The Book of the Aquarium/Part 3/Chapter 4
CHAPTER IV.
LARVA.
The great class of insects comprises many remarkable and diverse forms, among the 560,000 species which Dr. Imhoff estimates to be now known to naturalists. Yet, various as they are, it is by no means impossible to define what are the distinctive features by which this class is separated from those which approach it in conformation and habits. A true insect has the body divided into three parts—the head, the thorax, and the abdomen. It has never more than six legs, and these are attached to the thorax. The segments of the body seldom exceed thirteen in number, one of which forms the head, three the thorax, and the remaining nine the abdomen. The possession of one or two pairs of wings gives them their prominent characteristic to the eye, but it is the successive metamorphoses that arrests universal attention, and calls forth the admiration and wonder of mankind. In the progress of an insect from the minute egg to its completed form, we see the most remarkable series of developments which animal life ever displays in all its endless procession of forms—the egg, the worm, the chrysalis, the fly—a strange unfolding, for the first time accurately observed by Swammerdam, who detected, under the wrinkled skin of the disgusting worm, the complete outline of the lovely butterfly.
This metempsychosis may be studied in its several strange details by the aid of the Water-Cabinet. The first condition of the newly-hatched egg is that known as the grub, or caterpillar—scientifically called the larva. The larva generally bears no earthly resemblance to the imago, or perfect insect, into which it is to be hereafter developed, but leads a life of sensual enjoyment—it eats, eats! it is gluttony concentrated in type and act. It changes its skin several times, slips one coat off and acquires a new; growing, and eating, and changing garments, till, like man himself, it seeks a temporary tomb, from which it is to soar to the skies like a soul liberated. This second form is popularly known as the chrysalis, or aurelia, scientifically called the pupa. In this form the insect remains in a state of complete or partial torpidity for a few days, weeks, or months, according to the particular species.
The day of its deliverance arrives, its bonds burst, and it comes forth “ a thing of beauty ” to sport in sunbeams, and, for but a brief season, lead a life of joy—
“Fluttering round the jasmine stems,
Like winged flowers or flying gems.”
But beauty of the high poetic kind is not the inheritance of every member of the class insecta; and the water-cabinet presents us with many that have but analogical resemblances to the typical structure of the moth or the fly, though the naturalist finds beauty in a beetle, and points of profound interest in a maggot or grub.
Since larva are distributed through at least three elements, being, according to the species, inhabitants of earth, air, and water, the breathing apparatus arrests our attention, as constituting a distinct feature in the anatomy of the insect. A caterpillar may be regarded as all stomach, and the cravings of this immense digesting tube easily account for the voracity of larva of all kinds. In the larger animals, the food is elaborated into blood, and brought to the lungs to be oxygenated by means of contact with the air, but the insect does not breathe at the mouth, but at the other end, or by means of tubes arranged along the sides of the body. In a caterpillar there are usually eighteen of these tubes, the orifices of which may be seen in action. These tubes all run into two larger lateral tubes, or wind-pipes, arranged one on each side of the body; and from these lateral tubes innumerable smaller ones diverge, and convey air to the vessels in which the digested food is contained, and thus supply it with oxygen. Swammerdam was the first who successfully anatomised this class of animals, and to him we are indebted for the microscopic anatomy of grubs, and a revelation of their inner economy generally.
The Dragon Fly, Libellulidæ; (Leach), a well-known summer beauty—the mere mention of which is always sufficient to set one’s heart beating for rustic coolness, and the hushed music of the beechen shades—is, in its larva form, an interesting object for the cabinet. Between the larva and the imago of this insect, the difference is striking indeed; as a lady, not addicted to scientific studies, once characterised the larva—using Pope’s lines—as,
“A monster of such hideous mien,
That to be hated needs but to be seen.”
