A Spiracle open.The same shut.
We may, therefore, consider the question as settled—that larvæ do breathe. Strange it may appear to some, but neither in the larva nor in any other form of its existence do insects breathe by themouth. How, then, it may be asked? We shall now describe their breathing apparatus very briefly. Along each side of the body runs a delicate tube for the conveyance of air, called atrachea, or when both are spoken of, thetracheæ; these channels run underneath the muscles and skin, and open to the air by little branch tubes, the mouths of which are calledspiracles. Besidesthese branch tubes which open to the air, the large tubes send off a great number of smaller ones, which extend to all parts of the body, penetrating even to the legs and wings. In some insects, the spiracles, or breathing holes, are defended by a pair of flaps, which swing to and fro, so as to let in the air, and close upon it after each inspiration. By this simple means the breathing of insects is carried on; the air enters the tubes, passes along them, and is then conveyed to every part of the body; after which, having fulfilled its duties, it escapes again. In man, and the higher animals, there is a separate place for the air in the body, which we call the lungs, and here the blood is conveyed to the air, in order to be purified. But, as we have just seen, in insects it is very different, for in them the air is conveyed by innumerable minute pipes to every portion of the frame. Such is the breathing mechanism common in larvæ which live in the air.
It will be readily imagined that those larvæ which dwell in the water must be furnished somewhat differently, in order to enablethemto breathe. And here, as we shall now find, there are some very curious arrangements, in order to effect this object. They may be considered under two divisions. First, contrivances for breathing air while the larva is immersed in the water; and, second, apparatus for extracting the dissolved oxygen gas necessary for breathing from the water.
Breathing apparatus of an aquatic Larva.
Let us select a few of the most singular instances under the first of these divisions. "No better example," write the entertaining authors of the Introduction to Entomology, "can be selected than the gnat. You must have occasionally observed in tubs of rain-water, numerous little wriggling worm-like animals, which frequently ascend to the surface, there remain awhile, and then bending their head under the body, rapidly sink again to the bottom. These are the larvæ of some species of the genus just named; and if you take one out of the water and examine it, you will perceive that it is furnished, near the end of the body, with a singular organ, which varies in lengthaccording to the species, and forms an angle with the last segment but one. The mouth of this organ is funnel-shaped, and terminates in five points like a star; and by this it is usually suspended at the surface of the water, and preserves its communication with the atmosphere. In its interior is a tube which is connected with thetracheæ, and terminates in several openings, visible under a microscope at the mouth of the organ. The points or rays of this mouth are used to close it when the animal is disposed to sink in the water, and thus cut off its communication with the atmosphere. When the animal is immersed, a globule of air remains attached to the end of the tube, so that it is in fact of less specific gravity than water, and it is not without some effort that it descends to the bottom; but when it wishes to rise again, it has only to unclose the tube, and it rises without an effort to the surface, and remains suspended for any length of time. The extremity of the larva is clothed with bunches of hairs, which are furnished with some repellentmaterial which prevents their becoming wet. It is this repellent quality that probably causes a dimple or depression of the surface, which, if you look narrowly, you will discover round the mouth of the tube."
In summer time, the reader can scarcely fail of the gratification of watching these most curious creatures, for they abound in ditches, pools, and the stagnant waters of artificial fountains. They form a very curious object under the microscope, and particularly in the gas microscope, or solar microscope, where the image of the larvæ is thrown, as in the case of the magic-lantern, upon a white sheet, or wall. Their curious writhings to and fro, the energy they display in lashing about their tails, so as to throw the few drops of water in which they are imprisoned into mimic whirlpools, their occasional dartings down to the bottom, and subsequent gentle rise up to the surface, together with the terror into which countless minor occupants of the same watery prison with themselves are thrown, when one of these comparative giants plunges into the midst of them, form a spectacle as unique as it is interesting and diverting. A sharp eye, and a phial and cork, are thealone requisites for finding out these singular beings on most fine days in summer. But a little while ago the writer was amused to find, on a sunny day in June, the water of a small fountain playing before his study window, thickly populated with these active larvæ, and it became an amusement which seldom wearied, to watch their fantastic evolutions, and to trace their changes from the larva through the ulterior stage up to the perfect insect. To this, however, we shall probably have occasion to revert before the conclusion of this work.
