LETTER XXII.

_MOTIONS OF INSECTS._ (_Larva and Pupa._)

Amongst the means of defence to which insects have recourse, I have noticed their _motions_. These shall be the subject of the present letter. I shall not, however, confine myself to those by which they seek to escape from their enemies; but take a larger and more comprehensive survey of them, including not only every species of locomotion, but also the movements they give to different parts of their body when in a state of repose: and in order to render this survey more complete, I shall add to it some account of the various organs and instruments by which they move.

Whenever you go abroad in summer, wherever you turn your eyes and attention, you will see insects in motion. They are flying or sailing every where in the air; dancing in the sun or in the shade; creeping slowly, or marching soberly, or running swiftly, or jumping upon the ground; traversing your path in all directions; coursing over the surface of the waters, or swimming at every depth beneath; emerging from a subterranean habitation, or going into one; climbing up the trees, or descending from them; glancing from flower to flower; now alighting upon the earth and waters, and now leaving them to follow the impulse of their various instincts; sometimes travelling singly; at other times in countless swarms: these the busy children of the day, and those of the night. If you return to your apartment--there are these ubiquitaries--some flying about--others pacing against gravity up the walls or upon the ceiling--others walking with ease upon the glass of your windows, and some even venturing to take their station on your own sacred person, and asserting their right to the lord of the creation.

This universal movement and action of these restless little animals gives life to every part and portion of our globe, rendering even the most arid desert interesting. From their visitations every leaf and flower becomes animated; the very dust seems to quicken into life, and the stones, like those thrown by Deucalion and Pyrrha, to be metamorphosed into locomotive beings. In the variety of motions which they exhibit, we see, as Cuvier remarks[402], those of every other description of animals. They walk, run, and jump with the quadrupeds; they fly with the birds; they glide with the serpents; and they swim with the fish. And the provision made for these motions in the structure of their bodies is most wonderful and various. "If I was minded to expatiate," says the excellent Derham, "I might take notice of the admirable mechanism in those that creep; the curious oars in those amphibious insects that swim and walk; the incomparable provision made in the feet of such as walk or hang upon smooth surfaces; the great strength and spring in the legs of such as leap; the strong-made feet and talons of such as dig; and, to name no more, the admirable faculty of such as cannot fly, to convey themselves with speed and safety, by the help of their webs, or some other artifice, to make their bodies lighter than the air[403]."

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].

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.

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.

But the most extraordinary motion of pupæ is jumping. In the year 1810 I received an account from a very intelligent young lady, who collected and studied insects with more than common ardour and ability, that a friend had brought her a chrysalis endued with this faculty. It was scarcely a quarter of an inch in length; of an oval form; its colour was a semitransparent brown, with a white opake band round the middle. It was found attached, by one end, to the leaf of a bramble. It repeatedly jumped out of an open pill-box that was an inch in height. When put into a drawer in which some other insects were impaled, it skipped from side to side, passing over their backs for nearly a quarter of an hour with surprising agility. Its mode of springing seemed to be by balancing itself upon one extremity of its case. About the end of October one end of the case grew black, and from that time the motion ceased; and about the middle of April, in the following year, a very minute ichneumon made its appearance by a hole it had made at the opposite end.--Some time after I had received this history, I happened to have occasion to look at Reaumur's Memoir upon the enemies of caterpillars, where I met with an account of a similar jumping chrysalis, if not the same. Round the nests of the caterpillar of the processionary moth, before noticed[463], he found numerous little cocoons suspended by a thread three or four inches long to a twig or a leaf, of a shortened oval form, and close texture, but so as the meshes might be distinguished. These cocoons were rather transparent, of a coffee-brown colour, and surrounded in the middle by a whitish band. When put into boxes or glasses, or laid on the hand, they surprised him by leaping. Sometimes their leaps were not more than ten lines, at others they were extended to three or four inches, both in height and length. When the animal leaps, it suddenly changes its ordinary posture (in which the back is convex and touches the upper part of the cocoon, and the head and anus rest upon the lower), and strikes the upper part with the head and tail, before its belly, which then becomes the convex part, touches the bottom. This occasions the cocoon to rise in the air to a height proportioned to the force of the blow. At first sight this faculty seems of no great use to an animal that is suspended in the air; but the winds may probably sometimes place it in a different and unsuitable position, and lodge it upon a leaf or twig: in this case it has it in its power to recover its natural station. Reaumur could not ascertain the fly that should legitimately come from this cocoon, for different cocoons gave different flies: whence it was evident that these ichneumons were infested by their own parasite[464]. This might be the case with that of the lady just mentioned. Perhaps, properly speaking, in this last instance the motions ought rather to be regarded as belonging to a larva; but as it had ceased feeding, and had inclosed itself in its cocoon, I consider it as belonging to the present head.