But the gauze-winged and gaily-coloured fly merits all the praise bestowed upon it by the French, who call them Demoiselles, so light, fairy-like, and visionary are its form and movements. Scientific writers have applied many descriptive names to it, such as Calepteryx (pretty wing), Puella (girl), Sponsa (bride), and Virgo (virgin). The larva of the dragon fly exhibits, in a very striking manner, the mode of respiration in aquatic insects. It is not an active creature; for though it has six legs, it seldom uses these except in the capture of prey; its locomotion is chiefly performed by the tail in the action of breathing. When thrown into a jar with some fragments of weed and a few light chips, these will be seen to be drawn towards the tail of the creature, by the current occasioned by the absorption of water; and then again driven off, with considerable force, as the water is again ejected. The quantity drawn into the body by this hydrostatic action must be considerable, since the dimensions of the larva regularly change with the breathing action, the body becoming collapsed when the stream is ejected, and again swelled out with the suction that follows. If it be thrown into water, tinged with cochineal, and then quickly removed again into clear water, the coloured stream will be seen to be projected several inches, and with force sufficient to propel the creature forward by a series of successive jerks.
Besides the act of breathing, then, this anal pump has locomotive uses; and it also aids the creature in obtaining food by drawing minute creatures towards it in a manner similar to those animals which are furnished with cilia.
But the microscope reveals a still more curious fact, in the anatomy of this larva, which has been most faithfully described by Kirby and Spence. The under lip, when closed, entirely conceals the mouth, and it not only retains, but actually seizes, the animal’s prey, by means of a very singular pair of jaws with which it is furnished. Conceive your under lip (to have recourse, like Reaumur, on another occasion, to such a comparison) to be horny instead of fleshy, and to be elongated perpendicularly downward, so as to wrap over your chin, and extend to its bottom—that this elongation is there expanded into a triangular convex plate, attached to it by a joint, so as to bend upwards again and fold over the face as high as the nose, concealing not only the chin and the first mentioned elongations, but the mouth and part of the cheeks. Conceive, moreover, that to the end of this last-mentioned plate are fixed two other convex ones, so broad as to cover the whole nose and temples—that these can open at pleasure, transversely, like a pair of jaws, so as to expose the nose and mouth, and that their inner edges, where they meet, are cut into numerous sharp teeth, or spines, or armed with one or more long sharp claws—you will have as accurate an idea, as my powers of description can give, of the strange formation of the under lip in the larva of Libellulina, which conceals the mouth and face, precisely as I have supposed a similar construction of lip would do yours. You will probably admit that your own visage would present an appearance not very engaging, while concealed by such a mask; but it would strike still more awe into the spectators, were they to see you first open the two upper jaw plates, which would project from the temples like the blinders of a horse; and next, having, by means of the joint of your chin, let down the whole apparatus and uncovered your face, employ them in seizing any food that presented itself, and conveying it to your mouth. Yet this procedure is that adopted by the larvæ of the dragon-fly, provided with this strange organ. While it is at rest, it applies close to and covers the face. When the insects would make use of it, they unfold it like an arm, catch the prey at which they aim, by means of the mandibuliform plates, and then partly refold it, so as to hold the prey to the mouth, in a convenient position for the operation of the two pairs of jaws with which they are provided. The form of this masked jaw is represented, but not very clearly, in Rennies’ “Insect Transformations,” p. 164.
When its season of larva life is over, it retires to the bottom of the vessel to repose, and becomes a pupa. When the crumpled form of the gaudy fly begins to
TRANSFORMATION OF THE DRAGON FLY.
1. The Fly just emerging. 2. The Fly nearly free, and forming an arch. 3. The Fly
liberated, and with its body bent, to hasten the drying of the wings.
expand within, and to knock at the door of its sepulchre, the pupa quits the watery element for ever, and betakes itself to the dry land, or to the slip of cork placed in the jar for its use. There, the apparently painful process of its unfolding takes place, and the fly slowly emerges. The envelope bursts asunder, and the head of the lovely, but blood-thirsty damsel, emerges to the light. Next appear the legs, not in action, but gathered up to the breast, as if in spasm, and, for a time, the effort is suspended, and the helpless, half-formed beauty hangs back her head, as if languid with the exhaustion of pain. Once more she pants for freedom, sighing to sun herself in the blue ether, and another struggle is made. This time she clutches at the pupa case with a convulsive grasp, and drags forth the whole of her delicate body from the grave, and there remains motionless, still clinging to it, as if contemplating the baseness of her origin—for beauty is ever the offspring of the dust. She is free at last—but, ah! how helpless. Her wings are damp, and closely folded, and would not yield to the wish for flight, even were she already possessed of the power to stir them into action. She is on the threshold of a new world—a creature born of the dust, just escaped from the dust; and now as we watch her wings dry and expand, away she goes—a thing of light and loveliness soaring heavenward. Like the mortal ark, out of which the spirit of man escapes, we may, without losing sight of the disparity of the subjects, speak of the chrysalis of the dragon fly as—
“A worn-out fetter, which the soul
Has broken, and thrown away.”
