Chapter VIII. The Caddis Worm
Jean-Henri Fabre The Life of the Fly
Caddis Fly Larva
Whom shall I lodge in my glass trough, kept permanently wholesome by the action of the water weeds? I shall keep caddis worms, those expert dressers. Few of the self-clothing insects surpass them in ingenious attire. The ponds in my neighborhood supply me with five or six species, each possessing an art of its own. Today, but one of these shall receive historical honors.
I obtain it from the muddy bottomed, stagnant pools crammed with small reeds. As far as one can judge from the habitation merely, it should be, according to the specialists, Limnophilus flavicornis, whose work has earned for the whole corporation the pretty name of Phryganea, a Greek term meaning a bit of wood, a stick. In a no less expressive fashion, the Provencal peasant calls it lou portofais, lou porto-caneu. This is the little grub that carries through the still waters a faggot of tiny fragments fallen from the reeds.
Its sheath, a travelling house, is a composite and barbaric piece of work, a megalithic pile wherein art, retires in favor of amorphous strength. The materials are many and sundry, so much so that we might imagine that we had the work of dissimilar builders before our eyes, if frequent transitions did not tell us the contrary.
With the young ones, the novices, it starts with a sort of deep basket in rustic wicker-work. The twigs employed present nearly always the same characteristics and are none other than bits of small, stiff roots, long steeped and peeled under water. The grub that has made a find of these fibers saws them with its mandibles and cuts them into little straight sticks, which it fixes one by one to the edge of its basket, always crosswise, perpendicular to the axis of the work.
Picture a circle surrounded by a bristling mass of tangents, or rather a polygon with its sides extended in all directions. On this assemblage of straight lines we place repeated layers of others, without troubling about similarity of position, thus obtaining a sort of ragged fascine, whose sticks project on every side. Such is the bastion of the child grub, an excellent system of defense, with its continuous pile of spikes, but difficult to steer through the tangle of aquatic plants.
Sooner or later, the worm forsakes this kind of caltrop which catches on to everything. It was a basket maker, it now turns carpenter; it builds with little beams and joists — that is to say, with round bits of wood, browned by the water, often as wide as a thick straw and a finger's-breadth long, more or less — taking them as chance supplies them.
For the rest, there is something of everything in this rag bag: bits of stubble, fag ends of rushes, scraps of plants, fragments of some tiny twig or other, chips of wood, shreds of bark, largish grains, especially the seeds of the yellow iris, which were red when they fell from their capsules and are now black as jet.
The heterogeneous collection is piled up anyhow. Some pieces are fixed lengthwise, others across, others aslant. There are angles in this direction and angles in the other, resulting in sharp little turns and twists; the big is mixed with the little, the correct rubs shoulders with the shapeless. It is not an edifice, it is a frenzied conglomeration. Sometimes, a fine disorder is an effect of art. This is not so here: the work of the Caddis worm is not a masterpiece worth signing.
And this mad heaping up follows straight upon the regular basket work of the start. The young grub's fascine did not lack a certain elegance, with its dainty laths, all stacked crosswise, methodically; and, lo and behold, the builder, grown larger, more experienced and, one would think, more skilful, abandons the orderly plan to adopt another which is wild and incoherent! There is no transition stage between the two systems. The extravagant pile rises abruptly from the original basket. But that we often find the two kinds of work placed one above the other, we would not dare ascribe to them a common origin. The fact of their being joined together is the only thing that makes them one, in spite of the incongruity.
But the two storeys do not last indefinitely. When the worm has grown slightly and is housed to its satisfaction in a heap of joists, it abandons the basket of its childhood, which has become too narrow and is now a troublesome burden. It cuts through its sheath, lops off and lets go the stern, the original work. When moving to a higher and roomier flat, it understands how to lighten its portable house by breaking off a part of it. All that remains is the upper floor, which is enlarged at the aperture, as and when required, by the same architecture of disordered beams.
Side by side with these cases, which are mere ugly faggots, we find others just as often of exquisite beauty and composed entirely of tiny shells. Do they come from the same workshop? It takes very convincing proofs to make us believe this. Here is order with its charm, there disorder with its hideousness; on the one hand a dainty mosaic of shells, on the other a clumsy heap of sticks. And yet it is all produced by the same laborer.
