There are three classes of fliers in this order, the form of whose bodies, as well as the shape and circumstances of their wings, is different. First are the slender flies—the gnats, gnat-like flies, and crane-flies (Tipulariæ). The bodies of these are light, their wings narrow, and their legs long, and they have no winglets. Next are those whose bodies, though slender, are more weighty—the Asilidæ, Conopsidæ, &c.; these have larger wings, shorter legs, and very minute and sometimes even obsolete winglets. Lastly come the flies, the Muscidæ, &c., and their affinities, whose bodies being short, thick, and often very heavy, are furnished not only with proportionate wings and shorter legs, but also with conspicuous winglets. From these comparative differences and distinctions, we may conjecture in the first place—since the lightest bodies are furnished with the longest legs, and the heaviest with the shortest—that the legs act as poisers and rudders, that keep them steady while they fly, and assist them in directing their course[576]; and in the next—since the winglets are largest in the heaviest bodies, and altogether wanting in the lightest—that one of their principal uses is to assist the wings when the insect is flying.
The flight of the Tipularian genera is very various. Sometimes, as I have observed, they fly up and down with a zigzag course; at others in vertical curves of small diameter, like some birds; at others, again, in horizontal curves:—all these lines they describe with a kind of skipping motion. Sometimes they would seem to flit in every possible way—upwards, downwards, athwart, obliquely, and sometimes almost in circles. The common gnat (Culex pipiens) seems to sail along also in various directions. The motion of its wings, if it does not fly like a hawk, is so rapid as not to be perceptible. When the crane-fly (Tipula oleracea) is upon the wing, its fore-legs are placed horizontally, pointing forwards, and the four hind ones stretched out in an opposite direction, the one forming the prow and the other the stern of the vessel, in its voyage through the ocean of air. The legs of another insect of this tribe (Hirtæa Marci) all point towards the anus in flight, the long anterior pair forming an acute angle with the body:—thus, perhaps, it can better cut the air.
I have often been amused in my walks with the motions of the hornet-fly (Asilus crabroniformis), belonging to the second division just mentioned. This insect is carnivorous, living upon small flies. When you are taking your rambles, you may often observe it alight just before you;—as soon as you come up, it flies a little further, and will thus be your avant-courier for the whole length of a long field. This usually takes place, I seem to have observed, when a path lies under a hedge; and perhaps the object of this manœuvre may be the capture of prey. Your motions may drive a number of insects before you, and so be instrumental in supplying it with a meal. Other species of the genus have the same habit.
The aërial progress of the fly tribes, including the gad-flies (Œstridæ); horse-flies (Tabanidæ); carrion-flies (Muscidæ), and many other genera—which constitute the heavy horse amongst our two-winged fliers—is wonderfully rapid, and usually in a direct line. An anonymous observer in Nicholson's Journal[577] calculates that, in its ordinary flight, the common house-fly (Musca domestica) makes with its wings about 600 strokes, which carry it five feet, every second. But if alarmed, he states their velocity can be increased six- or seven-fold, or to thirty or thirty-five feet, in the same period. In this space of time a race-horse could clear only ninety feet, which is at the rate of more than a mile in a minute. Our little fly, in her swiftest flight, will in the same space of time go more than the third of a mile. Now compare the infinite difference of the size of the two animals (ten millions of the fly would hardly counterpoise one racer), and how wonderful will the velocity of this minute creature appear! Did the fly equal the race-horse in size, and retain its present powers in the ratio of its magnitude, it would traverse the globe with the rapidity of lightning.
It seems to me, that it is not by muscular strength alone that many insects are enabled to keep so long upon the wing. Every one who attends to them must have noticed, that the velocity and duration of their flights depend much upon the heat or coolness of the atmosphere: especially the appearance of the sun. The warmer and more unclouded his beam, the more insects are there upon the wing, and every diurnal species seems fitted for longer or more frequent excursions. As these animals have no circulating fluid except the air in their tracheæ and bronchiæ, their locomotive powers, with few exceptions, must depend altogether upon the state of that element. When the thermometer descends below a certain point they become torpid, and when it reaches a certain height they revive; so that the air must be regarded, in some sense, as their blood, or rather the caloric that it contains; which when conveyed by the air, it circulates quickly in them, invigorates all their motions, enters into the muscles and nervures of their wings, maintaining their tension, and by the greater or less rapidity of its pulsations accelerating or diminishing their action.
Having given you all the information that I can collect with respect to the motions of perfect insects in the air, I must next say something concerning their modes of locomotion in or upon the water. These are of two kinds, swimming and walking. Observe—I call that movement swimming, in which the animal pushes itself along by strokes—while in walking, the motion of the legs is not different from what it would be if they were on land. Most insects that swim have their posterior legs peculiarly fitted for it, either by a dense fringe of hairs on the shank and foot, as in the water-beetles (Dytiscus)[578], or the water-boatmen (Notonecta); or by having their terminal joints very much dilated—as in the whirlwig (Gyrinus)—so as to resemble the paddle of an oar[579]. When the Dytisci rise to the surface to take in fresh air—a silver bubble of which may often be seen suspended at their anus—they ascend, as it should seem, merely in consequence of their being specifically lighter than the water; but when they descend or move horizontally, which they do with considerable rapidity, it is by regular and successive strokes of their swimming legs. While they remain suspended at the surface, these legs are extended so as to form a right angle with their body. The water-boatmen swim upon their back, which enables them to see readily and seize the insects that fall upon the water, which are their prey. Sigara, however, a cognate genus separated from Notonecta by Fabricius, swims in the ordinary way. As the Gyrini are usually in motion at the surface, whirling round and round in circles, it is probable that their legs are best adapted to this movement. They dive down, however, with great ease and velocity when alarmed. The common water-bug (Gerris lacustris), though it never goes under water, will sometimes swim upon the surface, which it does by strokes of the intermediate and posterior legs[580]. These, however, are neither fringed nor dilated, but very long and slender, with claws, not easily detected, situated under the apex of the last joint of the foot, which covers and conceals them. The underside of their body—as is the case with Elophorus, and many other aquatic insects—is clothed with a thick coat of gray hairs like satin, which in certain lights have no small degree of lustre, and protect its body from the effects of the water. Some insects, that are not naturally aquatic, if they fall into the water will swim very well. I once saw a kind of grasshopper (Acrydium), which by the powerful strokes of its hind legs pushed itself across a stream with great rapidity.
