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].