Volume 2 — LETTER XXIII.

MOTIONS OF INSECTS. (Imago.)

III. The motions of insects in their perfect or imago state are various, and for various purposes; and the provision of organs by which they are enabled to effect them is equally diversified and wonderful. It will be convenient to divide this multifarious subject; I shall therefore consider their motions under two principal heads:—motions of insects reposing—and motions of insects in action;—and this last head I shall further subdivide into motions whose object is change of place, and sportive motions.

The first of these, motions of insects reposing, will not detain us long. The most remarkable is that of the long-legged gnats or crane-flies (Tipulæ).—When at rest upon any wall or ceiling, sometimes standing upon four legs, and sometimes upon five, you may observe them elevate and depress their body alternately. This oscillating movement is produced by the weight of their body and the elasticity of their legs, and is constant and uninterrupted during their repose. Unless it be connected with the respiration of the animal, it is not easy to say what is the object of it. Moths, when feeling the stimulus of desire, or under alarm, set their whole body into a tremor[468]. A living specimen of the hawk-moth of the willow being once brought me, upon placing it upon my hand, after ejecting a milky fluid from its anus, it put its wings and body into a most rapid vibration, which continued more than a minute, when it flew away. A butterfly, called by Aurelians "The large skipper," (Hesperia Sylvanus,) when it alights, which it does very often, for they are never long on the wing, always turns half-way round; so that, if it settles with its head from you, it turns it towards you.

Others of the motions in question are merely those of parts. Butterflies, when standing still in the sun, as you have doubtless often observed,

thus, it should seem, unless this motion be connected with their respiration, alternately warming and cooling their bodies. You have probably noticed a very common little fly, of a shining black, with a black spot at the end of its wings (Seioptera vibrans[469]). It has received its trivial name (vibrans) from the constant vibration which, when reposing, it imparts to its wings. This motion also, I have reason to think, assists its respiration.—Some insects when awake are very active with their antennæ, though their bodies are at rest. I remember one evening attending for some time to the proceedings of one of those caseworm-flies (Leptocerus), that are remarkable, like certain moths, for their long antennæ. It was perched upon a blade of grass, and kept moving these organs, which were twice as long as itself, in all directions, as if by means of them it was exploring every thing that occurred in its vicinity.—Many Tipulæ, and likewise some mites (Acarus vibrans and Gamasus motatorius), distinguished by long anterior legs, from this circumstance denominated pedes motatorii by Linné, holding them up in the air impart to them a vibratory motion, resembling that of the antennæ of some insects[470].—I scarcely need mention, what must often have attracted your attention, the actions of flies when they clean themselves; how busily they rub and wipe their head and thorax with their fore legs, and their wings and abdomen with their hind ones.—Perhaps you are not equally aware of the use to which the rove-beetles (Staphylinus, L.) put their long abdomen. They turn it over their back not only to put themselves in a threatening attitude, as I lately related[471], but also to fold up their wings with it, and pack them under their short elytra.

With respect to the motions of insects in action, they may be subdivided, as was just observed, into motions whose object is change of place—and sportive motions.

The locomotions of these animals are walking, running, jumping, climbing, flying, swimming, and burrowing. I begin with the walkers.

The mode of their walking depends upon the number and kind of their legs. With regard to these, insects may be divided into four natural classes; viz. Hexapods, or those that have only six legs: such are those of every order except the Aptera of Linné, of which only three or four genera belong to this class.—Octopods, or those that have eight legs, including the tribes of mites (Acarina); spiders (Araneidæ); long-legged spiders (Phalangidæ); and scorpions (Scorpionidæ):—Polypods, or those that have fourteen legs, consisting of the woodlouse tribe (Oniscidæ);—and Myriapods, or those that have more than fourteen legs—often more than a hundred—composed of the two tribes of centipedes (Scolopendridæ) and millepedes (Julidæ). The first of these classes may be denominated proper, and the rest improper insects. The legs of all seem to consist of the same general parts; the hip, trochanter, thigh, shank, and foot; the four first being usually without joints (though in the Araneidæ, &c. the shank has two), and the foot having from one to above forty[472].

In walking and running, the hexapods, like the larvæ that have perfect legs, move the anterior and posterior leg of one side and the intermediate of the other alternately, as I have often witnessed. De Geer, however, affirms that they advance each pair of legs at the same time[473]; but this is contrary to fact, and indeed would make their ordinary motions, instead of walking and running, a kind of canter and gallop. Whether those that have more than six feet move in this way—which is not improbable—from the difficulty of attending at the same time to the movements of so many members, is not easily ascertained.

The dog-tick (Ixodes Ricinus), if when young and active it moves in the same way that it does when swoln to an enormous size with blood, seems to afford an exception to the mode of walking just described. It first uses, says Ray, its two anterior legs as antennæ to feel out its way, and then fixing them, brings the next pair beyond them, which being also fixed, it takes a second step with the anterior, and so drags its bloated carcase along[474]. Redi observes, that when scorpions walk they use those remarkable comb-like processes at the base of their posterior legs to assist them in their motions, extending them and setting them out from the body, as if they were wings: and his observation is confirmed by Amoreux, who calls them ventral swimmers[475]. I have often noticed a millepede (Julus terrestris), frequently found under the bark of trees, and where there is not a free circulation of air, the motions of which are worthy of attention. Observed at a little distance, it seems to glide over the surface, like a serpent, without legs; but a nearer inspection shows how its movement is accomplished. Alternate portions of its numerous legs are extended beyond the line of the body, so as to form an obtuse angle with it, while those in the intervals preserve a vertical direction. So that, as long as it keeps moving, little bunches of the legs are alternately in and out from one end to the other of its long body; and an amusing sight it is to see the undulating line of motion successively beginning at the head and passing off at the tail.—The motion of centipedes (Scolopendra), as well as that of this insect and its congeners, is retrogressive as well as progressive. Put your finger to the common one (Lithobius forficatus), and it will immediately retrograde, and with the same facility as if it was going forwards. This difference, however, is then observable—it uses its four hind legs, which, when it moves in the usual way, are dragged after it. Almost all the other apterous insects, as well as many of those in the other orders, can move in all directions; backwards, and towards both sides, as well as forwards. Bonnet mentions a spider (not a spinner) that always walked backwards when it attacked a large insect of its own tribe; but when it had succeeded in driving it from a captive fly, which however it did not eat, it walked forwards in the ordinary way[476].

Insects vary much in their walking paces: some crawling along; others walking slowly; and others moving with a very quick step. The field cricket (Gryllus campestris) creeps very slowly—the bloody-nose beetle (Timarcha tenebricosa) and the oil-beetle (Meloe Proscarabæus) march very leisurely; the spider-wasps (Pompilus) walk by starts, as it were, vibrating their wings, at the same time, without expanding them; while flies, ichneumons, wasps, &c., and many beetles, walk as fast as they can. One insect, a kind of snake-fly (Mantispa pagana), is said to walk upon its knees. The crane-flies (Tipula oleracea) and shepherd-spiders (Phalangium) have legs so disproportionately long, that they seem to walk upon stilts; but when we consider that they have to walk over and amongst grass,—the former laying its eggs in meadows,—we shall see the reason of this conformation. Insects do not always walk in a right line; for I have often observed the little midges (Psychoda, Latr.), when walking up glass, moving alternately from right to left and from left to right, as humble-bees fly, so as to describe small zigzags.

