Popular Science Monthly/Volume 24/February 1884/Defenses of the Lesser Animals
|DEFENSES OF THE LESSER ANIMALS.|
ALL organic beings are, in the course of their lives, subject to a series of dangers and destructive influences arising from the conditions of climate and temperature, and from the competition of their fellow-beings, the universality and power of which are well illustrated in Darwin's phrase, "the struggle for existence." Yet all creatures are adjusted with most wonderful art and adaptation to the conditions of their existence and the state of the world around them. Among these adaptations are the means given to the most helpless animal existences for securing themselves against the depredations of their enemies. It is proper to observe, in considering this subject, that the protection enjoyed by the lower animal organisms is not absolute and individual, but that it is generally effective principally for the preservation of the species against destruction. For where peculiar means of protection are given to one creature, corresponding means for overcoming it are often given to another, its enemy. To the protective sharp sight of the rodents and birds are opposed the equally sharp sight of the fox and the long range of vision of the hawk. It is only in averaging the mass of such animals that we find they are secured as a whole against danger, while numerous individuals are overtaken by their enemies.
Some of the higher animals illustrate the manner in which Nature contrives to furnish special measures of precaution for its little-gifted, unalert, unarmed, and helpless creatures. The absence of teeth in the edentates is offset by shields or scale-armor; helpless beetles are furnished with hard wing-cases; the pheasants, quails, and larks of the fields are hidden from the keen vision of birds of prey by their earthy color, birds of the river and sea-shore by their resemblance in color to the sand and shingle.
Protection is required by the lower animals chiefly against the weather and against parasites and other external enemies. Frequently the place of their abode is their only and ordinarily a sufficient protection, as is the case with earth-worms and burrowing larvæ, wood-worms and fruit-borers. But such animals appear to be afflicted with particular enemies peculiarly fitted to hunt them out in their otherwise secure fortresses—in the shape of moles, mole-crickets, long-nosed hedgehogs, shrew-mice, and swine, hook-billed lapwings, and sharp-tongued woodpeckers. Frequently, also, each animal is defended by some special relation peculiar to its species. Insects, which in their comparatively brief state of maturity are secured by their powers of flight, have to be guarded, in their three previous conditions of egg larva, and pupa, against hosts of enemies to which they would wise be an easy prey and a palatable food. In the condition of the apparently lifeless and really helpless egg, they are covered by their obscurity and littleness, or by being deposited in holes and cracks, or covered with slime or hairy or silken veils and cocoons, under which they escape all but the sharpest search and rare accidents.
More curious are the many-sided and diversified means provided for the security of the young insect during the helpless larval condition. In this state, when it is destitute of eyes and wings, it is either furnished with hairy bristles or spiny envelopes, like those of numerous caterpillars, or with covers composed of fine chips, bud-scales, or other fragments, compactly woven together with a few threads of silk; or else it is screened from the sun and from parasites and birds by a plaster of mud. A group of insects, described sometimes as sack-weavers or sack-moths, make a kind of sack or pocket out of fragments of leaves and splinters, within which they perfect their growth. The case-moths make thick and close-fitting garments for their bodies, out of leaves loosely strung together, within which they hang, head downward, from the skeletons of the leaves they have attacked, undistinguishable to birds and parasites from a long bud-scale or from a dry splinter; and clothes-moths conceal themselves in similar cases made from the hair-dust or wool of the fabrics of which they have taken possession. Some beetles envelop themselves and go through their changes in balls of earth within which they inclose themselves. The larvae of one group protect themselves by a kind of foam which they manufacture from the juice of the plants they suck. The woolly aphides are well cared for with the great tufts of wool with which they are provided, under the cover of which they suck the juices of plants and bring forth their multitudinous offspring, which given to the winds, the same hairy envelopes serve them as sails on which they are borne afar to new plantations. A species that feeds on the ash-tree takes possession of the galls that form upon it, and can not be removed without taking off the whole limb, for birds will not attack insects thus protected. These and other aphides, which are particularly injurious to fruit-trees, are so carefully protected against the ordinary attacks of external enemies that man is left to contend against them alone. The bark-lice or scale-insects are particularly difficult to reach, and seem to multiply in perfect security against all ordinary attacks.
A whole series of gall-insects provide security for their posterity by colonizing them in the swellings or knots that are produced on the trees wherever they sting the bark and lay their eggs. The larvae, continuing to irritate the tissues of the tree, cause the knots to swell and grow correspondingly with their own growth, and thus find themselves in a well-fortified home exactly fitted to their wants. Within the galls, the naked, helpless worms are at once protected from exterior assaults of every kind and provided with an unfailing supply of food which they can reach without effort, so that their development goes on without obstruction of any kind. According to A. Schenck, the gall-nuts of the rose are adapted to the shelter and support of the larvæ of more than two hundred species of flies, and those of the oak are also the home of numerous varieties. Malpighi, who died near the end of the seventeenth century, remarked that there was no part of the plant on which galls did not arise. The roots, runners, stalks, leaf-stems, leaves, buds, flower-stems, flowers, and fruit, all are made to serve as the nest or place of transformation for the young of one or more species of insect; but only the aphis lives upon them permanently.
