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Animals possess the power of feeling, and of effecting certain movements, by the exercise of a muscular apparatus with which their bodies are furnished. They are distinguished from the organisations of the vegetable kingdom by the presence of these attributes. Every one is aware, that when the child sees some strange and unknown object he is observing start suddenly into motion, he will exclaim: 'It is alive!' By this exclamation, he means to express his conviction that the object is endowed with animal life. Power of voluntary and independent motion and animal organisation are associated together, as inseparable and essentially connected ideas, by even the earliest experience in the economy and ways of nature.

The animal faculty of voluntary motion, in almost every case, confers upon the creature the ability to transfer its body from place to place. In some animals, the weight of the body is sustained by immersion in a fluid as dense as itself. It is then carried about with very little expenditure of effort, either by the waving action of vibratile cilia scattered over its external surface, or by the oar-like movement of certain portions of its frame especially adapted to the purpose. In other animals, the weight of the body rests directly upon the ground, and has, therefore, to be lifted from place to place by more powerful mechanical contrivances.

In the lowest forms of air-living animals, the body rests upon the ground by numerous points of support; and when it moves, is wriggled along piecemeal, one portion being pushed forward while the rest remains stationary. The mode of progression which the little earthworm adopts, is a familiar illustration of this style of proceeding. In the higher forms of air-living animals, a freer and more commodious kind of movement is provided for. The body itself is raised up from the ground upon pointed columns, which are made to act as levers as well as props. Observe, for instance, the tiger-beetle, as it runs swiftly over the uneven surface of the path in search of its dinner, with its eager antennÆ thrust out in advance. Those six long and slender legs that bear up the body of the insect, and still keep advancing in regular alternate order, are steadied and worked by cords laid along on the hollows and grooves of their own substance. While some of them uphold the weight of the superincumbent body, the rest are thrown forwards, as fresh and more advanced points of support on to which it may be pulled. The running of the insect is a very ingenious and beautiful adaptation of the principles of mechanism to the purposes of life.

But in the insect organisation, a still more surprising display of mechanical skill is made. A comparatively heavy body is not only carried rapidly and conveniently along the surface of the ground, it is also raised entirely up from it at pleasure, and transported through lengthened distances, while resting upon nothing but the thin transparent air. From the top of the central piece—technically termed thoracic—of the insect's body, from which the legs descend, two or more membraneous sails arise, which are able to beat the air by repeated strokes, and to make it, consequently, uphold their own weight, as well as that of the burden connected with them. These lifting and sustaining sails are the insect's wings.

The wings of the insect are, however, of a nature altogether different from the apparently analogous organs which the bird uses in flight. The wings of the bird are merely altered fore-legs. Lift up the front extremities of a quadruped, keep them asunder at their origins by bony props, fit them with freer motions and stronger muscles, and cover them with feathers, and they become wings in every essential particular. In the insect, however, the case is altogether different. The wings are not altered legs; they are superadded to the legs. The insect has its fore-legs as well as its wings. The legs all descend from the under surface of the thoracic piece, while the wings arise from its upper surface. As the wings are flapping above during flight, the unchanged legs are dangling below, in full complement. The wings are, therefore, independent and additional organs. They have no relation whatever to limbs, properly so called. But there are some other portions of the animal economy with which they do connect themselves, both by structure and function. The reader will hardly guess what those wing-allied organs are.

There is a little fly, called the May-fly, which usually makes its appearance in the month of August, and which visits the districts watered by the Seine and the Marne in such abundance, that the fishermen of these rivers believe it is showered down from heaven, and accordingly call its living clouds, manna. Reaumur once saw the May-flies descend in this region like thick snow-flakes, and so fast, that the step on which he stood by the river's bank was covered by a layer four inches thick in a few minutes. The insect itself is very beautiful: it has four delicate, yellowish, lace-like wings, freckled with brown spots, and three singular hair-like projections hanging out beyond its tail. It never touches food during its mature life, but leads a short and joyous existence. It dances over the surface of the water for three or four hours, dropping its eggs as it flits, and then disappears for ever. Myriads come forth about the hour of eight in the evening; but by ten or eleven o'clock not a single straggler can be found alive.

From the egg which the parent May-fly drops into the water, a six-legged grub is very soon hatched. This grub proceeds forthwith to excavate for himself a home in the soft bank of the river, below the surface of the water, and there remains for two long years, feeding upon the decaying matters of the mould. During this aquatic residence, the little creature finds it necessary to breathe; and that he may do so comfortably, notwithstanding his habits of seclusion, and his constant immersion in fluid, he pushes out from his shoulders and back a series of delicate little leaf-like plates. A branch of one of the air-tubes of his body enters into each of these plates, and spreads out into its substance. The plates are, in fact, gills—that is, respiratory organs, fitted for breathing beneath the water. The little fellow may be seen to wave them backwards and forwards with incessant motion, as he churns up the fluid, to get out of it the vital air which it contains.

When the grub of the May-fly has completed his two years of probation, he comes out from his subterranean and subaqueous den, and rises to the surface of the stream. By means of his flapping and then somewhat enlarged gills, he half leaps and half flies to the nearest rush or sedge he can perceive, and clings fast to it by means of his legs. He then, by a clever twist of his little body, splits open his old fishy skin, and slowly draws himself out, head, and body, and legs; and, last of all, from some of those leafy gills he pulls a delicate crumpled-up membrane, which soon dries and expands, and becomes lace-netted and brown-fretted. The membrane which was shut up in the gills of the aquatic creature, was really the rudiment of its now perfected wings.