The Gnat—Culex pipiens (Linnæus)
In the operation of dragging, many curious larva are brought out, and the mud should be searched carefully for them before washing the net for another cast. The larva
DRAGON FLIES.
a. Virgin Dragon Fly—Calepteryx Virgo.
b. Green Dragon Fly—Æshna varia.
of the gnat is one of the most interesting of these, and during summer may be obtained in hundreds if a little of the brook water be dipped to fill the jar with, and a few light weeds thrown in to supply oxygen. These larva are the produce of eggs deposited in a curious manner.
The gnats repair to the water soon after day-break, and commence an operation of a truly naval kind, such as would have delighted the savage heart of Peter the Great, could he have witnessed it in the midst of his dreams of achieving naval power. In fact, the mother gnats construct rafts of eggs, and each egg is added as a separate timber of the vessel, till a boat-like structure is produced. The skill as well as the necessity of the construction is well tested by the fact that each separate egg would of itself sink to the bottom, whereas being protruded one by one into the angle formed by the hind legs which serve as stocks for the future vessel, and successively glued to each other by the fluid which exudes with them, they gradually assume, under her guidance, a neat boat-like form of about three hundred minute pyramidal eggs.
“The most violent agitation,” says Kirby, “cannot sink it, and what is more extraordinary, and a property still a disideratum in our life-boats, though hollow it never becomes filled with water, even though exposed.”
The grubs at last come forth, and lead a very merry sort of life under the shadow of the sedges. Placed in the jars they appear at first sight like newly hatched fry of fishes, but we soon detect the segments of their pellucid bodies, and, as might be expected in water larva, they breathe at the wrong end, and hence most of their merry movements are performed between the surface and the bottom; every time they descend they carry with them a minute bubble from the surface. Under a good lens the pretty creature changes its form considerably, and comes out in the pantomime style, with huge horns, goggle eyes, and starched frills of shaggy hair; but then the tail becomes the object of attraction, and we watch the breathing action of the curious funnel which breaks away at an angle from the last segment of the body.
LARVA OF THE GNAT, NATURAL SIZE AND MAGNIFIED.
Swammerdam first observed that the breathing tube and tail are both anointed with oil, and that if the larva is handled roughly the oil is removed, and the grub “can no longer suspend itself on the surface of the water. I have, on these occasions, observed it put its tail in its mouth, and afterwards draw it back, as a water fowl will draw its feathers through its bill to prepare them for resisting water.” I have now (July, 1856) some thousands of the larva of this and other species of gnats, and they are the most lively creatures in my collection. The flies come off in large numbers, and escape through the open window; or, if the window be closed, they swarm on the glass, and keep up a musical humming, closely resembling that of a swarm of bees at a distance.
A more elegant example of this kind of breathing apparatus is seen in the grub of the two-winged fly,
LARVA OF STRATIOMYS.
Stratiomys Chamæleon. The funnel tail spreads into a beautiful star of thirty distinct rays, and with this structure, the creature suspends itself to the surface of the water, as if it were a ceiling, and as it moves to and fro its changes of position may be noted by the shifting of the dimple on the surface.
In the little gnat, Corethra plumicornis (Meigen) we have some further examples of the peculiar conformation of the larva, to enable it to respire in the water; but the larva is so transparent that it requires an expert microscopist, and an effective instrument to work out the details as represented by Reaumur and Dr. Goring. The larva of this species is plentiful in our brooks, and worthy of close scrutiny by the aid of the microscope. Just now the gnats are abundant in meadows near streams, and to them we are indebted for that soft humming which has been appropriately termed the “music of the wild,” and on which good old Gilbert White exhausted his ingenuity to find an explanation. The social communities of these ephemeral creatures are strictly music parties, and whenever we suffer them to assemble about our heads, when rambling in the hedgerows, we are entertained by their fairy-like performances. Expertum est.
