Since the motions, and instruments of motion, of insects are usually very different in their preparatory states, from what they are in the imago or perfect state, I shall therefore consider them separately, and divide my subject into—motions of larvæ,—motions of pupæ,—and motions of perfect insects.
I. Amongst larvæ there are two classes of movers—Apodous larvæ, or those that move without legs,—and Pedate larvæ, or those that move by means of legs. I must here observe, that by the term legs, which I use strictly, I mean only jointed organs, that have free motion, and can walk or step alternately; not those spurious legs without joints, that have no free motion, and cannot walk or take alternate steps; such as support the middle and anus of the larvæ of most Lepidoptera and saw-flies (Serrifera).
Apodous larvæ seldom have occasion to take long journeys; and many of them, except when about to assume the pupa, only want to change their place or posture, and to follow their food in the substance, whether animal or vegetable, to which, when included in the egg, the parent insect committed them. Legs therefore would be of no great use to them, and to these last a considerable impediment. They are capable of three kinds of motion;—they either walk, or jump, or swim. I use walking in an improper sense, for want of a better term equally comprehensive: for some may be said to move by gliding; and others (I mean those that, fixing the head to any point, bring the tail up to it, and so proceed) by stepping.
The motion of serpents was ascribed by some of the ancients (who were unable to conceive that it could be effected naturally, unless by the aid of legs, wings, or fins,) to a preternatural cause. It was supposed to resemble the "incessus deorum," and procured to these animals, amongst other causes, one of the highest and most honourable ranks in the emblematical class of their false divinities[404]. Had they known Sir Joseph Banks's late discovery,—that some serpents push themselves along by the points of their ribs, which Sir E. Home has found to be curiously constructed for this purpose,—their wonder would have been diminished, and their serpent-gods undeified. But though serpents can no longer make good their claim to motion more deorum, some insects may take their places; for there are numbers of larvæ, that having neither legs, nor ribs, nor any other points by which they can push themselves forward on a plane, glide along by the alternate contraction and extension of the segments of their body. Had the ancient Egyptians been aware of this, their catalogue of insect divinities would have been wofully crowded. In this annular motion, the animal alternately supports each segment of the body upon the plane of position, which it is enabled to do by the little bundles of muscles attached to the skin, that take their origin within the body[405].
I shall begin the list of walkers, the movements of which are aided by various instruments, with one which is well known to most people,—the grub of the nut-weevil (Balaninus Nucum). When placed upon a table, after lying some time, perhaps, bent in a bow, with its head touching its tail, at last it begins to move, which, though in no certain direction, it does with more speed than might be expected. Rösel fancied that this animal had feet furnished with claws; but in this, as De Geer justly observes, he was altogether mistaken, since it has not the least rudiment of them, its motion being produced solely by the alternate contraction and extension of the segments of the body, assisted, perhaps, by the fleshy prominences of its sides.—Other larvæ have this annular motion aided by a slimy secretion, which gives them further hold upon the plane on which they are moving, and supplies in some degree the place of legs or claws. That of the weevil of the common figwort (Cionus Scrophulariæ) is always covered with slime, which enables it,—though it renders its appearance disgusting,—to walk with steadiness, by the mere lengthening and shortening of its segments, upon the leaves of that plant[406].—Of this kind also are those larvæ, mentioned above[407], received by De Geer from M. Ziervogel, which, adhering to each other by a slimy secretion, glide along so slowly upon the ground as to be a quarter of an hour in going the breadth of the hand, whence the natives call their bands Gårds-drag[408].
As a further help, others again call in the assistance of their unguiform mandibles. These, which are peculiar to grubs with a variable membranaceous, or rather retractile, head[409], especially those of the fly tribe (Muscidæ), when the animal does not use them, are retracted not only within the head, but even within the segments behind it[410]; but when it is moving they are protruded, and lay hold of the surface on which it is placed. They were long ago noticed by the accurate Ray. "This blackness in the head," says he, speaking of the maggot of the common flesh-fly, "is caused by two black spines or hooks, which when in motion it puts forth, and fixing them in the ground, so drags along its body[411]."—The larvæ of the aphidivorous flies (Syrphus, &c.), the ravages of which amongst the Aphides I have before described to you[412], transport themselves from place to place in the same way, walking by means of their teeth. Fixing their hind part to the substances on which they are moving, they give their body its greatest possible tension; and, if I may so speak, thus take as long a step as they can: next, laying hold of it with their mandibles, by setting free the tail and relaxing the tension, the former is brought near the head. Thus the animal proceeds, and thus will even walk upon glass[413]. Some grubs, as the lesser house-fly (Anthomyia canicularis), have only one of these claw-teeth; and in some they have the form as well as the office of legs[414]. Bonnet mentions an apodous larva, that, before it can use its mandibles, is obliged to spin, at certain intervals, little hillocks or steps of silk; of which it then lays hold by them, and so drags itself along.