Another very frequently observed means of securing young insect broods is by envelopes formed, sometimes with great apparent skill, by rollings or foldings of the leaf. Some weevils have the art of cutting out patterns of leaves, and, without wholly severing their attachment, rolling them up into a scroll, within which they deposit their eggs; and they do the whole with such mathematical accuracy that their constructions have been made the subjects of formal monographs, like those of Drs. Heis and Debey on the funnel-rollers. Specimens of these scrolls are familiar enough, as they have been observed on the hazel, beech, hornbeam, alder, birch, aspen, and vine, where the operations of the insects are in some seasons attended with injury to the crop. The caterpillars of many butterflies and moths are also sheltered in the same manner; while other caterpillars associate themselves together and spin webs for their nests, in the air between the leaves and twigs of trees. Nests of this kind are frequently found on fruit-trees and shrubbery, and afford a very good degree of protection to their inhabitants against late frosts, storms, birds, and parasites. The nest of the procession-spinner serves, curiously, only as a resting-place for the insect in the larval state, though it finally becomes the common home of the pupæ. The caterpillars, to satisfy their hunger, are accustomed to leave the nest in a kind of orderly procession, climbing up the stem of the tree to wander all over the crown of the foliage, and, after they have done their work, to return again in procession to their nest. They are avoided by man on account of the irritation produced by the sting of their hairs, and are for the same reason safe against all birds but the cuckoo. A carnivorous beetle, the Calosoma sycophanta, also despises their fortress and their weapons, and breaks voraciously into their communities, like a wolf into a sheep-fold. We must remember here, the consummate architectural skill with which honey-bees build up their combs of waxen cells closely joined one to another. Their whole manner of life and their professional division of labor, in which they remind us of civilized human life, provoke the query, Whence the mechanical and technical skill and the intelligence of these little creatures?
A considerable number of our insects are burrowers, and during the period of their larval development excavate, under the epidermis of the leaves and other green parts of plants, passages, small at first, but which widen as the larvæ grow, feeding themselves from the parenchyma in which they work, and at the same time obtaining a defense against external injurious influences and disturbances. They usually leave their burrow, when about to assume the chrysalis state, by a little hole that may be found at the extreme end of the excavation, and either fall to the ground or make a cocoon, attached to some plant, in the air. Other burrowing larvæ bury themselves in the ground.
For the preservation of the chrysalis, Nature has provided many insect-larvae with the faculty of spinning, and organs for the purpose. This function is so extraordinarily developed in the larvæ of the butterflies that a whole group of that order have been called "the spinners"; while many of these spinners—the silk-worms—have been made serviceable to human civilization. Before the spinning larva advances to its last change of skin, it selects a sheltered, dry spot—between leaves, on bark, in a hedge, in turf, or on a post—and then, drawing from the spinning-glands situated under its neck and between its head and fore-feet fine silken threads, it prepares an ample, firm, and intricate web of flock-silk for its envelope. Having completed its cocoon, it shakes off its old skin, and lays itself to sleep in this soft but solidly-made bed, while its pupa-skin hardens and it awaits the time for its next transformation; and only when disturbed from without does it show by some spasmodic motion of the posterior segment that it can still feel, and that its pupa-rest is not a death-sleep, but only a temporary repose. If the larva is provided with a hairy skin or bristles, they become interwoven with the cocoon, and a composite texture is formed, which man must be careful how he touches, or the bristles will sting his fingers and make them smart. Naked caterpillars, or larvae, weave, like the real silk-worm, cocoons of pure silk, or, like the false-caterpillars, and the larvae of wasps, ants, and bees, transparent, cylindric-oval envelopes of a consistency like that of parchment or waxed paper. The naked caterpillars of the Hermione moth make a kind of roof of pieces of bark over a hollow which they have excavated in the ground for their bed; and a hairy larva provides for itself in a similar manner. Many other larvae go for the security of their pupae into or upon the ground, where they prepare, from leaf-dust, moss, and grains of sand, a ball rough on the outside but smoothly finished within, or simply a hole in the ground, as an envelope.
Arrived at last at its perfect and free state, the insect is efficiently protected by that "mimicry" which has been much discussed by Wallace and other writers, or the likeness in color, and sometimes in other qualities, which it presents to objects that are associated with its most accustomed haunts. Some instances of this mimicry may be observed among higher animals, but it is most conspicuous and significant with insects. We need only refer to the appearance of different butterflies resting with their wings folded together on flowers, leaves, bark, old walls, dead wood, etc., and to the thousands of instances daily in which insects pass unobserved by being confounded in their general harmony with the objects that are nearest to them.
The shells which serve as houses to land-snails, and which the animals close in winter by their opercula, or doors, are known to all. Many snails are not provided with shells, and they secure themselves by creeping under dead leaves, stones, or pieces of wood, or into the sod and the ground.
If we regard the animals in the water we shall find that they are furnished with safeguards as well adapted to their wants as those of their fellows of the air. The larvæ hide, like those of the Ephemerœ, with their whole bodies in the ground, and thus escape destruction by the fish; or they live, like the larvæ of the May-flies, in cases made of splinters of wood, pieces of rush, seeds, bits of shells, or hollow straws and stalks of weeds. Other larvæ conceal themselves in leaf-rollings on the surface of the water or beneath the floating leaves of water-plants The soft animals of the water find their protection in shells of limestone, either spirally coiled or double-valved and kept tightly closed by a strong muscle. Crustaceans are protected by the peculiar armor which gives the class its name, and which they change once a year for a suit of larger size; some members of the family take possession of deserted shells, and concealing their hinder parts within them live thus, and carry their acquired houses about with them, as Diogenes did his tub. The coral-polyps of the ocean build from their secretions solid, branching masses of limestone, within which they conceal their jelly-like forms, furnishing another striking example of the care Nature takes for all its creatures. The boring-worms of the sea, the Serpulæ, and the borers of oyster and other shells, the Sabellœ and the Terchellœ, offer other examples of a similar kind. And the Arenicolœ or sand-worms, like the earth-worms of the land, find their security simply by being under the cover of the sand as they crawl around for their food.—Translated and abridged for the Popular Science Monthly from Die Natur.