Another instance of a curious contrivance for assisting the breathing of a larva, whose body is wholly immersed in water, may also be found near home. Most probably the majority of our readers have seen, or at any rate are well acquainted with the apparatus by which the diver descends and works under water. Lest some should not, we may mention that it consists of a water-tight dress, which covers the diver from head to foot, while his head is enclosed in a helmet, or hood, furnished with a pipe or hose, which is long enough to reach to the surface of the water, and is there generally connected withan air-pump, which forces down fresh air, and so enables the man to breathe almost as comfortably as if he were walking on land, instead of at the bottom of the sea. Let it be conceived now that there were no air-pump at the top of the hose, but simply a buoy of cork to keep it floating, and that the diver had the power of shortening or lengthening the hose at his pleasure, we should then have a tolerably fair representation of the contrivance with which Infinite Wisdom has supplied the humble creatures of whom we are about to speak.
By carefully examining the water running from manure heaps, or water which has become putrid, and is much loaded with mud and ooze, the reader will pretty generally be successful in discovering a number of the larvæ of a two-winged fly of the shape represented in the cut. When discovered, the singularity of their structure will amply repay him for the trifling unpleasantness attending the search after them. After washing them in a little fair water, and putting them into a tall ale-glass, or German beer-glass, which must be three parts filled with water, they will be in the best condition for our examination. WhenRéaumur first beheld them, he gave them the name ofVers à queue de rat; that is, larvæ with a rat's tail; and when the reader looks at them, as shown in the representation annexed, he will be inclined to think with us, that a more appropriate title could not have been found.
Rat-tailed Larvæ.
On watching them they will be seen to wriggle about until they are comfortably settled, darting from place to place, as if uneasy in their new position and in the pure element. By-and-by, however, they become reconciled to their lot, and lie perfectly quiet at the bottom of the glass in the position represented. From the tail of each will now be seen to rise a beautifully delicate and minute tube, which comes up to the surface of the water, and here, whatever be the movements of the larva below, it remains floating and free. Just as the diver may, while his companions above are pouring down torrents of fresh air, move about, and perform various duties under,water, so the larva may move from one position to another, and assume almost any variety of attitude; but its little air-pipe still retains its perpendicular direction, and does not quit the surface of the fluid.
A singular experiment may now be tried with them. Let a little water be poured into the glass by means of a funnel, the end of which dips under the water so as not to give them unnecessary disturbance, and by this means the level of the water in the glass becomes insensibly raised half an inch or so. In a few seconds it will be found that these little tails have all lengthened themselves, and are again at the surface of the water, even at its higher level. More water may be poured in until perhaps the level is raised three or four inches, and still the same thing takes place, the larvæ still prolong their curious tails, until it seems as if they would stretch to an indefinite length, for they are already eight or ten times longer than the body! But, as we might jestingly say—the longesttailcomes to an end at last; for, on continuing to pour more water in, the larvæ stretch their tails to the utmost, and then, finding the level of the water still above their reach, they become uneasy, andall quit the bottom of the glass and attach themselves to its sides.
The Tail magnified.
Let us now examine more minutely this very singular apparatus. By a good lens we may readily do this, as the organ is very transparent, permitting its internal structure to be beautifully seen. Placing the creature in a convenient position for our examination, and in a good light, it will be presently seen that the tail is composed in reality of two tubes, of which one slides up and down within the other, just like the tubes of a telescope (see cut). By this means the larva is provided with an instrument capable of being made to reach to a great length, and yet also possessing the capability of being packed up again in a very small compass. The proper breathing tube is the small inner one; at its extremity is apair of spiracles, or breathing pores, and surrounding these are placed five minute bristles, which are supposed to act somewhat as a buoy, and to retain the tail in its position. Where the tail joins on to the body of the larva, two tubes thrown into folds are placed, which are connected with the breathing tubes of the tail, at one end, and with the breathing pipes which run along the body of the larva at the other. When the tail is pushed out to its farthest, these zig-zag tubes are stretched quite straight; when it is drawn in again, they are thrown into folds. Both these appearances are shown in the plate. It has been supposed that the larva causes the tube to be pushed out by forcing air into it, and thus causing the zig-zag tubes to straighten themselves. Perhaps the tail is drawn in by the same tubes contracting again, and thus folding together and dragging back the tube to which they are connected.
