Wings equally ample, forming the quadrant of a circle, and with five or six nervures diverging from their base, distinguish the strepsipterous tribe. When unemployed, these are folded longitudinally[555].
Probably in the next order (Orthoptera), the Tegmina, or wing-covers—since they are usually of a much thinner substance than elytra—assist them in flying. They are however quite covered by irregular reticulations, produced by various nervures sent forth by the longitudinal ones, and running in all directions. When at rest, the inner part of one laps over that of the other[556]: but in different genera there is a singular variation in this circumstance. Thus in Blatta, Phasma, and male Acridæ, and generally speaking, but not invariably, in Locusta and Truxalis,—the left elytrum laps over the right: but in Mantis; Mantispa; some female Acridæ; Gryllus; and Gryllotalpa; the right is laid over the left. The wings in this order, though always ample and larger than the tegmina, do not invariably form a quadrant of a circle, falling often short of it. They are extended by means of nervures, which, like so many rays, diverge from the base of the wing, and are intersected alternately by transverse ones, which thus form quadrangular areas, arranged like bricks in a wall. When at rest, they are longitudinally folded. The flight of these insects, as far as it has been observed, much resembles, it is said, that of certain birds. Ray tells us that both sexes of the house-cricket (Gryllus domesticus) fly with an undulating motion, like a woodpecker, alternately ascending with expanded wings, and descending with folded ones[557]. The field- and mole-crickets (Gryllus campestris and Gryllotalpa vulgaris), as we learn from Mr. White[558],—and, since the structure of their wings is similar, probably the other Orthoptera,—fly in the same way.
Hemipterous insects, with respect to their Hemelytra, may be divided into two classes. Those in which they are all of the same substance—varying from membrane to a leathery or horny crust[559]—and those in which the base and the apex are of different substances; the first being generally corneous, and the latter membranaceous[560]. The former or homopterous division includes the Cicadariæ, Latr.; Aphis; Chermes; Thrips; and Coccus;—and the latter the heteropterous division, comprehending besides the Geocorisæ, Latr., Notonecta; Sigara; Nepa; Ranatra; and Naucoris of Fabricius. The posterior tibiæ of some of this last division (Lygæus phyllopus, foliaceus, &c. F.) are furnished on each side with a foliaceous process—which may act the part of outriggers, and assist them in their flight[561]. I can give you no particular information with respect to the aërial movements of the insects of this order: the British species that belong to it are generally so minute that it is not easy to trace them with the naked eye; and unless some kind optician, which is much to be wished, would invent a telescope by which the proceedings of insects could be examined at a distance, there is no other way of studying them.
The four wings of the next order, the Trichoptera or case-worm flies, both in their shape and nervures resemble those of many moths[562]; only instead of scales they are usually covered with hairs, and the under wings, which are larger than the upper, fold longitudinally. Some of these flies, I have observed, move in a direct line, with their legs set out, which makes them look as if they were walking in the air. In flying they often apply their antennæ to each other, stretching them out straight, and thus probably are assisted in their motion.
The Lepidoptera vary so infinitely in the shape, comparative magnitude, and appendages of their wings, that I should detain you too long did I enlarge upon so multifarious a subject. I shall therefore only observe, that one species is described, both by Lyonet and De Geer[563] (Lobophora hexaptera), as having six wings; for besides the four ordinary ones, it has a winglet (Alula) attached to the base of the lower one, and placed, when the wings are folded, between it and the upper. These organs in this order you know are covered with scales of various shapes[564]. Their nervures are diverging rays, which issue either from a basal area or from the base itself, and terminate in the exterior margin[565]. The wings of many male butterflies, hawk-moths, and moths, are distinguished by a remarkable apparatus, noticed by De Geer, and since by many other naturalists[566] for keeping them steady and underanged in their flight. The upper wings, on their underside near their base, have a minute process, bent into a hook (Hamus), and covered with hairs and scales. In this hook one or more bristles (Tendo), attached to the base of the under wing, have their play. When the fly unfolds its wings, the hook does not quit its hold of the bristle, which moves to and fro in it as they expand or close. The females, which seldom fly far, often have the bristles but never the hook. The hairy tails of some insects, Sesia, belonging to the hawk-moth tribe, are expanded when they fly, so as to form a kind of rudder, which enables them to steer their course with more certainty.