The Case Ply Phryganea grandis.—There are several species in the family of Phryganea, which is the only tribe in the order Trichoptera, but such strong resemblances exist between the several members of the family that some entomological experience is necessary to enable the student to distinguish them. In an aquarium, the caddis worms are very amusing, and since they thrive there, they are very suitable additions to the happy family. We see them busy at the bottom, adding fragments of weed, pebbles, minute shells, even if the snails within them are alive, and any small debris that their fingers can seize hold of. Last season I had amongst a large number of cads, one that had his case nearly destroyed by accidentally falling from the table. I removed from him what remained of his case, and threw him into a jar with a soldier plant and a few Lymnea. He set to work to repair his tabernacle, and the Lymnea helped him, for they nibbled a plant of Stratoides into shreds. These shreds the cad gathered, and every day he added a fresh piece, so that, in about ten days, he appeared in a suit of green, his clothes bulged out to an enormous size, and everywhere studded with points and corners, the most comical sight that could be imagined. Since he could find nothing of a small neat pattern, he took what he could, and became a perfect Jack in the green, nearly an inch and a half in length, and thicker than a carpenter’s lead pencil.
The movements of these creatures are as comical as their specimens of tailoring. We see them mounting a stem or leaf with great gravity, when suddenly up goes the tail, the legs hold tight, and the case turns completely over, as if on the first of May, Jack-in-the-green were to dance on his head. When the creature is hidden, and the case sways to and fro like a buoy attached by too short a rope, the sight is very curious. This case-maker is the larva from a fly which bears resemblances to the two families which stand on either side of it—the Lepidoptera, or true butterflies, and the Neuroptera, of which the dragon flies and other membranaceous winged insects are members. As soon as he enters the world, he begins to show his skill in tailoring, and by means of silken threads and gluten constructs his case of bits of stick, straw, dead leaves, or shells, in fact, whatever he can get, and as long as he retains the worm-like form the case is his castle, and he can defy the world. The case outside is generally a rough affair, but if you draw out the cad you
LARVÆ AND IMAGO OF CASE FLY.
will see that inside it is perfectly cylindrical, smooth, and polished, and around the doorway, through which the larva makes acquaintance with the world, it is neatly finished with a very circular rim. When you have removed a cad, if you throw him into a tank you will learn in an instant what is the use of his case, for his soft nakedness is no sooner exposed, than the minnows finish him, and find the flavour excellent. But to see a cad in his proper uniform molested is a very rare sight indeed. He passes his larva life innocently, and is an amusing fellow; when he feels the numbness of death creeping over him, the cad draws in his six legs, and sets to work inside to weave a winding-sheet and to shut the shutters, for he knows that his time is come, and there is no one to do such melancholy offices for him. All alone in his solitary cell, the hermit works day and night, and hourly his fingers grow more feeble. We look and find the shutters closed, and by this time the larva has changed into a pupa.
GRATING OF CASE-WORM, MAGNIFIED.
The mode in which the worm closes its cell is curious enough. Over its entrance it weaves a grating of silk, which hardens in water and remains insoluble. It may be seen very plainly by the naked eye, but under a good lens increases in interest. The grating is placed a little inside the margin of the opening, and fits exactly within it, and its object is to protect the pupa from invasion, and at the same time, to admit water for respiration. De Geer describes one of these gratings in which the pierced holes were disposed in concentric circles, as represented in the engraving. This, however, is not, as far as I am aware, the usual form of the grating, many that I have examined were formed in regular rays from a centre like the spokes of a wheel.
But the escape of the pupa, when about to undergo its last metamorphosis, is as interesting as the fact of its closing the shutters to announce its own death. It is provided width a pair of hooked mandibles, with which to gnaw through the grating, and no sooner have these accomplished their purpose than they fall off, and the pupa takes its last shape of a four winged fly, as represented in the cut.