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Popular Science Monthly/Volume 53/September 1898/A Game of Hide and Seek


NOTHING illustrates more vividly the change which has taken place during the present century in the attitude of naturalists toward the objects of their study than the colors of plants and animals. To the dried-specimen systematist of a hundred years ago color was an immutable factor in Nature. The delicate beauty of the butterfly, the iridescent hues of the paradise bird, the tawny stripes of the tiger, the somber shades of the reptile, the whiteness of the lily and the redness of the rose—these and the myriad other color phases in the living world were believed to exist now as the Creator designed them a few thousand years ago. To the naturalist they were chiefly valuable in enabling him to separate species from species in dreary Latin tomes. To the theologian they served to show the goodness of God in adorning man's passing abode. To the artist beauty was its own excuse for being.

But there were not wanting interpreters of Nature who saw that in many cases colors were useful to the organisms possessing them. Sprengel expressed his belief that the colors of flowers served to attract insects which aided in the fertilization of the seeds. Other observers showed that dull hues in animals might assist a persecuted species to escape from enemies, or a persecuting species to steal upon prey. Such views were at once utilized by the promoters of the celebrated "argument from design," who contended that these creatures were so made when created, the useful coloring being conclusive evidence of a designing intelligence on the part of the Creator. In those days all paths in the fields of science led to the domains of theology, where it was unsafe to PSM V53 D682 Catocala moth at rest.pngCatocala Moth at Rest. wander without the guidance of a keeper appointed by the church. To this individual puzzling problems were easy of solution: "God intended it so, and who shall dare to question the actions of Infinity?" Natural phenomena were given supernatural explanations which were accepted in a way that seems incredible to us to-day. But gradually the law of parsimony in logic—the law which in this case directed that when a phenomenon can be explained in natural terms, we must not appeal to supernatural ones—became accepted, and reasons that had long held good were rejected as worthless. The hold of dogmatism and priestcraft upon science gradually relaxed as the church meddled less in secular matters and religion became an affair of the heart rather than of the intellect.

It is one of the triumphs of biology—the youngest of sciences—that she has already given an adequate explanation for the existence of most of the phases of animate color. We all now know that the colors of flowers exist, as Sprengel believed, to attract insects, and that insects are attracted to insure cross-fertilization—a fact of which Sprengel was not aware. All the world is familiar with accounts of mimicry—"the imposture of Nature"—as well as of protective and aggressive resemblance. Nearly every one has seen pictures of the kallima insect, which when in motion is a gaudy butterfly, and at rest becomes a dry and withered leaf; or of the various leaf insects and stick insects of the tropics. Many of us have seen in imagination the puff adder which Professor Drummond did not sit upon in the wilds of Africa, or have watched with Forbes the bird's-dropping mimicking spider of the eastern archipelago. We have also followed Bates and Wallace along the Amazon, or Belt in the wilds of Nicaragua, as they studied the tropical butterflies which mimic each other so strangely.

The fact that most published accounts of mimicry and protective resemblance deal with animals of tropical countries has led to a general impression that to observe these curious color phenomena one must travel to out-of-the-way places; that the creatures about us are commonplace and uninteresting. But to the seeing eye and the attentive mind there are as many facts of interest in a stroll through northern woods as through the Everglades. Richard Jeffries and John Burroughs found in England and New York most interesting phases of tropical life, and had besides the exhaustless treasures of their temperate out-of-doors to draw upon. The tropics are full of strangeness to northern eyes, and possess many phases of life that seem wonderful to unaccustomed minds. But the luxuriance of vegetable life is almost oppressive; it is always in full glory; one does not see the bursting buds and the greening leaves because the full foliage overshadows all else.

The animals of the north show numberless color phases of interest. One of the most curious of these is exhibited by several families of insects in which the outer wings are protectively colored in dull

PSM V53 D683 Catocala moth in flight.png
Catocala Moth in Flight.

hues and the under wings brightly colored. For example, there are many species of moths belonging to the genus Catocala found throughout the United States. These are insects of good size, the larger ones measuring three inches in expanse of wings, and the majority of them being at least two thirds that size. Most of them live during the day on the bark of trees, with their front wings folded together over the back. The colors and markings of these wings, as well as of the rest of the exposed portions of the body, are such as to assimilate closely with the bark of the tree upon which the insect rests. In such a situation it requires a sharp eye to detect the presence of the moth, which, unless disturbed, flies only at night, remaining all day exposed to the attacks of many enemies. Probably

PSM V53 D684 Carolina locust at rest.png
Carolina Locust at Rest.

the most important of these are the birds, especially species like the woodpeckers, which are constantly exploring all portions of the trunks of trees.

The chief beauty of these Catocalas as they are seen spread out in the museum cabinet lies in the fact that the hind wings, which, when the moth is at rest in life, are concealed by the front ones, are brightly colored in contrasting hues of black, red, and white in various brilliant combinations. These colors, in connection with the soft and blended tones of the front wings, make a very handsome insect.

