Chapter XIV: Letter XXIII: MOTIONS OF INSECTS. (Imago.) (1)

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.

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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,

"Their golden pinions ope and close;"

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.

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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.

"...... The fine nets which oft we woven see
Of scorched dew,"

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

"Lovers who may bestride the gossamer
That idles in the wanton summer air,
And yet not fall"--

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,

"As light and thin as cobwebs that do fly
In the blew air, caus'd by the autumnal sun,
That boils the dew that on the earth doth lie,
May seem this whitish rag then is the scum;
_Unless that wiser men make't the field-spider's loom_[526]."