Numerous are the insects that run. Almost all the predaceous tribes, the black dors, clocks, or ground-beetles (Eutrechina), and their fellow destroyers the Cicindelæ, and other Eupterina—which Linné, with much propriety, has denominated the tigers of the insect world,—are gifted with uncommon powers of motion, and run with great rapidity. The velocity, in this respect, of ants is also very great.—Mr. Delisle observed a fly—so minute as to be almost invisible—which ran nearly three inches in a demi-second, and in that space made 540 steps. Consequently it could take a thousand steps during one pulsation of the blood of a man in health[477]. Which is as if a man, whose steps measured two feet, should run at the incredible rate of more than twenty miles in a minute! How astonishing then are the powers with which these little beings are gifted!—The forest-fly (Hippobosca), and its kindred genus Ornithomyia parasitic upon birds, are extremely difficult to take, as I have more than once experienced, from their extreme agility. I lost one from this circumstance two years ago that I found upon the sea-lark (Charadrius Hiaticula) and which appeared to be non-descript. Another most singular insect, which though apterous is nearly related to these—I mean the louse of the bat (Nycteribia Vespertilionis), is still more remarkable for its swiftness. Its legs, as appears from the observations of Colonel Montague, are fixed in an unusual position on the upper side of the trunk. "It transports itself," to use the words of the gentleman just mentioned, "with such celerity, from one part of the animal it inhabits to the opposite and most distant, although obstructed by the extreme thickness of the fur, that it is not readily taken."——"When two or three were put into a small phial, their agility appeared inconceivably great; for, as their feet are incapable of fixing upon so smooth a body, their whole exertion was employed in laying hold of each other; and in this most curious struggle they appeared actually flying in circles: and when the bottle was reclined, they would frequently pass from one end to the other with astonishing velocity, accompanied by the same gyrations: if by accident they escaped each other, they very soon became motionless: and as quickly were the whole put in motion again by the least touch of the bottle, or the movement of an individual[478]."—Incredibly great also is the rapidity with which a little reddish mite, with two black dots on the anterior part of its back (Gamasus Baccarum), common upon strawberries, moves along. Such is the velocity with which it runs, that it appears rather to glide or fly than to use its legs.

When insects walk or run, their legs are not the only members that are put in motion. They will not, or rather cannot, stir a step till their antennæ are removed from their station of repose and set in action. When the chafers or petalocerous beetles are about to move, these organs, before concealed, instantly appear, and the laminæ which terminate them being separated from each other as widely as possible, they begin their march. They employ their antennæ, however, not as feelers to explore surrounding objects,—their palpi being rather used for that purpose,—but, it should seem, merely to receive vibrations, or impressions from the atmosphere, to which these laminæ, especially in the male cockchafers, or rather tree-chafers (Melolonthæ) present a considerable surface. Yet insects that have filiform or setaceous antennæ appear often to use them for exploring. When the turnip-flea (Haltica oleracea) walks, its antennæ are alternately elevated and depressed.—The same thing takes place with some woodlice (Oniscidæ), which use them as tactors, touching the surface on each side with them, as they go along. This is not however constantly the use of this kind of antennæ; for I have observed that Telephorus lividus,—a narrow beetle with soft elytra, common in flowers,—when it walks vibrates its setaceous antennæ very briskly, but does not explore the surface with them. The parasitic tribes of Hymenoptera, especially the minute ones, when they move vibrate these organs most intensely, and probably by them discover the insect to which the law of their nature ordains that they should commit their eggs; some even using them to explore the deep holes in which a grub, the appropriate food of their larva, lurks[479]. But upon this subject I shall have occasion to enlarge when I treat of the senses of insects.—Antennæ are sometimes used as legs. A gnat-like kind of bug (Ploiera vagabunda) has very short anterior legs, or rather arms; while the two posterior pair are very long. Its antennæ also are long. When it walks, which it does very slowly, with a solemn measured step, its fore legs, which perhaps are useful only in climbing, or to seize its prey, are applied to the body, and the antennæ being bent, their extremity, which is rather thick, is made to rest upon the surface on which the animal moves, and so supply the place of fore-legs[480].—Mr. Curtis suspects that Xyela pusilla, a hymenopterous insect related to Xiphydria, uses its maxillary palpi as legs[481]. I have observed that mites often use the long hairs with which the tail of some species is furnished, to assist them in walking.

Another mode of motion with which many insects are endowed is jumping. This is generally the result of the sudden unbending of the articulations of the posterior legs and other organs, which before had received more than their natural bend. This unbending impresses a violent rotatory motion upon these parts, the impulse of which being communicated to the centre of gravity, causes the animal to spring into the air with a determinate velocity, opposed to its weight more or less directly[482]. Various are the organs by which these creatures are enabled to effect this motion. The majority do it by a peculiar conformation of the hind legs; others, by a pectoral process; and others, again, by means of certain elastic appendages to the abdomen.

The hind legs of many beetles are furnished with remarkably large and thick thighs. Of this description are several species of weevils; for instance, Orchestes and Ramphus; the whole tribe of skippers (Haltica), and the splendid Asiatic tribe of Sagra[483], &c. The object of these disproportioned and clumsy thighs is to allow space for more powerful muscles, by which the tibiæ, when the legs are unbent, are impelled with greater force. In the Orthoptera order all the grasshoppers, including the genera Gryllotalpa; Gryllus; Tridactylus; Locusta; Acrida; Pterophylla; Pneumora; Truxalis; Acrydium; Tetrix, &c.—are distinguished by incrassated posterior thighs; which however are much longer, more tapering and shapely, (they are indeed somewhat clumsy in the two first genera, the crickets,) than those of most of the Coleoptera that are furnished with them. When disposed to leap, these insects bend their hind leg so as to bring the shank into close contact with the thigh—which has often a longitudinal furrow armed with a row of spines on each side to receive it. The leg being thus bent, they suddenly unbend it with a jerk, when pushing against the plane of position, they spring into the air often to a considerable height and distance. A locust, which however is aided by its wings, it is said will leap two hundred times its own length[484].—Aristophanes, in order to make the great and good Athenian philosopher, Socrates, appear ridiculous, represents him as having measured the leap of a flea[485]. In our better times scientific men have done this without being laughed at for it, and have ascertained that, comparatively, it equalled that of the locust, being also two hundred times its length. Being effected by muscular force, without the aid of wings, this is an astonishing leap.—There are several insects, however, which, although they are furnished with incrassated posterior thighs, do not jump. Of this description are some beetles belonging to the genus Necydalis, (Oedemera, Oliv.) in which this seems a peculiarity of the male: and amongst the Hymenoptera, not to mention others, several species of Chalcis, and all that are known of that singular genus Leucospis.

