Generally speaking, however, in these apodous walkers the place of legs is supplied by fleshy and often retractile mamillæ or tubercles. By means of these and a slimy secretion, unaided by mandibular hooks, the caterpillar of a little moth (Apoda Testudo,) moves from place to place[424].—A subcutaneous larva belonging to the same order, that mines the leaves of the rose, moves also by tubercular legs assisted by slime. It has eighteen homogeneous legs, with which, when removed from its house of concealment, it will walk well upon any surface, whether horizontal, inclined, or even vertical[425]. But the greatest number of legs of this kind that distinguish any known larva, is to be observed in that of a two-winged fly (Syrphus Pyrastri) that devours the Aphides of the rose. This animal has six rows of tubercular feet, with which it moves, each row consisting of seven, making in all forty-two[426].—The grub of the weevil of the dock (Hypera Rumicis) has twenty-four tubercular legs; but, what is remarkable, the six anterior ones, being longer than the rest, seem to represent the real legs, while the others represent the spurious ones, of lepidopterous larvæ. These legs, however, are all fleshy tubercles, and have no claws, the place of which is supplied by slime which covers all the underside of the body, and hinders the animal from falling[427]. Another weevil (Lixus paraplecticus,) produces a grub inhabiting the water-hemlock, which has only six tubercles that occupy the place and are representatives of the legs of the perfect insect[428].
Some larvæ have these tubercles armed with claws. The maggot of a fly described by De Geer (Volucella plumata,) has six pair of them, each of which has three long claws. This animal has a radiated anus, and seems related to those flies that live in the nests of humble-bees[429].
Insects in the peculiarities of their structure, as we have seen in many instances, sometimes realize the wildest fictions of the imagination. Should a traveller tell you that he had seen a quadruped whose legs were on its back, you would immediately conclude that he was playing upon your credulity, and had lost that regard to truth which ought to distinguish the narratives of persons of his description. What then will you say to me, when I affirm, upon the evidence of two most unexceptionable witnesses, Reaumur and De Geer, that there are insects which exhibit this extraordinary structure? The grub of a little gall-fly, appearing to be Cynips Quercus inferus of Linné—which inhabits a ligneous gall resembling a berry to be met with on the underside of oak-leaves—was found by the former to have on its back, on the middle of each segment, a retractile fleshy protuberance that resembled strikingly the spurious legs of some caterpillars. A little attention will convince any one, argues Reaumur, that the legs of insects circumstanced like the one under consideration, if it has any, should be on its back. For this grub—inhabiting a spherical cavity, in which it lies rolled up as it were in a ring—when it wants to move, will be enabled to do so, in this hollow sphere, with much more facility, by means of legs on the middle of its back, than if they were in their ordinary situation[430]. So wisely has Providence ordered every thing.—Another similar instance is recorded by De Geer, which indeed had previously been noticed, though cursorily, by the illustrious Frenchman[431]. There is a little larva, he observes, to be found at all seasons of the year, the depth of winter excepted, in stagnant waters, which keeps its body always doubled as it were in two, against the sides of ditches or the stalks of aquatic plants. If it is placed in a glass half full of water, it so fixes itself against the sides of it, that its head and tail are in the water while the remainder of the body is out of it; thus assuming the form of a siphon, the tail end being the longest. When this animal is disposed to feed, it lifts its head and places it horizontally on the surface of the water, so that it forms a right angle with the rest of the body, which always remains in a situation perpendicular to the surface. It then agitates, with vivacity, a couple of brushes, formed of hairs and fixed in the anterior part of the head, which producing a current towards the mouth, it makes its meal of the various species of animalcula, abounding in stagnant waters, that come within the vortex thus produced. As these animals require to be firmly fixed to the substance on which they take their station, and their back is the only part, when they are doubled as just described, that can apply to it,—they are furnished with minute legs armed with black claws, by which they are enabled to adhere to it. They have ten of these legs: the four anterior ones, which point towards the head and are distant from each other, are placed upon the fourth and fifth dorsal segments of the body; and the six posterior ones, which point to the anus and are so near to each other as at first to look like one leg, are placed on the eighth, ninth, and tenth. When the animal moves, the body continues bent, and the sixth segment, which is without feet and forms the summit of the curve, goes first[432]. De Geer named the fly it produces Tipula amphibia: it seems not clear, from his figure, to which of the modern genera of the Tipulariæ it belongs, nor is it referred to by Meigen.