The wings of the insect are then a sort of external lungs, articulated with the body by means of a movable joint, and made to subserve the purposes of flight. Each wing is formed of a flattened bladder, extended from the general skin of the body. The sides of this bladder are pressed closely together, and would be in absolute contact but for a series of branching rigid tubes that are spread out in the intervening cavity. These tubes are air-vessels; their interiors are lined with elastic, spirally-rolled threads, that serve to keep the channels constantly open; and through these open channels the vital atmosphere rushes with every movement of the membraneous organ. The wing of the May-fly flapping in the air is a respiratory organ, of as much importance to the wellbeing of the creature in its way, as the gill-plate of its grub prototype is when vibrating under the water. But the wing of the insect is not the only respiratory organ: its entire body is one vast respiratory system, of which the wings are offsets. The spirally-lined air-vessels run everywhere, and branch out everywhere. The insect, in fact, circulates air instead of blood. As the prick of the finest needle draws blood from the flesh of the backboned creature, it draws air from the flesh of the insect. Who will longer wonder, then, that the insect is so light? It is aerial in its inner nature. Its arterial system is filled with the ethereal atmosphere, as the more stolid creature's is with heavy blood.

If the reader has ever closely watched a large fly or bee, he will have noticed that it has none of the respiratory movements that are so familiar to him in the bodies of quadrupeds and birds. There is none of that heaving of the chest, and out-and-in movement of the sides, which constitute the visible phenomena of breathing. In the insect's economy, no air enters by the usual inlet of the mouth. It all goes in by means of small air-mouths placed along the sides of the body, and exclusively appropriated to its reception. Squeezing the throat will not choke an insect. In order to do this effectually, the sides of the body, where the air-mouths are, must be smeared with oil.

In the vertebrated animals, the blood is driven through branching tubes to receptacles of air placed within the chest; the air-channels terminate in blood extremities, and the blood-vessels cover these as a net-work. The mechanical act of respiration merely serves to change the air contained within the air-receptacles. In the insects, this entire process is reversed; the air is carried by branching tubes to receptacles of blood scattered throughout the body; the blood-channels terminate in blood-extremities, and a capillary net-work of air-vessels is spread over these. Now, in the vertebrated creature, the chest is merely the grand air-receptacle into which the blood is sent to be aËrated; while in the insect, the chest contains but its own proportional share of the great air-system. In the latter case, therefore, there is a great deal of available space, which would have been, under other circumstances, filled with the respiratory apparatus, but is now left free to be otherwise employed. The thoracic cavity of the insect serves as a stowage for the bulky and powerful muscles that are required to give energy to the legs and wings. The portion of the body that is almost exclusively respiratory in other animals, becomes almost as exclusively motor in insects. It holds in its interior the chief portions of the cords by which the moving levers and membranes are worked, and its outer surface is adorned by those levers and membranes themselves. Both the legs and wings of the insect are attached to the thoracic segment of its body.

The extraordinary powers of flight which insects possess are due to the conjoined influences of the two conditions that have been named—the lightness of their air-filled bodies, and the strength of their chest-packed muscles. Where light air is circulated instead of heavy blood, great vascularity serves only to make existence more ethereal. Plethora probably takes the insect nearer to the skies, instead of dragging it towards the dust. The hawk-moth, with its burly body, may often be seen hovering gracefully, on quivering wings, over some favourite flower, as if it were hung there on cords, while it rifles it of its store of accumulated sweets by means of its long unfolded tongue. The common house-fly makes 600 strokes every second in its ordinary flight, and gets through five feet of space by means of them; but when alarmed, it can increase the velocity of its wing-strokes some five or six fold, and move through thirty-five feet in the second. Kirby believed, that if the house-fly were made equal to the horse in size, and had its muscular power increased in the same proportion, it would be able to traverse the globe with the rapidity of lightning. The dragon-fly often remains on the wing in pursuit of its prey for hours at a stretch, and yet will sometimes baffle the swallow by its speed, although that bird is calculated to be able to move at the rate of a mile in a minute. But the dexterity of this insect is even more surprising than its swiftness, for it is able to do what no bird can: it is able to stop instantaneously in the midst of its most rapid course, and change the direction of its flight, going sideways or backwards, without altering the position of its body.

As a general rule, insect wings that are intended for employment in flight are transparent membranes, with the course of the air-tubes marked out upon them as opaque nervures. These air-tubes, it will be remembered, are lined by spires of dense cartilage; and hence it is that they become nervures so well adapted to act like tent-lines in keeping the expanded membranes stretched. In the dragon-flies, the nervures are minutely netted for the sake of increased strength; in the bees, the nervures are simply parallel. Most insects have two pairs of these transparent membraneous wings; but in such as burrow, one pair is converted into a dense leather-like case, under which the other pair are folded away. In the flies, only one pair of wings can be found at all, the other pair being changed into two little club-shaped bodies, called balancers.

Butterflies and moths are the only insects that fly by means of opaque wings; but in their case the opacity is apparent rather than real, for it is caused by the presence of a very beautiful layer of coloured scales spread evenly over the outer surface of the membranes. When these scales are brushed off, membraneous wings of the ordinary transparent character are disclosed. The scales are attached to the membrane by little stems, like the quill-ends of feathers, and they are arranged in overlapping rows. The variegated colours and patterns of the insects are entirely due to them. If the wings of a butterfly be pressed upon a surface of card-board covered with gum-water to the extent of their own outlines, and be left there until the gum-water is dry, the outer layer of scales may be rubbed off with a handkerchief, and the double membranes and intervening nervures may be picked away piecemeal with a needle's point, and there will remain upon the card a most beautiful representation of the other surface of the wings, its scales being all preserved by the gum in their natural positions. If the outlines of the wings be carefully pencilled first, and the gum-water be then delicately and evenly brushed on, just as far as the outlines, a perfect and durable fac-simile, in all the original variety of colour and marking, is procured, which needs only to have the form of the body sketched in, to make it a very pretty and accurate delineation of the insect.