End of the Tail.
We need scarcely stop to mention how useful is this admirable contrivance to the insect. Having to seek for their food at the bottom of ditches,drains, or puddles, they are necessarily exposed to the risk of suffocation, unless fresh air can be conveyed to them; and it is difficult for us to imagine how any apparatus could have been contrived which would have adapted itself to all the varying depths of water in which the insect must be constantly living, as it changes from place to place. He must have been a clever engineer who could have successfully met this difficulty. None of our present diving apparatus does so. The organ we have been considering, the creation of an Infinite Mind, small and despicable as it may seem in our eyes, fulfils perfectly every function for which it was formed. It admits of free movement from place to place, it admits, moreover, of free change from one level to another in the fluid by which the larva is surrounded, and it is at the entire disposal of the insect, which can, without the smallest inconvenience, accommodate it to the various circumstances in which it may be placed.
Chameleon-fly Larva.
Possibly the same phial in which was brought home the last-named larvæ with the rat's tail, will furnish us with an equally elegant instance of larva-respiration under water, in the case of the larva of a fly, called the chameleon fly. Thislittle creature, if it can be found, and though not as common as the last, it may yet be detected, in summer by the edge of ponds, &c., has truly elegant habits. The great Swammerdam, who first described it, was lost in admiration at it. The extremity of its body is furnished with a coronet of about thirty elegantly feathered hairs. These are under the control of the insect, and are capable of being folded up, so as to enclose a minute bubble of air. When the larva is weary of remaining at the top of the water, and wishes to dive to the still and cool depths below, it causes these hairs so to fold up and close upon each other as to include the air-bubble, so that it cannot escape. Thus furnished with a pearl in its tail, it plunges downward, and thus breathes under water until the purity of the captive air-bubble is impaired, when it returns to the surface to repeat the same operations. The appearance of the little bubble shining, with a brilliant silvery lustre under the water, is very pleasing, and accounts for theenthusiastic expressions used by Swammerdam, in speaking of the visible manifestation of the skill of God in this insect. The fact of an insect coming to the surface for air, and stealing away a portion, then diving down again into the waters, is not uncommon in insect history, although few are possessed of so elegant a contrivance for effecting it as the larva in question.
We must pass on to mention an instance or two of larvæ which, from the peculiarity of their respiration, come under the second of our divisions, that is, they live in the water, and do not breathe air, but obtain the gas necessary for their respirationfrom the water, which holds it in a state of solution. We shall first speak of the larva of the common May-fly, the perfect insect of which is so well known to, and highly prized by, anglers, under the various names of "duns" and "drakes," than which in some streams no insect that flies is a better bait to the unwary fish. Should the reader wish to examine them for himself, they may be found in May or June, hiding themselves during the day in the mud, or underneath stones, or in little holes in the banks, or taking a sub-aqueous tour upon the bed of the brook, while theclear and purling waters roll smoothly on above them. Their little under-water cells are shown in the cuts, together with the larva.
Cells of May-fly Larva.
Larva and section of its cell.
Their organs for breathing the water, if the expression, which is in some measure correct, may be admitted, are very like the gills of a fish. But instead of forming a part of the structure of the mouth and head, as those of the fish, in the larva they are placed along its body, and are arranged in pairs. A delicate little air-tube enters into each of these gills, and there branches out into a great many directions; this tube is connected withthe large trachea, or breathing vessel of the insect, running along its body. The air entering these gills, and thus being exposed to a large surface of water containing dissolved gas, loses its impurities, and absorbs the pure gas from the water, whence it is then conveyed to the various parts of the body. In many species of these larvæ the whole series of gills are moved to and fro incessantly, and so quickly as to be scarcely perceptible to the eye. Some of them use them as oars, moving with inconceivable rapidity, and being capable of darting both backwards and forwards with great quickness. They have also the power of leaping or springing in the water to a considerable distance; and feed on minute aquatic larvæ, and on vegetable matters.
