-fangs, or by the secretion of an acrid, ill-tasting fluid in the body. Many caterpillars have been found, by observation in nature and by experiment, to be distasteful to insectivorous birds. Now it is obvious that it would be advantageous to these caterpillars to be readily recognized by birds. After a few trials the bird learns by experience to let these distasteful larvæ alone; their conspicuous markings serve as warning colors. The black-and-yellow-banded caterpillar of the common milkweed or monarch butterfly (Anosia plexippus) is a good example of such protection by a combination of distastefulness and warning coloration. The little lady-bird beetles are mostly distasteful to birds; they are brightly and conspicuously marked. Certain little Nicaraguan frogs have a bright livery of red and blue, in strong contrast to the dull concealing colors of other frogs in their region. By offering these little blue and red frogs to hens and ducks the naturalist Belt found that they are distasteful to the birds.

Fig. 164.—The larva of the pen-marked sphinx-moth, Sphinx chersis, showing terrifying attitude. (After Comstock.)

Certain animals which are without special means of defence and are not distasteful are yet so marked or shaped, and so behave as to present a threatening or terrifying appearance. The large green caterpillars of the sphinx-moths, the tomato- and tobacco-worms, are familiar examples, each larva having a sharp horn on the back of the next to last body-segment (fig. [164]). When disturbed the caterpillar assumes a threatening attitude, and the horn seems to be an effective weapon of defence. As a matter of fact it is not at all a weapon of defence, being weak, not provided with poison, and altogether harmless.

But it would plainly be to the advantage of a defenceless animal, one without poison-fangs or sting and without an ill-tasting substance in its body, to be so marked and shaped as to mimic some other specially defended or inedible animal sufficiently to be mistaken for it and thus to escape attack. Such cases have been noted, especially among insects. This kind of protective resemblance may be called mimicry. A most striking case is that presented by the familiar monarch and viceroy butterflies (fig. [165]). The monarch (Anosia plexippus) is perhaps the most abundant and widespread butterfly of our country. It is a fact well known to entomologists that it is distasteful to birds and is let alone by them. It is conspicuous, being large and chiefly red-brown in color. The viceroy (Basilarchia archippus), also red-brown and patterned almost exactly like the monarch, is not, as its appearance would seem to indicate, a very near relation of the latter, but on the contrary it belongs to a genus of butterflies all of which, except the viceroy and one other, are black and white in color and of different pattern from the monarch. The viceroy is not distasteful to birds, but by its extraordinary simulation or mimicking of the monarch it is not distinguished from it and so is not molested. In the tropics there have been discovered numerous examples of mimicry among insects. The members of two large families of butterflies (Danaidæ and Heliconidæ) are distasteful to birds and are mimicked by members of other butterfly families (especially the Pieridæ).