With respect to the larvæ that have only perfect legs, having just given you an account of these organs, I have nothing more to state relating to their structure. I shall therefore now consider the motions of pedate larvæ, under the several heads of walking or running, jumping, climbing, and swimming.
Amongst those that walk, some are remarkable for the slowness of their motion, while others are extremely swift. The caterpillar of the hawk-moth of the Filipendula (Zygæna Filipendulæ) is of the former description, moving in the most leisurely manner; while that of Apatela leporina, a moth unknown in Britain, is named after the hare, from its great speed. The caterpillar of another moth, the species of which seems not to be ascertained, is celebrated by De Geer for the wonderful celerity of its motions. When touched it darts away backwards as well as forwards, giving its body an undulating motion with such force and rapidity, that it seems to fly from side to side[448].—Cuvier observes, that the grubs of some coleopterous and neuropterous insects, which have only the six perfect legs, by means of them lay hold of any surrounding object, and, fixing themselves to it, drag the rest of their body to that point; and that those of many capricorn beetles and their affinities (but that of Callidium violaceum is an apode[449]) have these legs excessively minute and almost nothing; that they move in the sinuosities which they bore by the assistance of their mandibles, with which they fix themselves, and also of several dorsal and ventral tubercles, by which they are supported against the sides of their cavity, and push themselves along, in the same manner as a chimney-sweeper—by the pressure of his knees, elbows, shoulder-blades, and other prominent parts—pushes himself up a chimney[450]. The larva of the ant-lion (Myrmeleon)—with the exception of one species, which moves in the common way—always walks backwards, even when its legs are cut off.
The jumpers amongst pedate larvæ, as far as they are known, are not very numerous, and will not detain you long. When the caterpillar of Lithosia Quadra, a moth not uncommon, would descend from one branch or leap to another, it approaches to the edge of the leaf on which it is stationed, bends its body together, and retiring a little backwards, as if to take a good situation, leaps through the air, and, however high the jump, alights on its legs like a cat. That of another moth (Herminia rostralis) will also leap to a considerable height[451].
Another species of motion, which is peculiar to larvæ,—their mode I mean of climbing,—as it merits particular attention, will occupy more time. I have already related so many extraordinary facts in their history, that I promise myself you will not disbelieve me if I assert that insects either use ladders for this purpose, or a single rope. You may often have seen the caterpillar of the common cabbage-butterfly climbing up the walls of your house, and even over the glass of your windows. When next you witness this last circumstance, if you observe closely the square upon which the animal is travelling, you will find that, like a snail, it leaves a visible track behind it. Examine this with your microscope, and you will see that it consists of little silken threads, which it has spun in a zigzag direction, forming a rope-ladder, by which it ascends a surface it could not otherwise adhere to. The silk as it comes from the spinners is a gummy fluid, which hardens in the air; so that it has no difficulty in making it stick to the glass.—Many caterpillars that feed upon trees, particularly the geometers, have often occasion to descend from branch to branch, and sometimes, especially previously to assuming the pupa, to the ground. Had they to descend by the trunk, supposing them able to traverse with ease its rugged bark, what a circuitous route must they take before they could accomplish their purpose! Providence, ever watchful over the welfare of the most insignificant of its creatures, has gifted them with the means of attaining these ends, without all this labour and loss of time. From their own internal stores they can let down a rope, and prolong it indefinitely, which will enable them to travel where they please. Shake the branches of an oak or other tree in summer, and its inhabitants of this description, whether they were reposing, moving, or feeding, will immediately cast themselves from the leaves on which they were stationed; and however sudden your attack, they are nevertheless still provided for it, and will all descend by means of the silken cord just alluded to, and hang suspended in the air. Their name of geometer was given them, because they seem to measure the surface they pass over, as they walk, with a chain. If you place one upon your hand, you will find that they draw a thread as they go; when they move, their head is extended as far as they can reach with it; then fastening their thread there, and bringing up the rest of their body, they take another step; never moving without leaving this clue behind them; the object of which, however, is neither to measure, nor to mark its path that it may find it again; but thus, whenever the caterpillar falls or would descend from a leaf, it has a cord always ready to support it in the air, by lengthening which it can with ease reach the ground. Thus it can drop itself without danger from the summit of the most lofty trees, and ascend again by the same road. As the silky matter is fluid when it issues from the spinners, it should seem as if the weight of the insect would be too great, and its descent too rapid, so as to cause it to fall with violence upon the earth. The little animal knows how to prevent such an accident, by descending gradually. It drops itself a foot or half a foot, or even less, at a time; then making a longer or shorter pause, as best suits it, it reaches the ground at last without a shock. From hence it appears that these larvæ have power to contract the orifice of the spinners, so as that no more of the silky gum shall issue from it; and to relax it again when they intend to resume their motion downwards: consequently there must be a muscular apparatus to enable them to effect this, or at least a kind of sphincter, which, pressing the silk, can prevent its exit. From hence also it appears that the gummy fluid which forms the thread must have gained a degree of consistence even before it leaves the spinner, since as soon as it emerges it can support the weight of the caterpillar.—In ascending, the animal seizes the thread with its jaws as high as it can reach it; and then elevating that part of the back that corresponds with the six perfect legs, till these legs become higher than the head, with one of the last pair it catches the thread; from this the other receives it, and so a step is gained: and thus it proceeds till it has ascended to the point it wishes to reach. At this time if taken it will be found to have a packet of thread, from which, however, it soon disengages itself; between the two last pairs of perfect legs[452]. To see hundreds of these little animals pendent at the same time from the boughs of a tree, suspended at different heights, some working their way downwards and some upwards, affords a very amusing spectacle. Sometimes, when the wind is high, they are blown to the distance of several yards from the tree, and yet maintain their threads unbroken. I witnessed an instance of this last summer, when numbers were driven far from the most extended branches, and looked as if they were floating in the air.