The insects of this, and of every other order, except the Coleoptera, fly with their bodies in a horizontal position, or nearly so. As their wings are usually so ample, we need not wonder that the Lepidoptera are excellent fliers. Indeed they seem to flit untired from flower to flower, and from field to field; impelled at one while by hunger, and at another by love or maternal solicitude.—The distance to which some males will fly is astonishing. That of one of the silk-worm moths (Attacus Paphia) is stated to travel sometimes more than a hundred miles in this way[567].—Our most beautiful butterfly, the purple emperor (Apatura Iris), when he makes his first appearance fixes his throne on the summit of some lofty oak, from whence in sunny days, unattended by his empress, who does not fly, he takes his excursions. Launching into the air from one of the highest twigs, he mounts often to so great a height as to become invisible. When the sun is at the meridian his loftiest flights take place; and about four in the afternoon he resumes his station of repose[568].—The large bodies of hawk-moths (Sphinx, F.) are carried by wings remarkably strong both as to nervures and texture, and their flight is proportionably rapid and direct. That of butterflies is by dipping and rising alternately, so as to form a zigzag line with vertical angles, which the animal often describes with a skipping motion, so that each zigzag consists of smaller ones. This doubtless renders it more difficult for the birds to take them as they fly; and thus the male, when paired, often flits away with the female.
Amongst the Neuropterous tribes the most conspicuous insects are the dragon-flies (Libellulina), which—their metamorphosis, habits, mode of life, and characters considered—form a distinct natural order of themselves. Their four wings, which are nearly equal in size, are a complete and beautiful piece of net-work, resembling the finest lace, the meshes of which are usually filled by a pure, transparent, glassy membrane. In two of the genera belonging to this tribe, the wings, when the animal is at rest, are always expanded, so that they can take flight in an instant, no previous unfolding of these organs being necessary. In Agrion, the other genus of the tribe, the wings when they repose are not expanded. I have observed of these insects, and also of several others in different orders, that without turning they can fly in all directions—backwards, and to the right and left, as well as forwards. This ability to fly all ways, without having to turn, must be very useful to them when pursued by a bird. Leeuwenhoek once saw a swallow chasing an insect of this tribe, which he calls a Mordella, in a menagerie about a hundred feet long. The little creature flew with such astonishing velocity—to the right—to the left—and in all directions—that this bird of rapid wing and ready evolution was unable to overtake and entrap it; the insect eluding every attempt, and being generally six feet before it[569]. Indeed, such is the power of the long wings by which the dragon-flies are distinguished, particularly in Æshna and Libellula, and such the force of the muscles that move them, that they seem never to be wearied with flying. I have observed one of the former genus (Anax Imperator, Leach) sailing for hours over a piece of water—sometimes to and fro, and sometimes wheeling from side to side; and all the while chasing, capturing, and devouring the various insects that came athwart its course, or driving away its competitors—without ever seeming tired, or inclined to alight. Another species (Æshna variegata), very common in lanes and along hedges, which flies, like the Orthoptera, in a waving line, is equally alert and active after its prey. This however often alights for a moment, and then resumes its gay excursive flights. The species of the genus Agrion cut the air with less velocity; but so rapid is the motion of their wings, that they become quite invisible. Hawking always about for prey, the Agrions, from the variety of the colours of different individuals, form no uninteresting object during a summer stroll. With respect to the mode of flight of the other neuropterous tribes I have nothing to remark; for that of the Ephemeræ, which has been most noticed, I shall consider under another head.
