It is a mistake to suppose that it is unnecessary to consider other kinds of flight when studying that of birds. Even those who are not interested in the abstruse problems of the mechanism of bird’s flight, will find that comparisons made between birds, bats, butterflies and beetles when on the wing, are immensely interesting, and help to bring out the peculiarities of each.
During the twilight hours of a still summer evening one may compare, with advantage, the rushing swoop of the screaming swift, borne with lightning speed upon long, ribbon-like pinions, with the curiously erratic flight of the woolly bat with beaded eyes, who has ventured abroad for his evening meal. One cannot but feel astonishment at the marvellous dexterity with which he twists and turns, now shooting up into the sky, now darting downward. What bird can beat him, or even match him, in the art of doubling back on his tracks? And one can put his skill at lightning turns to the test if one attempts to catch him in a butterfly net. Often indeed have I attempted this feat, but never yet with success.
In the glare of noon-day this aerial athlete may perhaps be found in a deep slumber, hanging head downwards behind the shutters of a cottage window, or in some crevice of a barn-roof. Gently seize him and as gently stretch out his wing. The moment one opens it one sees that it is constructed upon a totally different plan from that of a bird. In the first place a thin membrane, or fold of skin is seen to take the place of the series of quill-feathers found in the wing of the bird. In the second it will be found that this membrane is stretched between a series of long and very slender bony rods. These are excessively attenuated fingers. And if the hinder border of the wing-membrane be traced inwards it will be found to be attached to the hind limb. In some species it will be found that this membrane passes backwards beyond the leg to attach itself to the tail. Here, then, is a wing as efficient for its purpose as that of a bird, but constructed on a totally different plan.
Ages ago, before even the birds or the beasts had appeared on the earth, the winged dragons, which the Men of Science call Pterodactyles, held the proud position of being, not only the first, but the only creatures blessed with a backbone that could fly. Their wings resembled those of the bats, but differed in this, that instead of the wing-membrane being stretched between all the fingers, leaving only the thumb free, it was attached only to the fifth finger, leaving the remaining fingers free, and these were reduced to mere vestiges. As with the birds, the breast-bone was very broad and was furnished with a keel, while in the bats it takes the form of a jointed rod, down which no more than a slight keel is ever developed.
But millions of years before the Flying-dragons, birds, and bats came into being, the stupendous problem of flight had been solved. Far away in the distant Devonian Epoch, when the distribution of land and water over the earth’s surface was totally different from that of to-day, dragon-flies and caddis-flies disported themselves in the summer sun, amid landscapes that would seem strange to our eyes. For there were no trees and flowering plants, such as we know.
The dragon-flies of that remote epoch were very like those of to-day, whose dancing flights and graceful, swooping movements are such a delight to watch by reed-fringed pools, or river-banks, during the sweltering days of summer. This flight is very different from that of a bird, though it would be hard to say precisely in what it differs. But we have no such difficulty in regard to the broad outlines of the mechanism of such flight. To begin with there are two pairs of wings, and these appear to be fashioned out of some curiously gauze-like material, a sort of mesh-work tissue, often strikingly coloured. And they are obviously driven after a very different fashion from those of the bird. For in the bird they are moved by quivering muscles, attached to a bony, internal skeleton. In the dragon-fly—as with all insects—the hard skeleton, composed of a material known as “chitin,” forms the outside of the body and encloses the muscles. Finally, for we may not dwell very long over this aspect of flight, it is clear that the wings cannot have been derived from modified fore-legs, like those of the bat, or the bird. Rather, it would seem, they have developed out of plate-like breathing organs.
The restful twilight hours of summer tempt not only bats from their hiding places, but a host of other winged creatures which are rarely to be seen, or heard, during the glare of noon. Among these is the lumbering dor-beetle, who, with lazy drone steers clear of solid objects only with difficulty. Many, indeed, are his failures. He and his kin are no match for bats and owls, who find them juicy morsels! On the next opportunity catch one and examine him. His wings are curiously interesting. There are the usual two pairs: but the fore-wings have been changed to serve as covers for the hind-wings. During flight they are spread outwards, and indirectly, no doubt, assist flight. But the hind-wings are the real propellers. And it will be noticed that when not in use they can be folded up in a perfectly wonderful manner, so as to lie completely underneath the fore-wings, or “elytra,” so that when the creature is crawling it appears to be wingless.
Now compare these with the transparent wings of the bee, or the gorgeously scale-covered wings of the butterfly. It is well worth while. If this examination be done very carefully, and with the aid of a magnifying glass, it will be found that the fore and hind wings are yoked together in the wing of the bee, by a delicate mechanism of hooks. In the moths, but not in the butterflies, a bristle, or sometimes two or three bristles, serve the same purpose. Further, in the case of the bee it will be found that the fore-wing, when at rest, is folded longitudinally back upon itself.