Having related to you what is peculiar in the motions of pedate larvæ upon the earth and in the air, I must next say something with respect to their locomotive powers in the water. Numbers of this description inhabit that element.—Amongst the beetles, the genera Dytiscus, Hydrophilus, Gyrinus, Limnius, Parnus, Heterocerus, Elophorus, Hydræna, &c. amongst the bug tribes, Gerris, Velia, Hydrometra, Notonecta, Sigara, Nepa, Ranatra, Naucoris; a few Lepidoptera; the majority of Trichoptera; Libellula, Aeshna, Agrion, Sialis, Ephemera, &c. amongst the Neuroptera; Culex and many of the Tipulariæ, Latr. from the dipterous insects; and from the Aptera, Atax, some Poduræ, and many of the Oniscidæ, &c.—All these, in their larva state, are aquatic animals.
The motions of these creatures in this state are various. Some walk on the ground under water; some move in midwater, either by the same motion of the legs as they use in walking, or by strokes, as in swimming; others for this purpose employ certain laminæ, which terminate their tails, as oars; others again swim like fish, with an equable motion; some move by the force of the water which they spirt from their anus; others again swim about in cases, or crawl over the submerged bottom; and others walk even on the surface of the water. I shall not now enlarge on all these kinds of water-motion, since many will come under consideration hereafter.
There are two descriptions of larvæ of Hydrophili, one furnished with swimmers or anal appendages, by means of which they are enabled to swim; the other have them not, and hence are not able to rise from the bottom[453]. The larvæ of Dytisci, by means of these natatory organs, will swim, though slowly, and every now and then rise to the surface for the sake of respiration. Those of Ephemeræ, when they swim, apply their legs to the body, and swim with the swiftness and motions of fish[454]. Those of the true may-fly (Sialis lutaria), on the contrary, use their legs in swimming, and at the same time, by alternate inflexions, give to their bodies the undulations of serpents[455]. But the larvæ of certain dragon-flies (Aeshna and Libellula,) will afford you the most amusement by their motions. These larvæ commonly swim very little, being generally found walking at the bottom on aquatic plants: when necessary, however, they can swim well, though in a singular manner. If you see one swimming, you will find that the body is pushed forward by strokes, between which an interval takes place. The legs are not employed in producing this progressive motion, for they are then applied close to the sides of the trunk, in a state of perfect inaction. But it is effected by a strong ejaculation of water from the anus. When I treat upon the respiration of insects, I shall explain to you the apparatus by which these animals separate the air from the water for that purpose; in the present case it is subsidiary to their motions, since it is by drawing in and then expelling the water that they are enabled to swim. To see this, you have only to put one of these larvæ into a plate with a little water. You will find that, while the animal moves forward, a current of water is produced by this pumping, in a contrary direction. As the larva, between every stroke of its internal piston, has to draw in a fresh supply of water, an interval must of course take place between the strokes. Sometimes it will lift its anus out of the water, when a long thread of water, if I may so speak, issues from it[456].
II. I am next to say something upon the motions of insects in their pupa state. This is usually to our little favourites a state of perfect repose; but, as I long since observed[457], there are several that, even when become pupæ, are as active and feed as rapaciously as they do when they are either larvæ or perfect insects. The Dermaptera, Orthoptera, Hemiptera, many of the Neuroptera, and the majority of the Aptera, are of this description. With respect to their motions, we may therefore consider pupæ as of two kinds—active pupæ, and quiescent pupæ.
The motions of most insects whose pupæ are active, are so similar in all their states, except where the wings are concerned, as not to need any separate account. I shall therefore request you to wait for what I have to say upon them, till I enter upon those of the imago. One insect, however, of this kind, moving differently in its preparatory states, is entitled to notice under the present head.—In a late letter, I mentioned to you a bug (Reduvius personatus) which usually covers itself with a mask of dust, and fragments of various kinds, cutting a very grotesque figure[458]. Its awkward motions add not a little to the effect of its appearance. When so disposed, it can move as well and as fast as its congeners; yet this does not usually answer its purpose, which is to assume the appearance of an inanimate substance. It therefore hitches along in the most leisurely manner possible, as if it was counting its steps. Having set one foot forward (for it moves only one leg at a time), it stops a little before it brings up its fellow, and so on with the second and third legs. It moves its antennæ in a similar way, striking, as it were, first with one, and then, after an interval of repose, with the other[459].—The pupæ of gnats also, as well as those of many other aquatic Diptera, retain their locomotive powers, not however the free motion of their limbs. When not engaged in action, they ascend to the surface by the natural levity of their bodies, and are there suspended by two auriform respiratory organs in the anterior part of the trunk, their abdomen being then folded under the breast; when disposed to descend the animal unfolds it, and by sudden strokes which she gives with it and her anal swimmers to the water, she swims, to the right and left as well as downwards, with as much ease as the larva[460].
Bonnet mentions a pupa which climbs up and down in its cocoon,—and that of the common glow-worm (Lampyris noctiluca) will sometimes push itself along by the alternate extension and contraction of the segments of its body[461].—Others turn round when disturbed. That of a weevil (Hypera Arator) which spins itself a beautiful cocoon like fine gauze, and which it fixes to the stalks of the common spurrey (Sagina arvensis), upon my touching this stalk, whirled round several times with astonishing rapidity.—The chrysalis of a scarce moth (Hypogymna dispar) when touched turns round with great quickness; but, as if fearful of breaking the thread by which it is suspended by constantly twisting it in one direction, it performs its gyrations alternately from left to right, and from right to left[462]. Generally speaking, quiescent pupæ when disturbed show that they have life, by giving their abdomen violent contortions.