=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. [Illustration: 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.