Technical Note.—For an appreciation of the reality of protective resemblances observations must be made in the field. Examples are easily found. Locusts, katydids, green caterpillars, lizards, crouching rabbits, and brooding birds are readily observed instances of general protective resemblance. For examples of variable resemblance examine specimens of a single locust species taken from different localities; the individuals of the various species of the genus Trimerotropis show much variation to harmonize with their surroundings. Collect a number of larvæ (caterpillars) of one of the swallow-tail butterflies (Papilio), and when ready to pupate put them separately into pasteboard boxes lined inside with differently colored paper. The chrysalids will show in their coloration the influence of the different colors of the lining paper, their immediate environment. As examples of special protective resemblance observe inch- or span-worms (larvæ of Geometrid moths). The walking-stick is not uncommon; many spiders that inhabit flower-cups show striking protective color patterns; and the Graptas or comma-butterflies which resemble dead leaves may be examined.

To illustrate warning colors, find, if possible, the larvæ (caterpillars) of the common milkweed or monarch butterfly (Anosia plexippus), and offer them to birds, at the same time offering other caterpillars, and note the results. For terrifying or threatening appearance find specimens of the large green tobacco- or tomato-worm (larva of the five-spotted sphinx-moth, Phlegethontius carolina), or other sphingid larvæ.

The butterflies illustrating the striking example of mimicry, described on p. 432, can be found in most parts of the country. Syrphid and other flies which mimic bees and wasps can readily be found on flowers.

Each student should search for himself for examples of protective resemblance.

Use of color.—The prevalence of color and the oftentimes striking and intricate coloration patterns of animals demand some explanation. As naturalists are accustomed to find the frequently bizarre and seemingly inexplicable shapes and general structure of animals readily explained by the principle of adaptation, that is, special modification of body-structure to fit special conditions of life, so they look to use as the chief explanation of color and markings. Some uses are obvious; bright colors and striking patterns may serve to attract mates or to avail as recognition marks by which individuals of a kind may readily recognize each other. The white color of arctic animals probably serves to help keep them warm by preventing radiation of heat from the body; on the other hand dark color may also help to keep animals warm by absorbing heat. "But by far the most widespread use of color is for another purpose, that of assisting the animal in escaping from its enemies or in capturing its prey."

It is common knowledge that the young and old, too, of many kinds of ground-inhabiting animals, when startled by an enemy will not run, but crouching close to the ground remain immovable, trusting to remain unperceived. But a blue or crimson rabbit, however still it might keep, would be easily seen by its enemy and killed. Rabbits, however, which are good examples of animals having this habit of lying close, are neither blue nor green nor red, but are colored very much like the ground on which they crouch. This harmonious coloration is as necessary to the success of this habit as is the keeping still. A grasshopper flying or leaping in the air is conspicuous; when it alights how inconspicuous it is! Unless one has followed it closely in its flight and has kept the eye fixed on it after alighting it is usually impossible to distinguish it from its surroundings. And this is greatly to the advantage of the grasshopper in its efforts to escape its enemies, that is, in its struggle for existence. On the other hand a green katydid would be very conspicuous in a dusty road. But dusty roads are precisely where katydids do not rest. They alight among the green leaves of a tree or shrub. The animals that live in deserts are almost all obscurely mottled with gray and brownish and sand-color so as to harmonize in color with their habitual environment. The arctic hares and foxes and grouse which live in regions of perpetual snow are pure white instead of red or brown or gray like their cousins of temperate and warm regions.

These cases of an animal's color and markings harmonizing with the usual environment are called instances of protective resemblance; that is, they are resemblances for a purpose, that purpose being to render the animal indistinguishable from its surroundings and thus to aid it in escaping its enemies. Such protective resemblances are obviously of great value to animals, and, like other advantageous modifications, have been produced by the action of natural selection. Those individuals of a species most conspicuous and hence most readily perceived by enemies are the first (under ordinary circumstances) to be captured and eaten. The less conspicuous live and produce young like themselves. Of these young the least conspicuous are again saved and so over and over again through thousands of generations until this natural selecting of the protectively colored results in the production of the wonderfully specialized examples of resemblance to which attention is called in the following paragraphs.

General, variable, and special protective resemblance.—In the brooks most fishes are dark olive or greenish above and white below. To the birds and other enemies which look down on them they are colored like the bottom. To their fish-enemies which look up from below they are like the white light above them in color and their forms are not clearly seen. The green tree-frogs and tree-snakes which live habitually among green foliage; the mottled gray and tawny lizards and birds and small mammals of the plains and deserts, and the white hares and foxes and owls and ptarmigan of the snowy arctic regions—all show a general protective resemblance.

Fig. 162.—The twig or walking-stick
insect, Diapheromera femorata.
(From specimen.)

Sometimes an animal changes color when its surroundings change. Certain hares and grouse of northern latitudes are white in winter when the snow covers all the ground, but in summer when much of the snow melts, revealing the brown and gray rocks and withered leaves, they put on a grayish and brownish coat of hair or feathers. A small insect called the toad-bug (Galgulus) lives abundantly on the banks of a pond on the campus of Stanford University. The shores of this pond are covered in some places with bits of bluish rock, in others with bits of reddish rock, and in still others with sand. Specimens of the toad-bug collected from the blue rocks are bluish or leaden in color, those from the red rocks are reddish, and those from the sand are sand-colored. Changes of color to suit the surroundings can be quickly made by some animals. The chameleons of the tropics change momentarily from green to brown, blackish, or golden. There is a little fish (Oligocottus snyderi) common in the tide-pools of the Bay of Monterey in California whose color changes quickly to harmonize with the rocks it happens to rest above. Such changing coloration to suit the surroundings may be called variable protective resemblance.

Very striking are those cases of protective resemblance in which the animal resembles in color and shape, sometimes in extraordinary detail, some particular object or part of its usual environment. This may be called special protective resemblance. The larvæ of the Geometrid moths called inch-worms or span-worms are twig-like in appearance, and have the habit, when disturbed, of standing out stiffly from the twig or branch on which they rest, so as to resemble in attitude as well as color and markings a short or broken twig. To increase this simulation the body of the larva often has a few irregular spots or humps resembling the scars left by fallen leaves, and it also lacks the middle prop-legs of the body common to other lepidopterous larvæ, which would tend to destroy the illusion so successfully carried out by it. The common twig-insect or walking-stick (fig. [162]) with its wingless, greatly elongate, brown or greenish body and legs is when at rest quite indistinguishable from the twigs on which it lies. Another excellent example of special protective resemblance is furnished by the famous green-leaf insect (Phyllium) of the tropics, which has broad leaf-like wings and body of a bright green color with markings which imitate the leaf-veins, and small irregular yellowish spots which simulate decaying or stained or fungus-covered spots in the leaf. Most striking of all, however, is the large dead-leaf butterfly Kallima (fig. [163]) of the East Indian region. The upper sides of the wing are dark with purplish and orange markings not at all resembling a dead leaf. But the butterflies when at rest hold their wings together over the back, so that only the under sides of them are exposed. These are exactly the color of a dry dead leaf with markings mimicking midrib and oblique veins, and, most remarkable of all, what are apparently two holes like those made in leaves by insects, but in the butterfly imitated by two small circular spots free from scales and hence clear and transparent. When Kallima alights it holds the wings in such position that the combination of all four produces with remarkable fidelity the simulation of a dead leaf still attached to the twig by a short pedicel or leaf-stalk (imitated by a short "tail" on the hind wings). The head and legs of the butterfly are concealed beneath the wings.