Popular Science Monthly/Volume 9/July 1876/Animal Powers of Offense and Defense
|ANIMAL POWERS OF OFFENSE AND DEFENSE.
THERE can hardly be any greater diversity observed in the animal series than that exemplified in the various means whereby animals are enabled to assume an offensive or defensive aspect. From the lowest to the highest grades of animal life—excepting, perhaps, man himself—we find ample provision made for the exigencies of animal existence, in so far as these exigencies demand the use of apparatus which gives its possessors some advantage or other in the "struggle for existence." Undoubtedly, in his superior intellectual organization, which enables man even in his rudest state to avail himself of almost every feature in his surroundings for advantage and defense, the human subject has been endowed above all other forms; and he therefore compensates himself by varied arts and stratagems for the want of the more rigid and natural appliances of lower forms. But if it be true that art is most to be admired when it closely imitates Nature, then the policy of man in his imitation, conscious or unconscious, of the many offensive arts of his humbler neighbors, must claim from us a fair share of favorable criticism.
Thus, it is a striking fact that very many human means of defense or offense find their prototypes, or at least strangely analogous features, in the extensive armory of the animal world at large. The lasso may be found in the apparatus whereby such a simple form as the Hydra, that tiny fresh-water polyp, secures its prey. Or, when human sharp-shooters think to conceal their whereabouts most effectually from the foes they purpose to annoy, and clothe themselves in garments of neutral tint, the hue of which shall most nearly resemble that of the objects amid which they are located, this principle of imitation of natural objects again finds a strict parallelism in the animal world. For it is a familiar fact to all observers of Nature that the color of most animals resembles more or less that of their natural surroundings. The color of the sand-grouse, for instance, and other species of grouse, of partridges and other birds inhabiting heaths, or of flounders and other fishes inhabiting the sand, strictly approximates in character to that of their dwelling-places, and serves to conceal and protect such beings. And, when we further discover that, in not a few cases, this principle of similarity to their surroundings is carried in some animals—such as the leaf insects and walking-stick insects—to the extent of close and actual mimicry, our surprise is increased.
Or, lastly, when we find, as in the latest phase of modern warfare, that the concealed torpedo is used as a subtile and powerful means for effecting the destruction of whole fleets, the fact cannot but call to mind the electrical apparatus of some fishes—and notably that of the torpedo or electric ray—which exists as a natural means of defense, the powers of which, few, if any, of their less favored neighbors care to test or provoke.
While the consideration of the more prominent and typical means of defense in animals may very reasonably occupy our brief attention, a few words on the subject of mimicry in the animal series may also prove interesting, more especially as this form of protection, through imitation of their surroundings, forms a simple yet effective means of defense to many organisms. We have already referred to the readily-perceived and very general correspondence in color seen throughout the animal world between animals and their abodes; and of the more general aspects of this condition nothing further need be said. The more special and striking developments of mimetic resemblances are found in cases in which not merely the general color of their environments is imitated, but where resemblances of a close, and sometimes of a very extraordinary kind, to other animals, to plants, or even to inorganic objects, are to be noted. In the leaf-insects, which are included in the same order as locusts, crickets, etc., for example, the wings are not only colored to resemble leaves, but their structure imitates in the most exact manner the appearance of the veins of the leaf. Nor does the principle of imitation end with this sufficiently remarkable effect. In some leaf-insects the colors of the leaf-like wings actually change with the season of the year; as if in the most perfect sympathy and harmony with the alteration of colors in the actual leaves. And the mimicry becomes of still more perfect kind, to our thinking, when we find that the wings of the leaf-insect exhibit even the characteristic markings we are familiar with in leaves as produced by the attacks of minute insects; Nature thus imitating, not merely the natural structure of the leaf, but the very imperfections to which the leaf is subject. It has been suggested that the little leaf-eating insects may be themselves deceived by the mimicry of their larger neighbors, and may actually eat into the wings of the latter, and thus produce the eroded appearance. But, if this latter view be correct, it only makes out a stronger case for the perfect reproduction of the leaves in the wings of the insect. Mr. Wallace has given us a very typical example of another such case of the imitation, not only of leaves, but of the natural parasites of leaves, in a butterfly, the wings of which, on their under-surfaces, resemble leaves; while the imitations of decay of leaves and of the fungi that appear thereon are so close that, as Mr. Wallace remarks, "it is impossible to avoid thinking at first sight that the "butterflies themselves have been attacked by real fungi."
The walking-stick insects, as they are called, in their turn imitate, in the skeleton-like structure of their bodies, the appearance of dried twigs; and it is a singular fact that even in their awkward, ungainly manner of walking, the resemblance to the chance movements of twigs is clearly perceptible; the mimicry being rendered more realistic through this latter phase. Then, also, we find certain harmless groups of moths imitating closely the outward appearance of species of stinging bees and hornets. And one remarkable case of mimicry is the well known instance of some perfectly inodorous South American butterflies, which perfectly reproduce the external appearance of other butterflies which emit a most offensive odor; the reason assigned for this latter phase of mimicry being the very feasible one that the inodorous forms are protected from the attacks of birds by their resemblance to their strong-smelling neighbors. As a last instance of this curious phase of animal organization, we may note the example furnished by those curious little fishes, the Hippocampi, or sea-horses—so named from the obvious resemblance of the form of the head to that of a horse—the bodies of which become covered with long streamers of certain kinds of seaweed; so that, when these fishes rest amid the seaweed-covered nooks of their marine grottoes, the presence of their streamers serves to render detection by their enemies no easy matter.