The only other instance of this kind that we shall quote will be that of the terrible larva of the dragon-fly, whose deeds of insect murder came before us on a former occasion. The apparatus provided for them is very singular, and has been described with his usual accuracy by the talented Réaumur. It exists in the tail of the insect, and may be readily watched in action by adopting the following expedient:—Pour into a small glass, inwhich the larva has been put, and which is half full of water, a few drops of a solution of carmine, by means of a little tube introduced into the water, and made to drop its contents near the tail of the creature. Immediately it will be seen that the coloured liquid is rapidly sucked in by the tail of the insect, and then, after the expiration of a little time, is forced back again in a tiny jet, with considerable violence, to a distance of several inches.[H]Réaumur tells us, that if we hold the insect between the finger and thumb gently, and let fall a drop of water on its tail, we shall presently see it eagerly sucked in, the body of the insect becoming sensibly larger; and by-and-by it is thrown out again. Sometimes it raises its tail above the surface of the water, and, whether for sport or not, it is hard to say, squirts the water in a mimic jet to some distance.
The apparatus has been described as resembling the piston or plunger of a force-pump, which moves to and fro within a cavity inside the insect's body, and by that means draws in and forces out the water. The tail is defended by a sort ofcoronet, composed of five hard bristle-like pieces, which are under the control of the creature, and may be opened and closed at pleasure. The larva appears capable of inflicting a somewhat painful pinch by this contrivance, so that, what with its wonderful mask, with its powerful squirt, and with its tail thus armed, it would be difficult to select another insect better prepared for battle with creatures of every aquatic tribe than it. The action of the pumping apparatus sends it rapidly forward through the water, somewhat like a paddle-wheel, or at least upon the same principle as that contrivance, namely, the resistance offered by the water to the stroke of this apparatus.
CHAPTER V.
MEANS OF DEFENCE OF THE LARVA.
There is, perhaps, no period in the life of an insect when it is so much in danger of the attacks of foes, as when it is in the larva state. Possessed, at the best, with but very feeble powers of locomotion, in comparison with those of winged insects, it is continually exposed to destruction on all sides, from which it has not the power to escape. Not only do insects of various species particularly select the larva for the object of their attacks, but birds and other creatures are also in the lists against them. Thus imperilled, what chance has the larva against foes so numerous, and in a contest so unequal? It has been calculated that a pair of sparrows, for their own consumption, and for that of their tender brood, destroy at least three thousand caterpillars in a week! and we well know what multitudes of larvæ fall into theclutches of the cawing rooks which follow the tail of the plough. The winged insects are tolerably secure, although many even of their ranks fall victims to the swift-winged swallow and sand-martin, and to the rapid leap of the artful trout. But the poor crawling larvæ are neither strong enough to fight, nor swift enough to flee, and fall by thousands into the unsparing hands of their various enemies.
Leaf-rolling Larvæ.
Leaf-rolling Larvæ.
Larva Case of Shells.
Larva Case of Shells.
Yet, the Creator, who has wisely appointed a large portion of their numbers for food to many creatures, has also taken many precautions to prevent their being entirely annihilated, as might otherwise have been the case. These are well deserving a brief notice, previous to our passing on to the last stage in the history of the larva. From what has been already said about the coats of various larvæ, it may be imagined that that must be a stout-hearted enemy which could venture to attack some of them. The spines and hard excrescences with which they are furnished must, undoubtedly, prove far from agreeable to the touch of a foe; and there can be no question that this sort of armour is, of itself, a good defence to many larvæ. Others form artificial coverings, which areequally useful, by concealing them from the notice of their enemies. Thus, as we have before remarked, some dwell in tubes of leaves, hid from every eye; others encase themselves in a shield of cemented gravel or shells, like those represented in the cut, presenting a most grotesque appearance; others hide themselves in variously-formed cells. Some, immediately on being touched, roll themselves up, like hedge-hogs, into a ball, which looks like a little globe of hairs, and is so slippery that it cannot be retained between the fingers; others, again, when they are touched, instantly drop down, and from their colour, resembling preciselythat of the herbage into which they have fallen, it becomes next to fruitless to search for them.