Besides their mandibular hooks, some of these grubs supply the want of legs by means of claws at their anus. Thus that of the flesh-fly, Ray tells us in the place just quoted, pushes itself by the protruded spines of its tail. The larva also of a long-legged gnat (Limnobia replicata), which in that state lives in the water, is furnished with these anal claws, which, in conjunction with its annular tension and relaxation, and the hooks of its mouth, assist it in walking over the aquatic plants[415].
A remarkable difference, according to their station, obtains in the bots of gad-flies (Œstridæ); those that are subcutaneous (Cuticolæ, Clark) having no unguiform mandibles; while those that are gastric (Gastricolæ, Clark), and those that inhabit the maxillary sinuses of animals (Cavicolæ, Clark), are furnished with them. In this we evidently see Creative Wisdom adapting means to their end. For the cuticular bots having no plane surface to move upon, and imbibing a liquid food, in them the mandibular hooks would be superfluous. But they are furnished with other means by which they can accomplish such motions, and in contrary directions, as are necessary to them; the anterior part of each segment being beset with numbers of very minute spines, not visible except under a strong magnifier, sometimes arranged in bundles, which all look towards the anus; and the posterior part is as it were paved with similar hooks, but smaller, which point to the head. Thus we may conceive, when the animal wants to move forward, that it pushes itself by the first set of hooks, keeping the rest, which would otherwise impede motion in that direction, pressed close to its skin—or it may depress that part of the segment; and when it would move backwards that it employs the second[416]. The other descriptions of bots, not being embedded in the flesh but fixed to a plane, are armed with the mandibles in question, by which they can not only suspend themselves in their several stations, but likewise, with the aid of the spines with which their segments also are furnished, move at their pleasure[417]. Other larvæ of flies, as well as the bots, are furnished with spines or hooks—by which they take stronger hold—to assist them in their motions. Those mentioned in my last letter as inhabiting the nests of humble-bees[418], besides the six radii that arm their anus, and which perhaps may assist them in locomotion, have the margin of their body fringed with a double row of short spines[419], which are, doubtless, useful in the same way.
The next order of walkers amongst apodous larvæ are those that move by means of fleshy tuberculiform or pediform prominences,—which last resemble the spurious legs of the caterpillars of most Lepidoptera. Some, a kind of monopods, have only one of such prominences, which being always fixed almost under the head, may serve, in some degree, the purpose of an unguiform mandible. The grub of a kind of gnat (Chironomus stercorarius), and also another, probably of the Tipularian tribe (found by De Geer in a subputrescent stalk of Angelica which he was unable to trace to the fly), have each a fleshy leg on the underside of the first segment, which points towards the head and assists them in their motions[420].—Others again go a little further, and are supported at their anterior extremity by a pair of spurious legs. An aquatic larva of a most singular form, and of the same tribe, figured by Reaumur, is thus circumstanced. In this case the processes in question proceed from the head, and are armed with claws[421]. Would you think it—another Tipularian grub is distinguished by three legs of this kind? It was first noticed by De Geer under the name of Tipula maculata (Tanypus monilis, Meig.), who gives the following account of its motions and their organs:—It is found, he observes, in the water of swampy places and in ditches, is not bigger than a horse-hair, and about a quarter of an inch in length. Its mode of swimming is like that of a serpent, with an undulating motion of the body, and it sometimes walks at the bottom of the water and upon aquatic plants. The most remarkable part of it are its legs, called by Latreille, but it should seem improperly, tentacula. They resemble, by their length and rigidity, wooden legs. The anterior leg is attached to the underside, but towards the head, of the first segment of the body. It is long and cylindrical, placed perpendicularly or obliquely, according to the different movements the animal gives it, and terminates in two feet, armed at their extremity by a coronet of long moveable hooks. These feet, like the tentacula of snails, are retractile within the leg, and even within the body, so that only a little stump, as it were, remains without. The insect moves them both together, as a lame man does his crutches, either backwards or forwards. The two posterior legs are placed at the anal end of the body. They are similar to the one just described, but larger, and entirely separate from each other, being not, like them, retractile within the body, but always stiff and extended. These also are armed with hooks. In walking, this larva uses these two legs much as the caterpillars of the moths, called Geometræ, do theirs. By the inflection of the anus it can give them any kind of lateral movement, except that it can neither bend nor shorten them, since like a wooden leg, as I have before observed, they always remain stiff and extended[422]. Lyonet had observed this larva, or a species nearly related to it; but he speaks of it as having four legs, two before and two behind. Probably, when he examined them, the common base, from which the feet are branches, was retracted within the body[423].