To explain these colors many suggestions have been made. The protective hues of the upper wings are easy to account for by accepted biological theories, but the bright colors of the under wings have presented more difficulties. Some biologists have supposed that the latter were without special significance, being produced by the tendencies of the insect to bright colors—the tendency not being kept down by the eliminating factors which would operate in the case of exposed portions of the body. On this principle Mr. Beddard would "expect that bright coloration would be the rule rather than the exception among nocturnal insects, for, however bright and varied, the colors would be invisible at night and could do their possessors no harm or good," an ingenuous assumption characteristic of many of the pages of that naturalist's recent book, in which the attempt to keep on both sides of the Darwinistic fence has left the author very much astraddle. Another English naturalist, Professor Poulton, has suggested that the bright under wings lead the pursuing bird to catch the insect by them, the wing membrane giving way without serious injury to the moth. But this seems to me a strained and inadequate explanation, much less satisfactory than the one afforded by the suggestion of another celebrated English entomologist, Lord Walsingham, who in a presidential address before the Entomological Society of London delivered the following passages:

"My attention was lately drawn to a passage in Herbert Spencer's Essay on the Morals of Trade. He writes: 'As when tasting different foods or wines the palate is disabled by something strongly flavored from appreciating the more delicate flavor of another thing afterward taken, so with the other organs of sense a temporary disability follows an excessive stimulation. This holds not only with the eyes in judging of colors, but also with the fingers in judging of texture.'

"Here I think we have an explanation of the principle on which protection is undoubtedly afforded to certain insects by the possession of bright coloring on such parts of their wings or bodies as can be instantly covered and concealed at will. It is an undoubted fact, and one which must have been observed PSM V53 D685 Carolina locust in flight.pngCarolina Locust in Flight. by nearly all collectors of insects abroad, and perhaps also in our own country, that it is more easy to follow with the eye the rapid movements of a more conspicuous insect soberly and uniformly colored than those of an insect capable of changing in an instant the appearance it presents. The eye, having once fixed itself upon an object of a certain form and color, conveys to the mind a corresponding impression, and, if that impression is suddenly found to be unreliable, the instruction which the mind conveys to the eye becomes also unreliable, and the rapidity with which the impression and consequent instruction can be changed can not always compete successfully with the rapid transformation effected by the insect in its effort to escape."

Lord Walsingham goes on to suggest that this intermittent display of bright coloring probably has as confusing an effect upon birds and other predaceous vertebrates as upon man; and that on this hypothesis such colors can be more satisfactorily accounted for than upon any other yet suggested.

This explanation is easy to understand and gives renewed emphasis to the oft-repeated statement that nothing in Nature is without significance. In the case of the Catocala moths one readily perceives that when driven to flight by a woodpecker or other bark-searching bird a moth which shows during a rapid irregular flight bright colors, and then alights, hiding the colors and instantly assuming entirely different hues, blending with the surroundings, would stand a better chance of escaping from a pursuing bird than a moth which had no bright colors with which to confuse the bird and prevent its seeing the place where the insect alights.

These insects are excellent illustrations of the combined action of the various forces which Darwin classed together under the term "natural selection." The factors involved are three—multiplication, variation, elimination. PSM V53 D686 Coral winged locust in flight.pngCoral-winged Locust in Flight. In nearly all organisms more young are produced than can mature. In these young there are infinite variations all directions. Some of these variations fit the individuals possessing them better to the conditions of life than variations in other directions. Consequently, the possessors of the latter will be eliminated in the struggle for existence and the former will escape elimination, and mature to reproduce. Their young will in part at least inherit the favorable variations, and thus have an advantage which will lead to their reproduction. Thus there is an ever-increasing tendency to a more perfect adaptation to environment.

On the rocky hills and sandy plains of New England there are several species of grasshoppers or locusts that also illustrate these principles. If you walk along a strip of sandy land in summer, you start to flight certain locusts which soon alight, and when searched for will be found closely to assimilate in color the sand upon which they rest. On a neighboring granite-ribbed hill you will find few if any of this species of locust, but instead there occur two or three quite different species, which when at rest closely resemble the lichen-covered rocks. This resemblance is very striking, and is found in all stages of the insect's existence. If now you go to a lowland meadow, still another color phase will be found to prevail—the green grass is swarming with the so-called "long-horned" grasshoppers which are green throughout, with linear bodies and long, slender legs and antennae.

Each of these three groups of insects is adapted to its particular habitat. All are constantly persecuted by birds, and have been so persecuted for unnumbered ages in the past. In every generation the individuals have varied, some toward a closer resemblance to environment, others in an opposite direction. The more conspicuous insects have been constantly taken, and the least conspicuous as constantly left to reproduce. Were the three groups to change places to-day, the green grasshoppers from the meadows going to sandy surfaces, the sand-colored locusts going to rocky hills, and the "mossbacks" from the hills to the lowland meadows, each would become conspicuous, and the birds would have such a feast as is seldom spread before them.

The species living on sand and rocks are often "flushed" by birds. Those which flew but a few feet would be likely to be captured by the pursuing bird; those which flew farther would stand a better chance of escaping. Similarly, those which flew slowly and in a straight line would be more likely to be caught than those which flew rapidly and took a zigzag course. As a consequence of the selection thus brought about through the elimination of those which flew slowly along the straight and narrow way that led to death, you will find that most locusts living in exposed situations when startled fly some distance in a rapid, zigzag manner.

But still another element of safety has been introduced by some species of these locusts through the adoption of the color tactics of the Catocala moths. The under wings of the common Carolina locust—the species most abundant along the highway—are black, bordered with yellowish white. The base of the hind wings of a related species living on the Western plains is bluish, while in the large coral-winged locust of the Eastern States the hind wings are red, bordered with black. In nearly all of the species of these locusts frequenting open localities where they are liable to disturbance by birds or other animals, the hind wings exhibit contrasting colors in flight. Most of them also fly in a zigzag line, and alight in a most erratic manner. Many times I have had difficulty in determining the exact landfall of one of these peculiar creatures, and I believe Lord Walsingham's suggestion is well exemplified in them.