The next order of insects, the Hymenoptera, attract also general attention as fliers, and from our earliest years. The ferocious hornet, with its trumpet of terror; the intrusive and indomitable wasp; the booming and pacific humble-bee, the frequent prey of merciless schoolboys; and that universal favourite, the industrious inhabitant of the hive,—all belonging to it,—are familiar to every one. And in summer-time there is scarcely a flower or leaf in field or garden, which is not visited by some of its numerous tribes. The four wings of these insects, the upper pair of which are larger than the under, vary much in their nervures. From the saw-flies (Serrifera), whose wings are nearly as much reticulated as those of some Neuroptera, to the minute Chalcis and Psilus, in which these organs are without nervures, there is every intermediate variety of reticulation that can be imagined[570]. It has been observed that the nervures of the wings are usually proportioned to the weight of the insect. Thus the saw-flies have generally bodies thicker than those of most other Hymenoptera, while those that have fewer nervures are more slender. This, however, does not hold good in all cases—so that the dimensions and cut of the wings, the strength of their nervures, and the force of their muscles, must also be taken into consideration. The wings of many of these insects when expanded, are kept in the same plane by means of small hooks (Hamuli) in the anterior margin of the under wing, which lay hold of the posterior margin of the upper[571]. Another peculiarity also distinguishes them. Base covers (Tegulæ), or small concavo-convex shields, protect the base of the wings from injury[572], or displacement.
The most powerful fliers in this order are the humble-bees, which, like the dung-chafers (Geotrupes), traverse the air in segments of a circle, the arc of which is alternately to right and left. The rapidity of their flight is so great, that could it be calculated, it would be found, the size of the creature considered, far to exceed that of any bird.—The aërial movements of the hive-bee are more direct and leisurely. When leaving the hive for an excursion, I have observed that as soon as they come out they turn about as if to survey the entrance, and then wheeling round in a circle, fly off. When they return to the hive, they often fly from side to side, as if to examine before they alight. When swarming, the heads of all are turned towards the group at the mouth of their dwelling; and upon rising into the air these little creatures fly so thick in every direction, as to appear like a kind of net-work with meshes of every angle. The queen also, upon going forth, when her object is to pair, after returning to reconnoitre, begins her flight by describing circles of considerable diameter, thus rising spirally with a rapid motion[573]. The object of these gyrations is probably to increase her chance of meeting with a drone.—I have not much to tell you with respect to the flight of other insects of this order, except that a spider-wasp (Pompilus viaticus) whose sting is redoubtable, and which often, when we are in the vicinity of sandy sunny banks, accompanies our steps, has a kind of jumping movement when it flies.
The next order, the Diptera, consists altogether of two-winged flies:—but to replace the under wings of the tetrapterous insects, they are furnished with poisers, and numbers of them also with winglets. The poisers (Halteres) are little membranaceous threads placed one under the origin of each wing, near a spiracle, and terminated by an oval, round, or triangular button, which seems capable of dilatation and contraction. The animal moves these organs with great vivacity, often when at rest, and probably when flying. Their winglets (Alulæ) are different from those of Dytiscus marginalis, and the moth before noticed. Like them, they are of rigid membrane, and fringed; but they consist generally of two concavo-convex pieces (sometimes surrounded by a nervure), situated between the wing and the poisers, which, when the insect reposes, fold over each other like the valves of a bivalve shell; but when it flies they are extended. The use of neither of these organs seems to have been satisfactorily ascertained. Dr. Derham thinks they are for keeping the body steady in flight; and asserts, that if either a poiser or winglet be cut off, the insect will fly as if one side overbalanced the other, till it falls to the ground; and that if both be cut off, they will fly awkwardly and unsteadily, as if they had lost some very necessary part[574]. Shelver cut off the winglets of a fly, leaving both wings and poisers, but it could no longer fly. He next cut off the poisers of another, leaving the wings and winglets, and the same result followed. He found, upon removing one of these organs, that they were not properly compared to balancers. Observing that a common crane-fly (Tipula crocata) moved the knee of the hinder tibia in connexion with the wing and poiser, he cut it off, and it could no longer fly: this last experiment, however, seems contradicted by the fact, which has been often observed, that the insects of this genus will fly when half their legs are gone. He afterwards cut off both its poisers, when it could neither fly nor walk. Hence he conjectures that the poisers are connected with the feet, and are air-holders[575]. I have often seen flies move their poisers very briskly when at rest, particularly Seioptera vibrans, before mentioned. This renders Shelver's conjecture—that they are connected with respiration—not improbable. Perhaps by their action some effect may be produced upon the spiracle in their vicinity, either as to the opening or closing of it.