Chamberss Edinburgh Journal, No. 437 | Page 9

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.