Many insects, that jump by means of their posterior legs, have not these thighs. This is said to be the case with Scaphidium, a little tribe of beetles[486]: and one of the same order, that seems to come between Anobium and Ptilinus, found by our friend the Rev. R. Sheppard, and which I have named after him Choragus Sheppardi, is similarly circumstanced.—In the various tribes of frog-hoppers (Cercopidæ, &c.) the posterior tibiæ appear to be principally concerned in their leaping. These are often very long, and furnished, on their exterior margin, with a fringe of stiff hairs, or a series of strong spines, by pressing which against the plane of position they are supposed to be aided in effecting this motion. On this occasion they bend their legs like the grasshoppers, and then unbending kick them out with violence[487]. Many of them, amongst the rest Cercopis spumaria, have the extremity of the above tibiæ armed with a coronet of spines; these are of great use in pushing them off when the legs are unbended. This insect, when about to leap, places its posterior thighs in a direction perpendicular to the plane of position, keeping them close to the body; it next with great violence pushes them out backwards, so as to stretch the leg in a right line. These spines then lay hold of the surface, and by their pressure enable the body to spring forwards, when, being assisted by its wings, it will make astonishing leaps, sometimes as much as five or six feet, which is more than 250 times its own length; or as if a man of ordinary stature should be able at once to vault through the air to the distance of a quarter of a mile. Upon glass, where the spines are of no use, the insect cannot leap more than six inches[488].—The species of another genus of the homopterous Hemiptera (Chermes), that jump very nimbly by pushing out their shanks, are perhaps assisted in this motion by a remarkable horn looking towards the anus, which arms their posterior hip.—Some bugs that leap well, Acanthia saltatoria, &c. seem to have no particular apparatus to assist them, except that their posterior tibiæ are very long.—Several of the minute ichneumons also jump with great agility, but by what means I am unable to say.—There is a tribe of spiders, not spinners, that leap even sideways upon their prey. One of these (Salticus scenicus), when about to do this, elevates itself upon its legs, and lifting its head seems to survey the spot before it jumps. When these insects spy a small gnat or fly upon a wall, they creep very gently towards it with short steps, till they come within a convenient distance, when they spring upon it suddenly like a tiger.—Bartram observed one of these spiders that jumped two feet upon a humble-bee. The most amusing account, however, of the motions of these animals is given by the celebrated Evelyn in his Travels. When at Rome, he often observed a spider of this kind hunting the flies which alighted upon a rail on which was its station. It kept crawling under the rail till it arrived at the part opposite to the fly, when stealing up it would attempt to leap upon it. If it discovered that it was not perfectly opposite, it would immediately slide down again unobserved, and at the next attempt would come directly upon the fly's back. Did the fly happen not to be within a leap, it would move towards it so softly, that its motion seemed not more perceptible than that of the shadow of the gnomon of a dial. If the intended prey moved, the spider would keep pace with it as exactly as if they were actuated by one spirit, moving backwards, forwards, or on each side without turning. When the fly took wing, and pitched itself behind the huntress, she turned round with the swiftness of thought, and always kept her head towards it, though to all appearance as immovable as one of the nails driven into the wood on which was her station: till at last, being arrived within due distance, swift as lightning she made the fatal leap and secured her prey[489]. I have had an opportunity of observing very similar proceedings in Salticus scenicus.

But the legs of insects are not the only organs by which they leap. The numerous species of the elastic beetles (Elater), skip-jacks as some call them, perform this motion by means of a pectoral process or mucro. These animals having very short legs, when laid upon their backs, cannot by their means recover a prone position. To supply this seeming defect in their structure, Providence has furnished them with an instrument which, when they are so circumstanced, enables them to spring into the air and recover their standing. If you examine the breast (pectus) of one of these insects, you will observe between the base of the anterior pair of legs a short and rather blunt process, the point of which is towards the anus. Opposite to this point, and a little before the base of the intermediate legs, you will discover in the after-breast (postpectus) a rather deep cavity, in which the point is often sheathed. This simple apparatus is all that the insect wants to effect the above purpose. When laid upon its back, in your hand if you please, it will first bend back, so as to form a very obtuse angle with each other, the head and trunk, and abdomen and metathorax, by which motion the mucro is quite liberated from its sheath; and then bending them in a contrary direction, the mucro enters it again, and the former attitude being briskly and suddenly resumed, the mucro flies out with a spring, and the insect rising, sometimes an inch or two into the air, regains its legs and moves off. The upper part of the body, by its pressure against the plane of position, assists this motion, during which the legs are kept close to its underside. Cuvier, when he says that man and birds are the only animals that can leap vertically[490], seems to have forgotten this leap of Elaters, which is generally vertical, the trunk being vertically above the organ that produces the leap.

Other insects again leap by means of the abdomen or some organs attached to it. An apterous species—belonging to the Ichneumonidæ, and to the genus Cryptus—takes long leaps by first bending its abdomen inwards, as De Geer thinks, and then pushing it with force along the plane of position[491]. There is a tribe of minute insects amongst the Aptera, found often under bark, sometimes on the water, and in various other situations, which Linné has named Podura, a term implying that they have a leg in their tail. This is literally the fact. For the tail, or anal extremity, of these insects is furnished with an inflexed fork[492], which, though usually bent under the body, they have the power of unbending; during which action, the forked spring, pushing powerfully against the plane of position, enables the animal to leap sometimes two or three inches. What is more remarkable, these little animals are by this organ even empowered to leap upon water. There is a minute black species (P. aquatica), which in the spring is often seen floating on that contained in ruts, hollows, or even ditches, and in such infinite numbers as to resemble gunpowder strewed upon the surface. When disturbed, these black grains are seen to skip about as if ignited, jumping with as much ease as if the fluid were a solid plane, that resists their pressure. The insects of another genus—separated from Podura by Latreille under the name of Sminthurus—have also an anal spring, which when bent under the body nearly reaches the head. These, which are of a more globose form than Podura, are so excessively agile that it is almost impossible to take them. Pressing their spring against the surface on which they stand, and unbending it with force, they are out of your reach before your finger can come near them. One of them, S. fuscus, besides the caudal fork, has a very singular organ, the use of which is to prevent it from falling from a perpendicular surface, on which they are often found at a great height from the ground. Between the ends of the fork there is an elevated cylinder or tube, from which the animal, when necessary, can protrude two long, filiform, flexible transparent threads covered with a slimy secretion. By these, when it has lost its hold, it adheres to the surface on which it is stationed[493]. Another insect related to the common sugar-louse, and called by Latreille Machilis polypoda, in some places common under stones[494], has eight pair of springs, one on each ventral segment of the abdomen, by means of which it leaps to a wonderful distance, and with the greatest agility.

Climbing is another motion of insects that merits particular consideration: since, as this includes their power of moving against gravity—as we see flies and spiders do upon our ceilings, and up perpendicular surfaces even when of glass—it affords room for much interesting and curious inquiry. Climbing insects may be divided into four classes.—Those that climb by means of their claws;—those that climb by a soft cushion of dense hairs, that, more or less, lines the underside of the joints of their tarsi, the claw-joint excepted;—those that climb by the aid of suckers, which adhere (a vacuum being produced between them and the plane of position) by the pressure of the atmosphere;—and those that are enabled to climb by means of some substance which they have the power of secreting.