A very singular larva which, both for offence and defence, adopts a most extraordinary procedure, must be mentioned by itself. This larva is not unfrequently to be found in our houses, and is said to feed upon our disgusting and annoying invaders—the bugs; but it so completely contrives to conceal its real character, that not once in a hundred times would it, probably, be recognised as an insect at all. It collects together a number of particles of sand, or fragments of wool, or silk, or earth, and with singular skill, contrives to form a sort of coat out of these, in which it envelopes itself. Thus attired, as may be imagined, it is not easy to discover what the object really is, and it thus, undoubtedly, escapes the undesirable notice of any of its enemies. But more than this: like those North American Indians, who hunt by concealing themselves in the skins of wolves, and so attract less attention from the herd than they would do if they were to expose themselves in their natural aspect to view, this crafty larva steals upon its prey in much the samemanner, although it can run with sufficient swiftness when necessary. It creeps forward with the greatest stealth, moving only one leg at a time, and thus quite insensibly approaches its victim, who is not at all alarmed at what merely appears to be a lump of dust. Soon, however, it is convinced of its fatal mistake, and discovers the terrible fact that it is in the embrace of an unsparing devourer.
Other larvæ escape notice in a singular manner, by their close resemblance to little pieces of stick. Thus, we are told by Messrs. Kirby and Spence, that "there is a certain tribe of caterpillars called surveyors (Geometræ), that will sometimes support themselves, for whole hours, by means of their posterior legs, solely upon their hinder extremity, forming an angle, of various degrees, with the branch on which they are standing, and looking like one of its twigs. The body is kept stiff and immovable, with the separations of the segments scarcely visible; it terminates in a knob, the legs being applied close, so as to resemble a bud at the end of a twig; besides which, it often exhibits intermediate tubercles, which increase the resemblance. Its colour, too, is usually obscure,and similar to that of the bark of a tree: so that, doubtless, the sparrows and other birds are frequently deceived by this manœuvre, and thus balked of their prey. Rösel's gardener, mistaking one of these caterpillars for a dead twig, started back in great alarm, when, upon attempting to break it off, he found it was a living animal."
Some, however, are actually armed, so as to offer an active resistance to the onslaught of any enemy. Mr. Abbot has described a terrible caterpillar, well known in certain districts in America, under the strange title of the "Hickory-horned Devil." This creature is furnished with seven or eight strong horn-like spines, upwards of half an inch in length, which project from its head and anterior segments. When threatened by danger, the caterpillar does not attempt to fly, but lifts up its head menacingly, and shakes it about in a very fierce manner. The negroes, in particular, are so terrified at its aspect, that they dread it as much as the rattlesnake. But, in reality, it is harmless; and Mr. Abbot, in order to convince the ignorant people that such was the case, took hold of it, in their presence, without receiving any injury. This, however,did not convince them, and they made answer that, although it did not sting him, it would undoubtedly sting them.
The weapon of another is thus pleasantly described by De Geer:—"In the month of July, I found a caterpillar resting upon a small twig, which I cut off in order to enable me to examine it more attentively. As I was scrutinizing its appearance, I happened, accidentally, to touch its body. Instantly there shot out, from a spot near the head, two streams of clear fluid, which struck me on the face, and partly entered one of my eyes, causing a very acute, but fortunately, not a very enduring pain. My surprise at this salute prevented my taking particular notice of the place from whence the jets proceeded. It appears probable that this caterpillar has been thus endowed with this squirting apparatus to enable it to defend itself against its enemies, or at least, to assist in frightening, or perhaps in hunting, its prey. For myself, I confess, after the reception I had met with, I felt some degree of fear at too nearly approaching it."
Squirting Larva. The Cleft and squirting Organ are shown separately.
Squirting Larva. The Cleft and squirting Organ are shown separately.
On examining them carefully at another opportunity, this entomologist found that these caterpillars were provided with a squirting apparatus, situated in a cleft in the neck. When the creature is irritated, it thrusts out a curious organ, divided into four branches, and drilled with holes, like the rose of a watering-pot. From this organ shoot the jets of fluid spoken of. After the discharge they are drawn in again, and covered over by the closing up of the cleft. Strange to say, when taken in-doors, the caterpillar lost the power of squirting altogether. It appeared as if it could only do so in the open air, where a number of insect enemies were arrayed against it. This caterpillar is commonly known as that of the Puss-moth, and has sometimes caused the most strange and foolish country tales by its striking appearance. It has been described as a horrible monster, having a head like a lion, jaws like a shark, a horn like a unicorn, and two tremendous stings in its tail!Certainly, its appearance is not the most engaging in the world, as may be conceived from the representation of it in the cut; and what adds to its singularity is, that it possesses the power of lashing its sides with its tail, so as to drive away flies. If the reader should be fortunate enough to find one, he may have some amusement in examining all the curious apparatus with which this rather forbidding looking insect is supplied.