I come now to the jumping apodes, and one of this description will immediately occur to your recollection,—that I mean which revels in our richest cheeses, and produces a little black shining fly (Tyrophaga Casei). These maggots have long been celebrated for their saltatorious powers. They effect their tremendous leaps—laugh not at the term, for they are truly so when compared with what human force and agility can accomplish—in nearly the same manner as salmon are stated to do when they wish to pass over a cataract, by taking their tail in their mouth, and letting it go suddenly. When it prepares to leap, our larva first erects itself upon its anus, and then bending itself into a circle by bringing its head to its tail, it pushes forth its unguiform mandibles, and fixes them in two cavities in its anal tubercles. All being thus prepared, it next contracts its body into an oblong, so that the two halves are parallel to each other. This done, it lets go its hold with so violent a jerk that the sound produced by its mandibles may be readily heard, and the leap takes place. Swammerdam saw one, whose length did not exceed the fourth part of an inch, jump in this manner out of a box six inches deep; which is as if a man six feet high should raise himself in the air by jumping 144 feet! He had seen others leap a great deal higher[433]. The grub of a little gnat lately noticed (Chironomus stercorarius) has a similar faculty, though executed in a manner rather different. These larvæ, which inhabit horse-dung, though deprived of feet, cannot move by annular contraction and dilatation; but are able, by various serpentine contortions, aided by their mandibles, to move in the substance which constitutes their food. Should any accident remove them from it, Providence has enabled them to recover their natural station by the power I am speaking of. When about to leap, they do not, like the cheese-fly, erect themselves so as to form an angle with the plane of position; but lying horizontally, they bring the anus near the head, regulating the distance by the length of the leap they mean to take; when fixing it firmly, and then suddenly resuming a rectilinear position, they are carried through the air sometimes to the distance of two or three inches. They appear to have the power of flattening their anal extremity, and even of rendering it concave: by means of which it may probably act as a sucker, and so be more firmly fixable[434].—The grub of a fly whose proceedings in that state I have before noticed[435] (Leptis Vermileo), will, when removed from its habitation, endeavour to recover it by leaping. Indeed this mode of motion seems often to be given to this description of larvæ by Providence, to enable them to return to their natural station, when by any accident they have wandered away from it.
Many apodous larvæ inhabit the water, and therefore must be furnished with means of locomotion proper to that element. To this class belongs the common gnat (Culex pipiens), which being one of our greatest torments, compels us to feel some curiosity about its history. Its larva is a very singular creature, furnished with a remarkable anal apparatus for respiration, by which it usually remains suspended at the surface of the water. If disposed to descend, it seems to sink by the weight of its body; but when it would move upwards again, it effects its purpose by alternate contortions of the upper and lower halves of it, and thus it moves with much celerity. The laminæ or swimmers, which terminate its anus[436], are doubtless of use to it in promoting this purpose. It does not, that I ever observed, move in a lateral direction, but only from the surface downwards, and vice versa.—Another dipterous larva (Corethra culiciformis), which much resembles that of the gnat in form, differs from it in its motions and station of repose. For, instead of being suspended at the surface with its head downwards, it usually, like fishes, remains in a horizontal position in the middle of the water. When it ascends to the surface, it is always by means of a few strokes of its tail, so that its motion is not equable, sed per saltus. It descends again gradually by its own weight, and regains its equilibrium by a single stroke of the tail[437].—A well known fly (Stratyomis Chamæleon), in its first state an aquatic animal, often remains suspended, by its radiated anus, at the surface of the water, with its head downwards. But when it is disposed to seek the bottom or to descend, by bending the radii of its tail so as to form a concavity, it includes in them a bubble of air, in brilliancy resembling silver or pearl; and then sinks with it by its own weight. When it would return to the surface it is by means of this bubble, which is, as it were, its air-balloon. If it moves upon the surface or horizontally, it bends its body alternately to the right and left, contracting itself into the form of the letter S; and then extending itself again into a straight line, by these alternate movements it makes its way slowly in the water[438].
I have dwelt longer upon the apodous larvæ, or those that are without what may be called proper legs, analogous to those of perfect insects, because the absence of these ordinary instruments of motion is in numbers of them supplied in a way so remarkable and so worthy to be known; and because in them the wisdom of the Creator is so conspicuously, or, I should rather say, so strikingly manifested—since it is doubtless equally conspicuous in the ordinary routine of nature. But aberrations from her general laws, and modes, and instruments of action, often of rare occurrence, impress us more forcibly than any thing that falls under our daily observation.