The first order of climbers—those that climb by means of their claws—includes a large proportion of insects, especially in the Coleoptera order—the majority of those that have five joints in their tarsi being of this description. The predaceous tribes, particularly the numerous and prowling ground-beetles (Eutrechina), often thus ascend the plants and trees after their prey. Thus one of them, the beautiful but ferocious Calosoma Sycophanta, mounts the trunk and branches of the oak to commit fearful ravages amongst the hordes of caterpillars that inhabit it[495]. By these the less savage but equally destructive tree-chafers (Melolonthæ), and those enemies of vegetable beauty the rose-chafers (Cetonia aurata), are enabled to maintain their station on the trees and shrubs that they lay waste. And by these also the water-beetles (Dytiscus, Hydrophilus, &c.) climb the aquatic plants.—But it is unnecessary further to enlarge upon this head; I shall only observe, that in most of the insects here enumerated, the claws appear to be aided by stiff hairs or bristles.

Other climbers ascend by means of foot-cushions (pulvilli) composed of hairs, as thickly set as in plush or velvet, with which the underside of the joints of their tarsi—the claw-joint, which is always naked, excepted—are covered. These cushions are particularly conspicuous in the beautiful tribe of plant-beetles (Chrysomelidæ). A common insect of this kind, before mentioned, called the bloody-nose beetle (Timarcha tenebricosa), by the aid of these is enabled to adhere to the trailing plants, the various species of bedstraw (Galium), on which it feeds; and by these will support itself against gravity; for both this and Chrysomela goettingensis will walk upon the hand with their back downwards, and it then requires a rather strong pull to disengage them from their station.—The whole tribe of weevils (Rhynchophora, Latr.) are also furnished with these cushions, but not always upon all their joints, some having them only at their apex; and the palm-weevil (Cordylia Palmarum) at the extremity solely of the last joint but one.—Those brilliant beetles the Buprestes have also these cushions, as have likewise the numerous tribes of capricorn-beetles (Longicornes, Latr.). The larvæ of these being timber-borers, the parent insect is probably thus enabled to adhere to this substance whilst it deposits its eggs. Indeed in some species of the former genus the cushions wear the appearance of suckers.—While the linear species of Helops are without them, they clothe all the tarsi of H. æneus (Chalcites K. Ms.)[496]. In two other genera of the same order, Silpha and Cicindela, the anterior tarsi of the males are furnished with them; in these therefore they may be regarded, like the suckers of the larger water-beetles (Dytisci), as given for sexual purposes. The three first joints of the anterior tarsi of many of the larger rove-beetles (Staphylinus, L.) are dilated so as to form, as in the last-mentioned insects, an orbicular patella, but covered by cushions. Since in them this is not peculiar to the males, it is probably given that they may be able to support their long bodies when climbing.

But the most remarkable class of climbers consists of those that are furnished with an apparatus by which they can form a vacuum, so as to adhere to the plane on which they are moving by atmospheric pressure. That flies can walk upon glass placed vertically, and in general against gravity, has long been a source of wonder and inquiry; and various have been the opinions of scientific men upon the subject. Some imagined that the suckers on the feet of these animals were spunges filled with a kind of gluten, by which they were enabled to adhere to such surfaces. This idea, though incorrect, was not so absurd as at first it may seem; since we have seen above in many instances, and very lately in that of the Sminthurus fuscus, that insects are often aided in their motions by a secretion of this kind. Hooke appears to have been one of the first who remarked that the suspension of these animals was produced by some mechanical contrivance in their feet. Observing that the claws alone could not effect this purpose, he justly concluded that it must be principally owing to the mechanism of the two palms, pattens, or soles as he calls the suckers; these he describes as beset underneath with small bristles or tenters, like the wire teeth of a card for working wool, which having a contrary direction to the claws, and both pulling different ways, if there be any irregularity or yielding in the surface of a body, enable the fly to suspend itself very firmly. That they walk upon glass, he ascribes to some ruggedness in the surface; and principally to a smoky tarnish which adheres to it, by means of which the fly gets footing upon it[497]. But these tenterhooks in the suckers of flies, and this smoky tarnish upon glass, are mere fancies, since they can walk as well upon the cleanest glass as upon the most tarnished. Reaumur also attributes this faculty of these animals to the hairs upon their suckers[498]. That learned and pious naturalist, Dr. Derham, seems to have been one of the first who gave the true solution of this enigma. "Flies," says he, "besides their sharp hooked nails, have also skinny palms to their feet, to enable them to stick on glass and other smooth bodies, by the pressure of the atmosphere[499]." He compares these palms to the curious suckers of male Dytisci, before alluded to, and illustrates their action by a common practice of boys, who carry stones by a wet piece of leather applied to their top. Another eminent and excellent naturalist, the late Mr. White, adopted this solution. He observes that in the decline of the year, when the mornings and evenings become chilly, many species of flies retire into houses and swarm in the windows: that at first they are very brisk and alert; but, as they grow more torpid, that they move with difficulty, and are scarcely able to lift their legs, which seem as if glued to the glass; and that by degrees many do actually stick till they die in the place. Then noticing Dr. Derham's opinion as just stated, he further remarks, that they easily overcome the atmospheric pressure when they are brisk and alert. But, he proceeds, in the decline of the year this resistance becomes too mighty for their diminished strength; and we see flies labouring along, and lugging their feet in windows as if they stuck fast to the glass[500].

Sir Joseph Banks, to whom every branch of Natural History becomes daily more indebted, has lately excited an inquiry, the results of which have confirmed Derham's system concerning this motion of animals against gravity. When abroad, he had noticed that a lizard, on account of the sound that it emits before rain, named the Gecko[501] (Lacerta Gecko) could walk against gravity up the walls of houses; and comparing this with the parallel motions of flies, he was desirous of having the subject more scientifically illustrated than it had been. This inquiry was put into the able hands of Sir Everard Home, so justly celebrated as a comparative anatomist, who was assisted in it by the incomparable pencil of Mr. Bauer: and it has been proved most satisfactorily, that it is by producing a vacuum between certain organs destined for that purpose and the plane of position, sufficient to cause atmospheric pressure upon their exterior surface, that the animals in question are enabled to walk up a polished perpendicular, like the glass in our windows and the chunam walls in India, or with their backs downward on a ceiling, without being brought to the ground by the weight of their bodies.

The instruments by which a fly effects this purpose are two suckers connected with the last joint of the tarsus by a narrow infundibular neck, which has power of motion in all directions, immediately under the root of each claw. These suckers consist of a membrane capable of extension and contraction: they are concavo-convex with serrated edges, the concave surface being downy, and the convex granulated. When in action they are separated from each other, and the membrane expanded so as to increase the surface: by applying this closely to the plane of position, the air is sufficiently expelled to produce the pressure necessary to keep the animal from falling. When the suckers are disengaged, they are brought together again so as to be confined within the space between the two claws. This may be seen by looking at the movements of a fly in the inside of a glass tumbler with a common microscope[502]. Thus the fly you see does no more than the leech has been long known to do, when moving in a glass vessel. Furnished with a sucker at each extremity, by means of these organs it marches up and down at its pleasure, or as the state of the atmosphere inclines it.