Other insects walk, as it were, in the water, moving their legs much in the same way as they would do on the land. Many smaller species of water-beetles, belonging to the genera Hydrophilus, Elophorus, Hydræna, Parnus, Limnius, &c. thus win their way in the waves.—Thus also the water-scorpion (Nepa) pursues its prey; and the little water-mites (Hydrachna) may be seen in every pool thus working their little legs with great rapidity, and moving about in all directions.—Some spiders also will not only traverse the surface of the waters, but, as you have heard with respect to one[581], descend into their bosom. There are other insects moving in this way that are not divers. Of this kind are the aquatic bugs (Gerris lacustris, Hydrometra Stagnorum, Velia Rivulorum, &c. Latr.). The first can walk, run, and even leap, which it does upon its prey, as well as swim upon the surface. The second, remarkable for its extreme slenderness, and for its prominent hemispherical eyes—which, though they are really in the head, appear to be in the middle of the body—rambles about in chase of other insects, in considerable numbers, in most stagnant waters. The Velia is to be met with chiefly in running streams and rivers, coursing very rapidly over their waves[582]. The two last species neither jump nor swim.
I am next to say a few words upon the motions of insects that burrow, either to conceal themselves or their young. Though burrowing is not always a locomotion, I shall consider it under this head, to preserve the unity of the subject. Many enter the earth by means of fore-legs particularly formed for the purpose. The flat dentated anterior shanks, with slender feet, that distinguish the chafers (Petalocera)—most of which in their first states live under ground, and many occasionally in their last—enable them to make their way either into the earth or out of it. Two other genera of beetles (Scarites and Clivina, Latr.)[583] have these shanks palmated, or armed with longer teeth at their extremity, for the same purpose. But the most remarkable burrower amongst perfect insects is that singular animal the mole-cricket (Gryllotalpa vulgaris)[584]. This creature is endowed with wonderful strength, particularly in its thorax and fore legs. The former is a very hard and solid shell or crust, covering like a shield the trunk of the animal; and the latter are remarkably fitted for burrowing, both by their strength and construction. The shanks are very broad, and terminate obliquely in four enormous sharp teeth[585], like so many fingers: the foot consists of three joints—the two first being broad and tooth-shaped, and pointing in an opposite direction to the teeth of the shank; and the last small, and armed at the extremity with two short claws. This foot is placed inside the shank, so as to resemble a thumb and perform the office of one[586]. The direction and motion of these hands, as in moles, is outwards; thus enabling the animal most effectually to remove the earth when it burrows. By the help of these powerful instruments, it is astonishing how instantaneously it buries itself. This creature works under ground like a field-mouse, raising a ridge as it goes; but it does not throw up heaps like its name-sake the mole. They will in this manner undermine whole gardens; and thus in wet and swampy situations, in which they delight, they excavate their curious apartments, before described. The field-cricket (Gryllus campestris) is also a burrower, but by means of different instruments; for with its strong jaws, toothed like the claws of a lobster, but sharper, in heaths and other dry situations it perforates and rounds its curious and regular cells. The house-cricket (G. domesticus), which, on account of the softness of the mortar, delights in new-built houses, with the same organs, to make herself a covered-way from room to room, burrows and mines between the joints of the bricks and stones[587].
But of all the burrowing tribes, none are so numerous as those of the order Hymenoptera. Wherever you see a bare bank, of a sunny exposure, you always find it full of the habitations of insects belonging to it;—and besides this, every rail and old piece of timber is with the same view perforated by them. Bees; wasps; bee-wasps (Bembex); spider-wasps (Pompilus); fly-wasps (Mellinus, Cerceris, Crabro), with many others, excavate subterranean or ligneous habitations for their young. None is more remarkable in this respect than the sand-wasp (Ammophila), or as it might be better named—since it always commits its eggs to caterpillars which it inhumes—the caterpillar-wasp. It digs its burrows, by scratching with its fore legs like a dog or a rabbit, dispersing with its hind ones, which are particularly constructed for that purpose, the sand so collected[588].
Since most of these burrows are designed for the reception of the eggs of the burrowers, I shall next describe to you the manner in which one of the long-legged gnats, or crane-flies (Tipula variegata,)—a proceeding to which I was myself a witness—oviposits. Choosing a south bank bare of grass, she stood with her legs stretched out on each side, and kept turning herself half round backwards and forwards alternately. Thus the ovipositor, which terminates her long cylindrical pointed abdomen, made its way into the hard soil, and deposited her eggs in a secure situation. All, however, were not committed to the same burrow; for she every now and then shifted her station, but not more than an inch from where she bored last. While she was thus engaged, I observed her male companion suspended by one of his legs on a twig, not far from her. The common turf-boring crane-fly (T. oleracea), when engaged in laying eggs, moves over the grass with her body in a vertical position, by the help—her four anterior legs being in the air—of her two posterior ones, and the end of her abdomen, which performs the office of another. Whether in boring, like T. variegata, she turns half round and back, does not appear from Reaumur's account[589].