The winter draws near, and sends its foretokening cold breathings to warn every creature, vegetable and animal, that the dark season is arriving, when food is no longer to be obtained. What is to become of the larva? Can it endure the penetrating tooth of frost, or the overwhelming shower of rain? Or can it endure to be from week to week, from month to month, without a mouthful of food? From the facts already stated with reference to the age of larva, it is very certain that many larvæ have to bear these trials; and some, not merely during one, but during several winters. We must, therefore, inquire how they have been defended, or strengthened, so as to enable them so to do.
Shaggy-coated Larvæ.
The alteration in the coats of animals during winter, or rather, just previous to it, and the fact that animals destined to inhabit the arctic regions are enveloped in coats of the shaggiest hair, are well known to every person. There can be no doubt, that in many instances in the insect world, where a larva has to be exposed to the frosts of winter, the same provision of a hairy covering, which has been given to them by the Creator, is intended to meet the same end, and to preserve their soft and tender bodies from the destructive influence of extreme cold. The followingstriking observation of a talented entomologist, who has not thought it beneath him to write a most interesting book only on the family of ants (M. Huber), shows very clearly that such is, in fact, the intention of providing the larva with a covering of this kind. He noticed that the larvæ of some species of ants destined to pass through the winter, were furnished with this kind of warm clothing, while those which were not so destined had smooth coats, that is, even in the same species: thus proving to us, beyond a doubt, that as the winter approached, the warm coat was put on by the larvæ. The engraving on the last page represents some extraordinary specimens of hairiness in caterpillars from Brazil, now in the British Museum.
A large number of larvæ become torpid during winter—they are not dead—they are not sensibly alive—they are plunged in that long half-death which affects not only them but also many large animals, who retire in winter, lie down to sleep for weeks, and awake to find the cold departing, and the spring-time nigh at hand. But the frost sometimes proves too severe for them, and the poor insects become sometimes congealed into solid lumpsof ice! Alas! we shall exclaim, then there is an end to their life when that takes place; and, undoubtedly, the severity of such a degree of cold destroys large numbers; yet, strange to say, some larvæ actually revive, even after they have been thus frozen into hard inanimate lumps. One observer states that he has found them in this condition; and so entirely congealed, that they chinked like small stones when dropped into a glass tumbler! yet they revived after careful management. In an experiment made during Sir John Ross's voyage to the arctic regions, upon the larvæ of a moth, they were four successive times exposed to the intense cold of 40° below zero, and four times they revived again on being brought into the warm atmosphere of the cabin. Experiments have also been made by other persons with a like result. It may, therefore, be considered certain that some larvæ will bear to be frozen into solid masses, so that they will snap asunder like an icicle, and yet return to life again; and even go through all their stages of development, into the perfect insect. How remarkable a fact this is, when we remember the deadly effect of frost upon man and the higher animals!
Many larvæ, however, form nests of various kinds, in which they comfortably spend the dreary days and nights of this stern season. "With this view," write the excellent observers, Kirby and Spence, "the larva ofCossus ligniperdaforms a covering of pieces of wood, lined with fine silk; those of some other moths excavate, under a stone, a cavity exactly the size of their bodies, to which they give all round a coating of silk; and the larvæ ofPieris Cratægienclose themselves, in autumn, in cases of the same material, and thus pass the cold season, in small societies of from two to twelve, under a common covering formed of leaves. Bonnet mentions a trait of the cleanliness of these insects, which is almost ludicrous. He observed in one of these nests a sort of sack, containing nothing but grains of excrement; and a friend assured him that he had seen one of these caterpillars partly protrude itself out of the case, the hind feet first, to eject a similar grain; so that it would seem the society have on their establishment a scavenger, whose business it is to sweep the streets, and convey the rejected matters to one grand repository. This, however singular, is rendered not improbable, from the fact that beavers dig, intheir habitations, holes solely destined for a like purpose, as do also badgers."