You may probably here feel some curiosity to be informed how the numerous larvæ that are buried in their pupa state, either in the heart of trees, under the earth, or in the waters, effect their escape from their various prisons and become denizens of the air, especially as you are aware that each is shrouded in a winding-sheet and cased in a coffin. In most, however, if you examine this coffin closely, you will see RESURGAM written upon it. What I mean is this. The _puparium_ or case of the animal is furnished with certain acute points (_adminicula_) generally single, but in some instances forked, looking towards the anus, and usually placed upon transverse ridges on the back of the abdomen, but sometimes arming the sides or the margins of the segments. By this simple contrivance, aided by new-born vigour, when the time for its great change is arrived, the included prisoner of hope, if under ground, pushes itself gradually upwards, till reaching the surface its head and trunk emerge, when an opening in the latter being effected by its efforts, it escapes from its confinement, and once more tastes the sweets of liberty and the joys of life. Those that are inclosed in trees and spin a cocoon, are furnished with points on the head, with which they make an opening in the former. The pupa of the great goat-moth (_Cossus ligniperda_) thus, by divers movements, keeps disengaging itself from this envelope, till it arrives at a hole in the tree which it had made when a caterpillar; when its anterior part having emerged, it stops short, and so escapes a fall that might destroy it. After some repose, in consequence of very violent efforts, the puparium opens, and it escapes from its prison[465].

The insects of the _Trichoptera_ order, or case-worm flies are _quiescent_ when they first assume the pupa, but become locomotive towards the close of their existence in that state. Since they inhabit the water when they become pupæ, Providence has furnished them with the means of quitting that fluid without injury, when they are to exchange it for the air; which in their winged state is their proper sphere of action. I have before described to you the grates which shut up their cases when they became quiescent[466]; if they had no means of piercing these grates, they would perish in the waters. The head of these pupæ is provided at first with a particular instrument, which enables them to effect this purpose; its anterior part is armed with a pair of hooks in form resembling the beak of a bird; and with this, previously to their last change, they make an opening in the grate which, though it once defended, now confines them. But at this moment, perhaps, the insect has a considerable space of water to rise through before she can reach the surface. This is all wisely provided for; before she leaves the envelope which covers her body, she emerges from the water, and fixes herself upon some plant or other object, the summit of which is not overflowed. But you will here, perhaps, ask--How can a pupa in her envelope, with all her limbs set fast, do this? This affords another instance of the wise provision of the beneficent Father of the universe for the welfare of his creatures. The antennæ and legs of this tribe of insects, when they are pupæ, are not included, as is the case with most that are quiescent in that state, in the general envelope; but each in a separate one, so as to allow it free motion. Thus the insect when the time is come for its last change can use them (except the hind-legs, which being partly covered by the wing-cases remain without motion) with ease. It then stretches out its antennæ, and steering with its legs makes for the surface. De Geer saw one just escaped from its case run and swim with surprising agility over the bottom of a saucer, in which he had put some cases of these flies; and at last when he held a piece of stick to it, it got upon it, and having emerged from the water, prepared to cast its envelope. It is remarkable, that the envelope of the intermediate tarsi, like the posterior ones of Dytisci, is fringed on one side with hairs, to enable the insects to use them as swimming feet[467], while those neither of the larva nor imago are so circumstanced.

I am, &c.

FOOTNOTES:

[402] _Anatom. Compar._ i. 144.

[403] _Physico-Theol._ Ed. 13. 363.

[404] _Encycl. Brit._, art. _Physiology_, 709.

[405] Cuvier, _Anat. Comp._ i. 430.

[406] De Geer, v. 210.

[407] See above, p. 7.

[408] De Geer, vi. 338.

[409] See MacLeay in _Philos. Mag. &c._ N. Ser. No. 9. 178.

[410] De Geer, vi. 65.

[411] _Hist. Ins._ 270.

[412] Vol. I. 265.

[413] Reaumur, iii. 369.