Referring to the explanation, if such can be afforded, of these mimetic resemblances, there can be little doubt that, viewed as to its ultimate use and purpose, the condition of mimicry serves in the most effective manner as a means of defense and protection to the animals so endowed. The resemblance of the colors of birds to that of their habitat presents an obvious instance of this purpose; as also does the more complicated example of the imitation, by scentless butterflies, of their odorous neighbors. But, as regards the exact means whereby the condition of mimicry is induced and perfected, or concerning the exact causes of its assumption and development, natural history science, in its practical aspect, remains silent; although the bolder march of theory and speculation may indeed lead us for a little way toward the solution of the problem. At any rate, there can be no difficulty to our clearly appreciating the workings of a great law of purpose and design in the production of mimicry, as serving to protect the weak and less powerful against stronger and better-provided animals.
Turning now to some lower forms of animal life, we find in such forms as the Hydrœ, or common fresh-water polyps, the zoöphytes, sea-anemones, jelly-fishes, and allied forms, excellent examples of very specific means of defense and offense in animals. Within the tissues of the bodies of the foregoing organisms, when these tissues are microscopically examined, numerous little sacs or cells, varying in size and form, may be observed. To these cells the appropriate name of "thread-cells," or cnidœ, has been given. When their structure is investigated, each little cell is seen to possess an elastic wall of double nature; the inner layer of the wall being strong, while the outer one is of thinner and more delicate texture. The upper or open extremity of the inner layer of the sac is prolonged to form a kind of sheath, which protects and gives origin to a thread-like filament, from the presence of which, indeed, these cells derive their name. This thread, in the ordinary condition of the cell, is coiled up within the interior of the sac, and around its own sheath; and in many cases both thread and sheath may be discerned to be provided with minute spines or hooks. The cell itself, in addition, contains a fluid, amid which the thread is submerged.
Such is the essential structure of a thread-cell in its normal state of what we may term repose. When such a structure, however, is pressed or irritated in any way, the cell ruptures or bursts, the contained fluid escapes, and the thread and its sheath are quickly protruded or thrown out from the opening in the cell. If, now, the thread and fluid are observed to come in contact with any body of appropriate and assailable kind, such a body will exhibit certain symptoms which will indicate to us the probable nature of these curious cells. Thus, when the tentacles or feelers of the sea-anemone, or of any of the zoöphytes, come in contact with a minute or susceptible organism adapted for food, the organism is first observed to struggle to escape from the entwining filaments which encircle its body. Soon, however, its active exertions cease, and the victim appears paralyzed and incapable of helping itself, or of struggling longer with its captor. The thread-cells, in other words, have been discharging their miniature darts or "threads" into the body attacked; the fluid—in all probability of acrid or poisonous nature—has been poisoning the tissues of the struggling organism; and the observation has revealed to us that the functions of the cells are undoubtedly analogous to those of the serpent's fangs and poison-gland, in that they serve to paralyze and kill the prey.
As might naturally be supposed, the power of the thread-cells varies in different species and groups of the animals that possess them; but there are some forms of Cœlenterata—for thus the Hydrœ, sea-anemones, and their allies, are collectively named—in which the stinging-cells are of sufficient size and power to inflict severe pain on man himself. Aristotle was fully aware of this latter fact when he named the jelly-fishes and their allies Acalephœ, or "nettle-like" animals. And bathers and swimmers, through instinct, if not thorough zoölogical knowledge, generally and wisely contrive to give the jelly-fishes a wide berth in their marine meanderings. The late Edward Forbes, in his humorsome manner, says of one species of jellyfish, that, "once tangled in its trailing 'hair,' the unfortunate, who has recklessly ventured across the graceful monster's path, too soon writhes in prickly torture. Every struggle," he continues, "but binds the poisonous threads more firmly round his body, and then there is no escape;" for, as the naturalist informs us, even when the arms or tentacles are cast loose from the body of the jelly-fish, they "sting as fiercely as if their original proprietor itself gave the word of attack." The Abbé Dicquemare, an observant French naturalist, found that some species can only sting the more sensitive parts of the body, such as the eyes. But Forbes's remark of the abbé's experiment, that most people would prefer "keeping their eyes intact, to poking medusæ into them," will coincide, we imagine, with the opinions of most of our readers. It is equally worthy of remark that "appearances" in natural history, as in ordinary life, are apt to be "deceptive;" and, looking at the grace and beauty of the jelly-fishes, we could hardly credit them with such virulent powers.
The most notable offenders of the jelly-fish class, in respect of their stinging powers, are the Physaliœ, or Portuguese-men-of-war, as they are popularly termed—a group of beautiful oceanic forms, met with floating far out at sea, especially in tropical latitudes, and presenting the appearance of a bladder-like structure, provided with a crest and trailing streamers, and colored of the most ethereal and beautiful of hues. When the tentacles of a physalia are allowed to come in contact with the human skin, the thread-cells—which are of large relative size, and measure in diameter about the three-thousandth of an inch—sting so severely that the effects of the irritation may persist for a considerable time, and may give rise in some cases to very painful after-effects. The thread-cells in the tentacles of the common species of sea-anemones have no effect on the skin of man; but, as the writer has frequently demonstrated on his own person, if the tentacle be allowed to touch the more delicate mucous membrane of the lips, a slight stinging sensation, accompanied by temporary numbness, may be felt. To the curious this is worth trying.
Passing in review the higher groups of the animal kingdom, we find an endless variety of contrivances subserving offensive purposes, or limited to the milder purposes of defense. Shells, scales, and plates of every kind, with special modifications for special purposes, may thus readily be selected as examples; spines and allied armaments of all shapes and sizes; poison-secretions and fangs of centipedes and serpents, and the sting of scorpions and bees, possessing sure and sometimes deadly effect on those they attack; and, in quadrupeds, strong claws and teeth united to equally powerful muscles—such are a few examples of the endless stores of weapons contained in animal armories.—Chambers's Journal.