Dipterous insects, which in general have these organs, and some three on each foot[503], are not exclusively gifted with them; for various others in different orders have them, and some in greater numbers. As I lately observed, the foot-cushions of the Buprestes are something very like them, particularly those of B. fascicularis.—A Brazilian beetle in my cabinet, belonging to the family of the Cleridæ, but not arranging well under any of Latreille's genera, which I have named Priocera variegata, has curious involuted suckers on its feet.—The strepsipterous genera Stylops and Xenos, are remarkable for the vesicles of membrane that cover the underside of their tarsi, which, though flaccid in old specimens, appear to be inflated in the living animal or those that are recent[504]. It is not improbable that these vesicles, which are large and hairy, may act in some degree as suckers, and assist it in climbing.

The insects of the Orthoptera order are, many of them, remarkable for two kinds of appendages connected with my present subject, being furnished both with suckers and cushions. The former are concavo-convex processes, varying in shape in different species—being sometimes orbicular, sometimes ovate or oblong, and often wedge-shaped—which terminate the tarsus between the claw, one on each foot. They are of a hard substance, and seem capable of free motion. In some instances[505], another minute cavity is discoverable at the base of the concave part, similar to that in Cimbex lutea[506]. The latter, the foot-cushions, are usually convex appendages, of an oblong form, and often, though not always, divided in the middle by a very deep longitudinal furrow, attached to the underside of the tarsal joints. Sir E. Home is of opinion that the object of these foot-cushions is to take off the jar, when the body of the animal is suddenly brought from a state of motion to a state of rest[507]. This may very likely be one of their uses, but there are several circumstances which militate against its being the only one. By their elasticity they probably assist the insects that have them in their leaps; and when they climb they may in some degree act as suckers, and prevent them from falling. But their use will be best ascertained by a review of the principal genera of the order. Of these the cock-roaches (Blatta), the spectres (Phasma), and the praying insects (Mantis), are distinguished by tarsi of five joints[508]. The grasshoppers with setaceous antennæ (Acrida) have four tarsal joints. Those with filiform antennæ (Locusta and Acrydium), those with ensiform (Truxalis[509]), and the crickets (Gryllus), have only three. In Blatta, the variations with respect to the suckers and cushions (for many species are furnished with both) are remarkable. The former in some (Blatta gigantea) are altogether wanting; in others (B. Petiveriana) they are mere rudiments; and in others (B. Maderæ) they are more conspicuous, and resemble those of the Gryllidæ. The foot-cushions also in some are nearly obsolete, and occupy the mere extremity of the four first tarsal joints (B. orientalis, americana, capensis, &c.). In B. Petiveriana there is none upon the first joint; but upon the extremity of the four last, not excepting the claw-joint, there is a minute orbicular concave one, resembling a sucker. In others (B. gigantea, &c.) they extend the length of the four first joints, and are very conspicuous. In some (B. Mouffeti, K.[510]), which have no claw-sucker, there appears to be a cavity in the extremity of the claw-joint, which may serve the purpose of one. These foot-cushions are usually of a pale colour; but in one specimen of a hairy female which I have, from Brazil, they are black. The spectre genus (Phasma) exhibits no particular varieties in this respect. The tarsal joints of the legs have cushions at their apex, which appear to be bifid. They have a large orbicular sucker between the claws. In Mantis the fore feet have neither of the parts in question, and the others have no suckers. They have cushions on the four first tarsal joints of the two last pair of legs, which, though smaller, are shaped much like those in Phasma. In Acrida the feet have no suckers between the claws, but they are distinguished by two oval, soft, concave, and moveable processes attached to the base of the first joint of the tarsus, which probably act as suckers[511]. In this genus there are two foot-cushions on the first joint of the tarsi, and one on each of the two following ones[512].—The species of the genus Locusta come next. This genus is called Acrydium by Latreille after Geoffroy; but, since it includes the true locust, it ought to retain the name Locusta given by Linné to the tribe to which it belongs[513]. All these insects have the terminal sucker between the claws, three foot-cushions on the first joint of the tarsus, and one on the second[514]; and the same conformation also distinguishes the feet of Truxalis[515]. In the species of Acrydium, F. (Tetrix, Latr.), the foot-cushions, I believe—for in the dead insect they are the reverse of conspicuous—are arranged nearly as in the two preceding genera, but these insects are without the claw-sucker. And lastly, Gryllus has neither suckers nor cushions. From this statement it seems to follow—since Blatta, Phasma, and Mantis, that do not leap, are provided with cushions; and Gryllus, a heavy tribe of insects that does, are without them—that their object cannot be exclusively to break the fall of the insects that have them. And for the same reason we may conclude, that they must have some further use than augmenting their elasticity when they jump. When we consider that the Blattæ—many of which have no suckers, or very small ones—are climbing insects (I have seen B. Germanica run up and down the walls of an apartment with great agility), and that the long and gigantic apterous spectres, &c. (Phasma) require considerable means to enable them to climb the trees in which they feed, and to maintain their station upon them, we may conclude that these cushions, by acting in some degree as suckers, may promote these ends.

Amongst the homopterous Hemiptera, Chermes and many of the Cercopidæ[516] are furnished with the claw-suckers; but the noisy Cicadæ, as well as the heteropterous section, at least as far as my examination of them has gone, have them not. De Geer has observed, speaking of a small fly of this order (Thrips Physapus), that the extremity of its feet is furnished with a transparent membranaceous flexible process, like a bladder. He further says that, when the animal fixes and presses this vesicle on the surface on which it walks, its diameter is increased, and it sometimes appears concave, the concavity being in proportion to the pressure; which made him suspect that it acted like a cupping-glass, and so produced the adhesion[517]. This circumstance affords another proof that the foot-cushions in the Orthoptera may act the same part; they appear to be vesicular; and in numbers of specimens, after death, I have observed that they become concave, particularly in Acrida viridissima.

In Cimbex, and others amongst the saw-fly tribes, the claw-sucker is distinguished by this remarkable peculiarity, that its upper surface is concave[518], so that before it is used it must be bent inwards. Besides these, at the extremity of each tarsal joint these animals are furnished with a spoon-shaped sucker, which seems analogous to the cushions in the Gryllina, Locustina, &c.: and, what is more remarkable, the two spurs (calcaria) at the apex of the shanks have likewise each a minute one[519].—Various other insects of this order have the claw-suckers. Amongst others the common wasp (Vespa vulgaris) is by these enabled to walk up and down our glass windows.

We learn from De Geer that several mites, to finish with the Aptera, have something of this kind. Among these is the cheese-mite (Acarus Siro): its four fore feet being terminated by a vesicle with a long neck, to which it can give every kind of inflexion. When it sets its foot down, it enlarges and inflates it; and when it lifts it up, it contracts it so that the vesicle almost entirely disappears. This vesicle is between two claws[520].—The itch Acarus (A. Scabiei) is similarly circumstanced.—Ixodes Ricinus and Reduvius have also these vesicles—which are armed with two claws—on all their feet[521].

