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.