[414] Vol. I. 137. De Geer, vi. 76. Reaumur, iv. 376. Swamm. _Bibl. Nat._ Ed. Hill, ii. 46. a. _t._ xxxix. _f._ 3, _h. h._

[415] De Geer, vi. 355.

[416] Reaum. iv. 416. _t._ xxxvi. _f._ 5. Comp. Clark _On the Bots_, &c. 48.

[417] Mr. Clark (ibid. 62) observed only rough points on the bots of the sheep, but these also have spines or hooks looking towards the anus. Reaum. iv. 556. _t._ xxxv. _f._ 11, 13, 15. I also observed them myself in the same grub.

[418] See above, p. 220.

[419] PLATE XIX. FIG. 11.

[420] De Geer, vi. _t._ xxii. f. 15, _i._ _t._ xviii. f. 8, _p._

[421] Reaum. v. _t._ vi. f. 5, _mm._

[422] De Geer, vi. 395--. PLATE XXIII. FIG. 7. Foreleg, _a._ Hind-legs, _bb._ Mr. W. S. MacLeay is of opinion that these legs are pedunculated spiracles, (_Philos. Mag._ N. Series, No. 9. 178.) but it is evident from De Geer's account that the animal uses them as legs, and like legs they are armed with hooks or claws.

[423] Lesser _L._ i. 96. note †.

[424] Klemann, _Beitrage_, 324.

[425] De Geer, i. 447-- _t._ xxxi. _f._ 17.

[426] De Geer, vi. 111.

[427] Ibid. v. 233.

[428] Ibid. 228.

[429] De Geer, vi. 137. _t._ viii. _f._ 8, 9.

[430] Reaum. iii. 496. _t._ xlv. _f._ 3.

[431] Ibid. _Mem. de l'Acad. Roy. des Sciences de Paris_, An. 1714. p. 203.

[432] De Geer, vi. 380-- _t._ xxiv. _f._ 1-9.

[433] Swamm. _Bibl. Nat._ Ed. Hill, ii. 64. b.

[434] De Geer, vi. 389--.

[435] VOL. I. 431.

[436] Reaum. iv. _t._ 43. _f._ 3. _nn._

[437] De Geer, vi. 375. _t._ xxiii. _f._ 4, 5.

[438] Swamm. _Bibl. Nat._ Ed. Hill, ii. 44. b. 47. a.

[439] For examples of larvæ having these joints, see De Geer, iv. 289. _t._ xiii. _f._ 20. _t._ xv. _f._ 14. ii. _t._ xii. _f._ 3. _t._ xvi. _f._ 5, 6. _t._ xix. _f._ 4, &c.

[440] Ibid. v. _t._ xi. _f._ 11. _t._ ix. _f._ 9. o.

[441] Lyonet, _Tr. Anat._ _t._ iii. _f._ 8.

[442] Mr. W. S. MacLeay, where quoted above, objects to this term; but as the organs in question are generally given to the animal to assist in its motions, and have been universally regarded as a kind of legs, it was judged best for the sake of distinction to give them a different name from perfect legs, and at the same time one that showed some affinity to them.

[443] Lyonet, 82-- _t._ iii. _f._ 10-16.

[444] Ibid. _t._ i. _f._ 4.

[445] De Geer, i. 379. _t._ xxv. _f._ 1. 3.

[446] VOL. I. 192--.

[447] De Geer, i. 12. 40. _t._ i. _f._ 27. _q. t._ vi. _f._ 11. _e._

[448] De Geer, i. 424.

[449] Kirby in _Linn. Trans._ v. 258.

[450] _Anatom. Comp._ i. 430.

[451] Rösel, I. iv. 112. vi. 14.

[452] Reaum. ii. 375--.

[453] Miger, _Ann. du Mus._ xiv. 441.

[454] De Geer, ii. 621.

[455] Ibid. 725--.

[456] De Geer, ii. 675-- Compare Reaum. vi. 393.

[457] VOL. I. 66.

[458] See above, p. 255.

[459] De Geer, iii. 284.

[460] Ibid. vi. 308.

[461] Ibid. iv. 43.

[462] Dumeril, _Trait. Element._ ii. 49. n. 603.

[463] VOL. I. 475; and above, p. 23.

[464] Reaum. ii. 450.

[465] Lyonet. _Trait. Anat._ 15--.

[466] See above, p. 264.

[467] De Geer, ii. 518--.