I am next to consider those climbers that ascend and descend, and probably maintain themselves in their station, by the assistance of a secretion which they have the power of producing. You will immediately perceive that I am speaking of the numerous tribes of spiders (Araneidæ), which, most of them, are endowed with this faculty. Every body knows that these insects ascend and descend by means of a thread that issues from them; but perhaps every one has not remarked—when they wish to avoid a hand held out to catch them, or any other obstacle—that they can sway this thread from the perpendicular. When they move up or down, their legs are extended, sometimes gathering in and sometimes guiding their thread[522]; but when their motion is suspended, they are bent inwards. These animals, although they have no suckers or other apparatus—except the hairs of their legs and the three claws of their biarticulate tarsi, to enable them to do it—can also walk against gravity, both in a perpendicular and a prone position. Dr. Hulse, in Ray's Letters, seems to have furnished a clue that will very well explain this. I give it you in his own homely phrase. "They," spiders, "will often fasten their threads in several places to the things they creep up; the manner is by beating their bums or tails against them as they creep along[523]." Fixing their anus by means of a web, the anterior part of their body, when they are resting, we can readily conceive, would be supported by the claws and hairs of their legs; and their motion may be accomplished by alternately fixing one and then the other. But you will remember I give you this merely as conjecture, having never verified it by observation.

It may not be amiss to mention here another apterous insect that reposes on perpendicular or prone surfaces, without either suckers or any viscous secretion by which it can adhere to them. I mean the long-legged or shepherd spiders (Phalangium). The tarsi of these insects are setaceous and nearly as fine as a hair, consisting sometimes of more than forty joints, those toward the extremity being very minute, and scarcely discernible, and terminating in a single claw. These tarsi, which resemble antennæ rather than feet, are capable of every kind of inflexion, sometimes even of a spiral one. These circumstances enable them to apply their feet to the inequalities of the surface on which they repose, so that every joint may in some measure become a point of support. Their eight legs also, which diverge from their body like the spokes from the nave of a wheel, give them equal hold of eight almost equidistant spaces, which, doubtless, is a great stay to them.

The next species of locomotion exhibited by perfect insects is flying. I am not certain whether under this head I ought to introduce the sailing of spiders in the air; but as there is no other under which it can be more properly arranged, I shall treat of it here. I shall therefore divide flying insects into those that fly without wings, and those that fly with them.

I dare say you are anxious to be told how any animals can fly without wings, and wish me to begin with them. As an observer of nature, you have often, without doubt, been astonished by that sight occasionally noticed in fine days in the autumn, of webs—commonly called gossamer webs—covering the earth and floating in the air; and have frequently asked yourself—What are these gossamer webs? Your question has from old times much excited the attention of learned naturalists. It was an old and strange notion that these webs were composed of dew burned by the sun.

says Spenser. Another, fellow to it, and equally absurd, was that adopted by a learned man and good natural philosopher, and one of the first fellows of the Royal Society, Robert Hooke, the author of Micrographia. "Much resembling a cobweb," says he, "or a confused lock of these cylinders, is a certain white substance which, after a fogg, may be observed to fly up and down the air: catching several of these, and examining them with my microscope, I found them to be much of the same form, looking most like to a flake of worsted prepared to be spun; though by what means they should be generated or produced is not easily imagined: they were of the same weight, or very little heavier than the air; and 'tis not unlikely, but that those great white clouds, that appear all the summer time, may be of the same substance[524]." So liable are even the wisest men to error when, leaving fact and experiment, they follow the guidance of fancy. Some French naturalists have supposed that these fils de la Vierge, as they are called in France, are composed of the cottony matter in which the eggs of the Coccus of the vine (C. Vitis) are enveloped[525]. In a country abounding in vineyards this supposition would not be absurd; but in one like Britain, in which the vine is confined to the fruit-garden, and the Coccus seldom seen out of the conservatory, it will not at all account for the phænomenon. What will you say, if I tell you that these webs (at least many of them) are air-balloons—and that the aëronauts are not

but spiders, who long before Montgolfier, nay, ever since the creation, have been in the habit of sailing through the fields of ether in these air-light chariots! This seems to have been suspected long ago by Henry Moore, who says,

Where he also alludes to the old opinion of scorched dew. But the first naturalists who made this discovery appear to have been Dr. Hulse and Dr. Martin Lister—the former first observing that spiders shoot their webs into the air; and the latter, besides this, that they were carried upon them in that element[527]. This last gentleman, in fine serene weather in September, had noticed these webs falling from the heavens, and in them discovered more than once a spider, which he named the bird. On another occasion, whilst he was watching the proceedings of a common spider, the animal suddenly turning upon its back and elevating its anus, darted forth a long thread, and vaulting from the place on which it stood, was carried upwards to a considerable height. Numerous observations afterwards confirmed this extraordinary fact; and he further discovered, that while they fly in this manner, they pull in their long thread with their fore feet, so as to form it into a ball—or, as we may call it, air-balloon—of flake. The height to which spiders will thus ascend he affirms is prodigious. One day in the autumn, when the air was full of webs, he mounted to the top of the highest steeple of York minster, from whence he could discern the floating webs still very high above him. Some spiders that fell and were entangled upon the pinnacles he took. They were of a kind that never enter houses, and therefore could not be supposed to have taken their flight from the steeple[528]. It appears from his observations, that this faculty is not confined to one species of spider, but is common to several, though only in their young or half-grown state[529]; whence we may infer, that when full-grown their bodies are too heavy to be thus conveyed. One spider he noticed that at one time contented itself with ejaculating a single thread, while at others it darted out several, like so many shining rays at the tail of a comet. Of these, in Cambridgeshire in October, he once saw an incredible number sailing in the air[530]. Speaking of his Ar. subfuscus minutissimis oculis, &c. he says, "Certainly this is an excellent rope-dancer, and is wonderfully delighted with darting its threads: nor is it only carried in the air, like the preceding ones; but it effects itself its ascent and sailing: for, by means of its legs closely applied to each other, it as it were balances itself, and promotes and directs its course no otherwise than as if nature had furnished it with wings or oars[531]." A later but equally gifted observer of nature, Mr. White, confirms Dr. Lister's account. "Every day in fine weather in autumn," says he, "do I see these spiders shooting out their webs, and mounting aloft: they will go off from the finger, if you will take them into your hand. Last summer one alighted on my book as I was reading in the parlour; and running to the top of the page and shooting out a web, took its departure from thence. But what I most wondered at was, that it went off with considerable velocity in a place where no air was stirring; and I am sure that I did not assist it with my breath. So that these little crawlers seem to have while mounting some locomotive power without the use of wings, and move faster than the air in the air itself[532]." A writer in the last number of Thomson's Annals of Philosophy[533], under the signature of Carolan, has given some curious observations on the mode in which some geometric spiders shoot and direct their threads, and fly upon them; by which it appears, that as they dart them out they guide them as if by magic, emitting at the same time a stream of air, as he supposes, or possibly some subtile electric fluid. One which was running upon his hand, dropped by its thread about six inches from the point of his finger, when it immediately emitted a pretty long line at a right angle with that by which it was suspended. This thread, though at first horizontal, quickly rose upwards, carrying the spider along with it. When it had ascended as far above his finger as it had dropped before below it, it let out the thread by which it had been attached to it, and continued flying smoothly upwards till it nearly reached the roof of the room, when it veered on one side and alighted on the wall. In flying, its motion was smoother and quicker than when a spider runs along its thread. He observes, that as the line lengthens behind them, the tendency of spiders to rise increases.—I have myself more than once observed these creatures take their flight, and find the following memorandum with respect to their mode of proceeding. "The spider first extends its thighs, shanks, and feet, into a right line, and then elevating its abdomen till it becomes vertical, shoots its thread into the air, and flies off from its station." It is not often, however, that an observer can be gratified with this interesting sight, since these animals are soon alarmed. I have frequently noticed them—for at the times when these webs are floating in the air they are very numerous—on the vertical angle of a post, or pale, or one of the uprights of a gate, with the end of their abdomen pointing upwards, as if to shoot their thread previously to flying off; when, upon my approaching to take a nearer view, they have lowered it again, and persisted in disappointing my wish to see them mount aloft. The rapidity with which the spider vanishes from the sight upon this occasion and darts into the air, is a problem of no easy solution. Can the length of web that they dart forth counterpoise the weight of their bodies; or have they any organ analogous to the natatory vesicles of fishes[534], which contributes at their will to render them buoyant in the air? Or do they rapidly ascend their threads in their usual way, and gather them up, till having collected them into a mass of sufficient magnitude, they give themselves to the air, and are carried here and there in these chariots? I must here give you Mr. White's very curious account of a shower of these webs that he witnessed. On the 21st of September 1741, intent upon field diversions, he rose before day-break; but on going out, he found the whole face of the country covered with a thick coat of cobweb, drenched with dew, as if two or three setting-nets had been drawn one over the other. When his dogs attempted to hunt, their eyes were so blinded and hoodwinked that they were obliged to lie down and scrape themselves. This appearance was followed by a most lovely day. About nine A. M. a shower of these webs (formed not of single floating threads, but of perfect flakes, some near an inch broad, and five or six long,) was observed falling from very elevated regions, which continued without interruption during the whole of the day;—and they fell with a velocity which showed that they were considerably heavier than the atmosphere. When the most elevated station in the country where this was observed was ascended, the webs were still to be seen descending from above, and twinkling like stars in the sun, so as to draw the attention of the most incurious. The flakes of the web on this occasion hung so thick upon the hedges and trees, that baskets-full might have been collected. No one doubts, he observes, but that these webs are the production of small spiders, which swarm in the fields in fine weather in autumn, and have a power of shooting out webs from their tails, so as to render themselves buoyant and lighter than the air[535]. In Germany these flights of gossamer appear so constantly in autumn, that they are there metaphorically called "Der fliegender Sommer" (the flying or departing summer); and authors speak of the web as often hanging in flakes like wool on every hedge and bush throughout extensive districts.