I come now to pedate larvæ, or those that move by means of proper or articulate legs. These legs (generally six in number, and attached to the underside of the three first segments of the body) vary in larvæ of the different orders: but they seem in most to have joints answering to the hip (coxa); trochanter; thigh (femur); shank (tibia); foot (tarsus), of perfect insects, the legs of which they include. Cuvier, speaking of Coleoptera and some Neuroptera, mentions only three joints. But many in these orders (amongst which he included the Trichoptera) have the joints I have enumerated. To name no more, the Lamellicornia, Dytisci, Silphæ, Staphylini, Cicindelæ, and Gyrini, &c. amongst coleopterous larvæ; and the Trichoptera, as well as the Libellulina and Ephemerina, amongst Cuvier's Neuroptera,—have these joints, and in many the last terminates in a double claw[439]. In some coleopterous genera the tarsus seems absent or obsolete. The larva of the lady-bird (Coccinella) affords an example of the former kind, and that of Chrysomela of the latter[440]. These joints are very visible in the legs of caterpillars of Lepidoptera, and their tarsus is armed with a single claw[441]. The larvæ that have these legs walk with them sometimes very swiftly. In stepping they set forward at the same time the anterior and posterior legs of one side, and the intermediate one of the other; and so alternately on each side.
Pedate larvæ are of two descriptions: those that to perfect legs add spurious ones with or without claws, and those that have only perfect legs. I begin with the former—those that have both kinds of legs. But first I must make a few remarks upon spurious legs. Because their muscles, instead of the horny substance that protects them in perfect legs, are covered only by a soft membrane, they have been usually denominated membranaceous legs: since, however, they are temporary, vanishing altogether when the insect arrives at its perfect state,—are merely used, for they do not otherwise assist in this motion, as props to hinder its long body, when it walks, from trailing on the ground; to push against the plane of position; and, by means of their hooks or claws, to fix itself firmly to its station when it feeds or reposes,—I shall therefore call them prolegs (propedes[442]). These organs consist of three or four folds, and are commonly terminated, though not always, by a coronet or semicoronet of very minute crooked claws or hooks. These claws, which sometimes amount to nearly a hundred on one proleg, are alternately longer and shorter. They are crooked at both ends, and are attached to the proleg by the back by means of a membrane, which covers about two-thirds of their length, leaving their two extremities naked. Of these the upper one is sharp, and the lower blunt. The sole, or part of the prolegs within the claws, is capable of opening and shutting. When the animal walks, that they may not impede its motion, it is shut, and the claws are laid flat with their points inwards; but when it wishes to fix itself, the sole is opened, becoming of greater diameter than before, and the claws stand erect with their points outwards. Thus they can lay stronger hold of the plane of position[443].
The number of these prolegs varies in different species and families. In the numerous tribes of saw-flies (Serrifera), the larvæ of which resemble those of Lepidoptera, and are called by Reaumur spurious caterpillars (fausses chenilles), one family (Lophyrus) has sixteen prolegs; a second (Hylotoma, &c.) fourteen; another (Tenthredo, F.) twelve; and a fourth (Lyda) none at all, having only the six perfect legs. The majority of larvæ of Lepidoptera have ten prolegs, eight being attached, a pair on each, to the sixth, seventh, eighth, and ninth segments of the body, and two to the twelfth or anal segment[444]. The caterpillar of the puss-moth (Cerura Vinula) and some others, instead of the anal prolegs, have two tails or horns. A hemigeometer, described by De Geer, has only six intermediate prolegs, the posterior pair of which are longer than the rest, to assist the anal pair in supporting the body in a posture more or less erect[445]. Other hemigeometers, of which kind is the larva of Plusia Gamma[446], have only six prolegs, four intermediate and two anal. The true geometers or surveyors (Geometræ) have only two intermediate and two anal prolegs. Many grubs of Coleoptera, especially those of Staphylinidæ, Silphidæ, &c. which are long and narrow, are furnished with a stiff joint at the anus, which they bend downwards and use as a prop to prevent their body from trailing. This joint, though without claws, may be regarded as a kind of proleg, which supports them when they walk[447]; and probably may assist their motion by pushing against the plane of position.