A singular variety of larva-nest is mentioned by a Mexican traveller. He says,—"After having ascended for about an hour, we came to the region of oaks, and other majestically tall trees, the names of which I could not learn. Suspended from their stately branches were innumerable nests, enclosed, apparently, in white paper bags, in the manner of bunches of grapes in England, to preserve them from birds and flies. I had the curiosity to examine one of them, which I found to contain numberless caterpillars. The texture is so strong that it is not easily torn; and the interior contained a quantity of green leaves to support the numerous progeny within."
The larvæ of some Australian insects form the most curious nests we have ever beheld. A representation, taken from specimens in the entomological collection at the British Museum, will show what extraordinary structures these are. They appear like minute bundles of faggots, and have been formed by the insect cutting short pieces of twigs, all nearly of the same size, and then cementing them together in the manner represented.Some of these are of a large size, as large as a moderate-sized pear; others are smaller, and these present the most beautifully regular appearance, consisting of a number of very small rods, bound together in a cylindrical form, like thefascesof the Roman soldiery.
Australian Larva-cases.
In this manner larvæ pass through the winter. Without a doubt many perish; but there remain infinite numbers at the end of this season, which, from the depths of their cells, become sensible to the influence of spring, so soon as the first footsteps of that season are felt upon the earth.The larva wakens to active life once more; and with an appetite rendered all the keener by its fast of four or five months, it leaves its dormitory, and begins to consume whatever suitable food may lie in its way; and so it continues, until the time of its change approaches.
CHAPTER VI.
PREPARATION FOR A CHANGE.
The last stage of the larva's history hastens on. Its career of activity and voracity is nearly ended, and the insect's days in this form are well nigh spent. Hitherto it has been an earthly, earth-loving being, without either the desire or the power to raise itself into the airy element, in which multitudes of its species are disporting the day long. If we may so speak, its highest desires have only been confined to the quality and quantity of its food; possessing these, it was content. But now, its hours of feasting are come to a close, and it has to make ready for a great and momentous change. The crawling, and, perhaps, repulsive creature which we turn from without regret, has a high destiny to accomplish, when the period of its sojourn in its vile body is fully accomplished; and then, that form which now creeps among the dust of theworld, shall mount up as on eagles' wings, and be carried whither it will through the pure regions of the sky.
The age to which the larva generally attains before this change takes place is exceedingly variable. To some it is permitted to reach the venerable age of four or five years; others, three; and many, not nearly so much. The dung and fungus-feeding insects, as if hastened forward by the warmth of their position, are at the end of their larva life in a week; others live a fortnight in this state; and others, several months. We are told of a curious anecdote of a beetle which had existed in a desk, set up in an office in London, for upwards of twenty years, half of which period, according to Messrs. Kirby and Spence, it must have lived as a larva. But this is quite an exception to the general rule.
For some days before, the larva loses its long distinguishing characteristic—its appetite—and now forsakes the plants upon which it formerly fed; or if it does not forsake them, no longer consumes any part of them. It becomes, apparently, in some degree sensible that it is approaching the most dangerous period of the history ofthe insect—the state of sleep or torpor in which it is to exist as a pupa—of which we shall speak more particularly in our next chapter. Before, although a weak and comparatively a defenceless creature, it was still able to exercise a vigilant watch against its enemies, and thus to avoid them, or to prepare for their reception; but now, its eyes are to be closed in a death-like slumber; its body to hang a tempting bait to any wandering foe, or hungry bird, and it can in no way resist the attacks of the feeblest adversary.
As if fully aware of this, the wonderful instinct of the larva meets the difficulty by teaching it to select the best place for its concealment from a dangerous notice. With this view, it sets out in search of a hiding-place, and rests not until it has obtained a city of refuge which may prove to it a defence against the enemy. It may be some little recess, covered by an irregular stone, and sheltered round the sides by moss and the upspringing grass; or, perhaps, a little hole in the earth has received the poor fugitive from future harm, and in order to conceal itself more completely, it has drawn over the mouth of the cave a covering of leaves. Here,safe from observation, the larva bids a farewell—to speak as the poets speak—to the busy staring world for awhile, and loses all recollection of its past history, in the night of torpidity upon which it has now entered. The larvæ of the lady-bird, and other insects, glue themselves fast to the under side of a twig or leaf, and here pass the days of their sleep in safety. Many of the larvæ of insects whose life, up to this period, has been passed beneath the surface of some glassy pool, now quit the pure element, and seek the shore, on which they are to pass the remaining portion of their existence.