Here we may inquire—Why is the ground in these serene days covered so thickly by these webs, and what becomes of them? What occasions the spiders to mount into the air, and do the same species form both the terrestrial and aërial gossamer?—And what causes the webs at last to fall to the earth? I fear I cannot to all these queries return a fully satisfactory answer; but I will do the best I can. At first one would conclude from analogy, that the object of the gossamer which early in the morning is spread over stubbles and fallows—and sometimes so thickly as to make them appear as if covered with a carpet, or rather overflown by a sea, of gauze, presenting, when studded with dew-drops, as I have often witnessed, a most enchanting spectacle—is to entrap the flies and other insects as they rise into the air from their nocturnal station of repose, to take their diurnal flights. But Dr. Strack's observations render this very doubtful; for he kept many of the spiders that produce these webs in a large glass upon turf, where they spun as when at liberty, and he could never observe them attempt to catch or eat—even when entangled in their webs—the flies and gnats with which he supplied them; though they greedily sucked water when sprinkled upon the turf, and remained lively for two months without other food[536]. As the single threads shot by other spiders are usually their bridges, this perhaps may be the object of the webs in question: and thus the animals may be conveyed from furrow to furrow or straw to straw less circuitously, and with less labour, than if they had travelled over the ground. As these creatures seem so thirsty, may we not conjecture that the drops of dew, with which they are always as it were strung, are a secondary object with them? So prodigious are their numbers, that sometimes every stalk of straw in the stubbles, and every clod and stone in the fallows, swarms with them. Dr. Strack assures us that twenty or thirty often sit upon a single straw, and that he collected about 2000 in half an hour, and could have easily doubled the number had he wished it: he remarks, that the cause of their escaping the notice of other observers, is their falling to the ground upon the least alarm.

As to what becomes of this immense carpeting of web there are different opinions. Mr. White conjectures that these threads, when first shot, might be entangled in the rising dew, and so drawn up, spiders and all, by a brisk evaporation, into the region where the clouds are formed[537]. But this seems almost as inadmissible as that of Hooke, before related. An ingenious and observant friend, thinking the numbers of the flying spiders not sufficient to produce the whole of the phenomenon in question, is of opinion that an equinoctial gale, sweeping along the fallows and stubbles coated with the gossamer, must bring many single threads into contact, which, adhering together, may gradually collect into flakes; and that being at length detached by the violence of the wind, they are carried along with it: and as it is known that such winds often convey even sand and earth to great heights, he deems it highly probable that so light a substance may be transported to so great an elevation, as not to fall to the earth for some days after, when the weather has become serene, or to descend upon ships at sea, as has sometimes happened. This, which is in part adopted from the German authors, is certainly a much more reasonable supposition than the other; but some facts seem to militate against it: for, in the first place, though gossamer often occurs upon the ground when there is none in the air, yet the reverse of this has never been observed; for gossamer in the air, as in the instance recorded by Mr. White, is always preceded by gossamer on the ground. Now, since the weather is constantly calm and serene when these showers appear, it cannot be the wind that carries the web from the ground into the air. Again, it is stated that these showers take place after several calm days[538]: now, if the web was raised by the wind into the air, it would begin to fall as soon as the wind ceased. Whence I am inclined to think that the cause assigned by Dr. Lister is the real source of the whole phenomenon. Though ordinary observers have overlooked them, he noticed these spiders in the air in such prodigious numbers, that he deemed them sufficient to produce the effect. I shall not, however, decide positively; but, having stated the different opinions, leave you to your own judgement.

The next query is, What occasions the spiders to mount their chariots and seek the clouds? Is it in pursuit of their food? Insects, in the fine warm days in which this phenomenon occurs, probably take higher flights than usual, and seek the upper regions of the atmosphere; and that the spiders catch them there, appears by the exuviæ of gnats and flies, which are often found in the falling webs[539]. Yet one would suppose that insects would fly high at all times in the summer in serene warm weather. Perhaps the flight of some particular species constituting a favourite food of our little charioteers—the gnats, for instance, which we have seen sometimes rise in clouds into the air[540]—may at these times take place; or the species of spiders that are most given to these excursions, may not abound in their young state—when only they can fly—at other seasons of the year.