But there are two other ways in which larvæ prepare for the state ofpupa, which are so curious, and prevail to such a large extent among insects, that we may be excused for following the example of all authors upon insect history, and treating our readers to the interesting facts which have been recorded by various observers upon this subject. One of these is the plan of hanging themselves up in the air, like sailors in their hammocks; and in other positions, this plan is commonly called thesuspensionof the larva. The other is the formation of beautiful cases of various kinds,which are calledcocoons, in which they wrap themselves up close during the period to be devoted to the pupa state. Let us speak of these each separately; and, first, of the case, orcocoon, as a place of concealment for the larva. It is right, however, to premise, that, while the greater number of insects are in a torpid or half-torpid state while in the pupa form, all are not so, as will be noticed in the next chapter; and those which are not do not of course require the protection of a cocoon, or the contrivance of a silken hammock.
The material of which the case or cocoon is composed is a beautiful silken fibre, which is furnished by a spinning apparatus placed either at the extremity of the body, or more frequently at the mouth of the insect, whence it proceeds in a filament of the utmost delicacy and transparency, but at the same time of considerable tenacity. By its means alone cocoons, of a number of different kinds, some of extraordinary beauty and ingenuity, are constructed; but in other cases the insect has recourse to various materials to assist in giving strength to the walls of its cell.
Cocoon of Silk-worm.
The best example, and the most familiar one we can select of a purely silken cocoon, is that of those
"——spinning wormsThat in their green shops weave the smooth-haired silk."
"——spinning wormsThat in their green shops weave the smooth-haired silk."
This beautiful and valuable cocoon, when examined externally, appears to be formed in two different ways; for the outer covering is loose and open, while the inner one is compact and close, and so in truth it is, although both coverings are formed of a single thread. The outer has been called thescaffolding, by means of which the inner and more solid covering is constructed. The larva stationing itself between two leaves, or in any other suitable spot, fastens its thread to one of the adjoining surfaces, and then proceeds to draw out the fibre and fasten it in various positionsuntil it has woven around itself an envelope, as it were, of loose gauze. After this has been done it begins to form the closer and more compact structure of the inner envelope, laying on the silk backwards and forwards in a series of zig-zags, until it has laid on several layers, and thus completely shut itself in. The loose outer portion forms what ladies so well know under the title of flos-silk; the inner fibres are the most continuous, admitting of being wound off without breaking, and they form the ordinary silk of commerce.
The Larva surrounded by its Palisade.
M. Lyonnet has given a most interesting account of the proceedings of another larva which far surpasses the silk-worm in the ingenuity of its manner of forming the cocoon. This larva is very small; in fact, not more than the sixth of an inch in length, and is found on the under side of oak-leaves. M. Lyonnet observes, that beyond all the larvæ he had ever watched, he considered this creature the most inconceivably ingenious workman. Its little cocoon is of a white colour, and of a long oval form, having its upper surface ornamented with seven upright ridges. The first thing the creature begins by doing, is to erect, inthe most curious manner, a series of little upright palisades of silk at a certain distance from each other, all round its body, so as to form a sort of oval fence, and probably the larva intends them to serve as a sort of hedge to keep off intruders while it is busy at work! After spinning half its case, it turns round, and begins at the opposite end, and thus forms the other half; it then unites it in the middle, and by so doing incloses itself in a very pretty cage of silken net-work, which it subsequently strengthens by adding a firm layer on the inside. The time taken in the construction of this ingenious cocoon is not more than half-an-hour. It is quite impossible to understand how the little being contrives to form the upright ridges on the surface of its cocoon; and as the larva is so minute it is almost hopeless to expect to discover it. Attempts have been made to unravel the difficulty, but not with complete success. When, looking at some magnificent works of man, we are tempted to become inflated with pride,let the remembrance of what this tiny insect can do keep us humble, and direct our admiration rather to Him who is the Author of all wisdom, the Giver of every good and perfect gift.
Upon the leaves of pear trees we may sometimes find a pretty cocoon of very slight and elegant texture, formed of silk, as shown in the cut.