Whether the same species that cover the earth with their webs produce those that fill the air, is to be our next inquiry. Did the appearance of the one always succeed that of the other, this might be reasonably concluded:—but the former, as I lately observed to you, often occurs without being followed by the latter. Yet, since it should seem that the aërial gossamer, though it does not always follow it, is always preceded by the terrestrial, this warrants a conjecture that they may be synonymous. Two German authors, Bechstein[541] and Strack[542], have described the spider that produces gossamer in Germany under the name of Aranea obtextrix[542]. But it is not clear, unless they have described it at different ages, when spiders often greatly change their appearance, that they mean the same species. The former describes his as of the size of a small pin's head, with its eight eyes disposed in a circle, having a black-brown body and light-yellow legs: while Dr. Strack represents his A. obtextrix as more than two lines in length; eyes four in a square, and two on each side touching each other; thorax deep brown with paler streaks; abdomen below dull white, above dark copper brown, with a dentated white spot running longitudinally down the middle. The first of these, if distinct, as I suspect they are, agrees very well with the young of one which Lister observed as remarkable for taking aërial flights[543]; and which I have most usually seen so engaged. The other may possibly be that before noticed, which he found in such infinite numbers in Cambridgeshire[544]. If this conjecture be correct, it will prove that the same species first produce the gossamer that covers the ground, and then, shooting other threads, mount upon them into the air.

My last query was, What causes these webs ultimately to fall to the earth? Mr. White's observation will I think furnish the best answer. "If the spiders have the power of coiling up their webs in the air, as Dr. Lister affirms, then when they become heavier than the air they will fall[545]." The more expanded the web, the lighter and more buoyant, and the more condensed, the heavier it must be.

I trust you will allow from this mass of evidence, that the English Arachnologists—may I coin this term?—were correct in their account of this singular phenomenon; and think, with me, that Swammerdam (who however admits that spiders sail on their webs), and after him De Geer, were rather hasty when they stigmatized the discovery that these animals shoot their webs into the air, and so take flight, as a strange and unfounded opinion[546]. The fact, though so well authenticated, is indeed strange and wonderful, and affords another proof of the extraordinary powers, unparalleled in the higher orders of animals, with which the Creator has gifted the insect world. Were indeed man and the larger animals, with their present propensities, similarly endowed, the whole creation would soon go to ruin. But these almost miraculous powers in the hands of these little beings only tend to keep it in order and beauty. Adorable is that Wisdom, Power, and Goodness, that has distinguished these next to nothings by such peculiar endowments for our preservation as if given to the strong and mighty would work our destruction.

After the foregoing marvellous detail of the aërial excursions of our insect air-balloonists, I fear you will think the motions of those which fly by means of wings less interesting. You will find, however, that they are not altogether barren of amusement. Though the wings are the principal instruments of the flight of insects, yet there are others subsidiary to them, which I shall here enumerate, considering them more at large under the orders to which they severally belong. These are wing-cases (Elytra, Tegmina, and Hemelytra); winglets (Alulæ); poisers (Halteres); tailets (Caudulæ); hooklets (Hamuli); base-covers (Tegulæ), &c. Besides, their tails, legs, and even antennæ, assist them in some instances, in this motion.

As wings are common to almost the whole class, I shall consider their structure here. Every wing consists of two membranes, more or less transparent, applied to each other: the upper membrane being very strongly attached to the nervures (Neuræ), and the lower adhering more loosely, so as to be separable from them. The nervures[547] are a kind of hollow tube,—above elastic, horny, and convex; and flat and nearly membranaceous below,—which take their origin in the trunk, and keep diminishing gradually, the marginal ones excepted, to their termination. The vessels contained in the nervures consist of a spiral thread, whence they appear to be air-vessels communicating with the tracheæ in the trunk.—The expansion of the wing at the will of the insect, is a problem that can only be solved by supposing that a subtile fluid is introduced into these vessels, which seem perfectly analogous to those in the wings of birds; and that thus an impulse is communicated to every part of the organ, sufficient to keep it in proper tension. We see by this, that a wing is supported in its flight like a sail by its cordage[548]. It is remarkable that those insects which keep the longest on the wing, the dragon-flies (Libellulina) for instance, have their wings most covered with nervures. The wings of insects in flying, like those of other flying animals, you are to observe, move vertically, or up and down.

In considering the flight of insects, I shall treat of that of each order separately, beginning with the Coleoptera or beetles. Their subsidiary instruments of flight are their wing-cases (Elytra), and in one instance, winglets (Alulæ). The former[549], which in some are of a hard horny substance, and in others are softer and more like leather, though they are kept immoveable in flight, are probably, by their resistance to the air, not without their use on this occasion. The winglets are small concavo-convex scales, of a stiff membranaceous substance, generally fringed at their extremity[550]. I know at present of only one coleopterous insect that has them (Dytiscus marginalis). They are placed under the elytra at their base. Their use is unknown; but it may probably be connected with their flight. The wings of beetles[551] are usually very ample, often of a substance between parchment and membrane. The nervures that traverse and extend them, though not numerous, are stronger and larger than those in the wings of insects of the other orders, and are so dispersed as to give perfect tension to the organ. When at rest—except in Molorchus, Atractocerus, Necydalis, and some other genera—they are folded transversely under the elytra, generally near the middle, with a lateral longitudinal fold, but occasionally near the extremity[552]. When they prepare for flight, their antennæ being set out, the elytra are opened so as to form an angle with the body and admit the free play of the wings, and they then fly off, striking the air by the vertical motion of these organs, the elytra all the while remaining immoveable. During their flight the bodies of insects of this order, as far as I have observed them, are always in a position nearly vertical, which gives to the larger sorts, the stag-beetle for instance, a very singular appearance. Olivier, probably having some of the larger and heavier beetles in his eye, affirms that the wings of insects of this order are not usually proportioned to the weight of their bodies, and that the muscular apparatus that moves them is deficient in force. In consequence of which, he observes, they take flight with difficulty, and fly very badly. The strokes of their wings being frequent, and their flight short, uncertain, heavy, and laborious, they can use their wings only in very calm weather, the least wind beating them down. Yet he allows that others, whose body is lighter, rise into the air and fly with a little more ease; especially when the weather is warm and dry, their flights however being short, though frequent. He asserts also, that no coleopterous insect can fly against the wind[553]. These observations may hold perhaps with respect to many species; but they will by no means apply generally. The cockchafer (Melolontha vulgaris), if thrown into the air in the evening, its time of flight, will take wing before it falls to the ground. The common dung-chafer (Geotrupes stercorarius)—wheeling from side to side like the humble-bee—flies with great rapidity and force, and, with all its dung-devouring confederates, directs its flight with the utmost certainty, and probably often against the wind, to its food. The root-devourers or tree-chafers (Melolontha, Hoplia, &c.) support themselves, like swarming bees, in the air and over the trees, flying round in all directions. The Brachyptera and Donaciæ, in warm weather, fly off from their station with the utmost ease;—their wings are unfolded, and they are in the air in an instant, especially the latter, as I have often found when I have attempted to take them. None are more remarkable for this than the Cicindelæ, which, however, taking very short flights, are as easily marked down as a partridge, and affords as much amusement to the entomologist, as the latter to the sportsman.—It is to be observed that many insects in this order have no wings, and the female glow-worms neither wings nor elytra.

Many persons are not aware that the insects of the next order, the Dermaptera, can fly: but earwigs (Forficula), their size considered, are furnished with very ample and curious wings, the principal nervures of which are so many radii, diverging from a common point near the anterior margin. Between these are others which, proceeding from the opposite margin, terminate in the middle of the wing[554]. These organs, when at rest, are more than once folded both transversely and longitudinally.

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.

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.