Proofs abound. On some case which offends the eye with the want of arrangement in its bits of wood, patches are apt to appear which are quite regular and made of shells; in the same way, it is not unusual to see a horrid tangle of joists braced to a masterpiece of shell work. One feels a certain annoyance at seeing the pretty sheath so barbarously spoilt.
This mixed construction tells us that the rustic stacker of wooden beams excels, when occasion offers, in making elegant shell pavements and that it practices rough carpentry and delicate mosaic work indifferently. In the latter instance, the scabbard is made, above all, of Planorbes, selected among the smaller of these pond snails and laid flat. Without being scrupulously regular, the work, at its best, does not lack merit. The pretty, close-whorled spirals, placed one against the other on the same level, have a very pleasing general effect. No pilgrim returning from Santiago de Compostella ever slung handsomer tippet from his shoulders.
But only too often the caddis worm dashes ahead, regardless of proportion. The big is joined to the small, the exaggerated suddenly stands out, to the great detriment of order. Side by side with tiny Planorbes, each at most the size of a lentil, others are fixed as large as one's fingernail; and these cannot possibly be fitted in correctly. They overlap the regular parts and spoil their finish.
To crown the disorder, the caddis worm adds to the flat spirals any dead shell that comes handy, without distinction of species, provided it be not excessively large. I notice, in its collection of bric-a-brac, the Physa, the Paludina, the Limnaea, the Amber snail (1) and even the Pisidium (2), that little twin-valved casket.
Land shells, swept into the ditches by the rains after the inmate's death, are accepted quite as readily. In the work made of the Mollusk's cast-off clothing, I find encrusted the spindle shell of the Clausilium, the key shell of the pupa, the spiral of the smaller Helix, the yawning volute of the Vitrina, or glass snail, the turret shell of the Bulimus (3), denizens all of the fields. In short, the caddis worm builds with more or less everything that comes from the plant or the dead mollusk. Among the diversified refuse of the pond, the only materials rejected are those of a gravelly nature. Stone and pebble are excluded from the building with a care that is very rarely absent. This is a question of hydrostatics to which we will return presently. For the moment, let us try to follow the construction of the scabbard.
In a tumbler small enough to allow of easy and precise observation, I install three or four caddis worms, extracted this moment from their sheaths with every possible precaution. After a number of attempts which have at last shown me the right road, I place at their disposal two kinds of materials, possessing opposite qualities; the supple and the firm, the soft and the hard. On the one hand, we have a live aquatic plant, such as watercress, for instance, or ombrelle d'eau, having at its base a tufty bunch of fine white roots about as thick as a horsehair. In these soft tresses, the caddis worm, which observes a vegetarian diet, will find at one and the same time the wherewithal to build and eat. On the other hand, we have a little faggot of bits of wood, very dry, equal in length and each possessing the thickness of a good sized pin. The two sorts of building material lie side by side, mingling their threads and sticks. The animal can make its choice from the lump.
A few hours later, having recovered from the shock of losing its sheath, the caddis worm sets to work to manufacture a new one. It settles across a bunch of tangled rootlets, which are brought together by the builder's legs and more or less arranged by the undulating movement of the hinder part. This gives a kind of incoherent and ill defined suspended belt, a narrow hammock with a number of loose catches; for the various bits of which it is made up are respected by the teeth and extended from place to place beyond the main cords of the roots. Here, without much trouble, is the support, suitably fixed by natural moorings. A few threads of silk, casually distributed, make the frail combination a trifle more secure.
And now to the work of building. Supported by the suspended belt, the caddis worm stretches itself and thrusts out its middle legs, which, being longer than the others, are the grapnels intended to seize things at a distance. It meets a bit of root, fastens on to it, climbs above the point gripped, as though it were measuring the piece to a requisite length, and then, with the fine scissors of its mandibles, cuts the string.
There is at once a brief recoil, which brings the animal back to the level of the hammock. The bit detached lies across the worm's chest, held in its forelegs, which turn it, twist it, wave it about, lay it down, lift it up, as though trying for the best position. Those forelegs make admirably dexterous arms. Being less long than the other two pairs, they are brought into immediate contact with those primordial implements, the mandibles and the spinneret. Their delicate terminal jointing, with a movable and crooked finger, is the caddis worm's equivalent of our hand. They are the working legs. The second pair, which are exceptionally long, serve to spear distant materials and to give the worker a firm footing when measuring a piece and cutting it with the pliers. Lastly, the hind legs, of medium length, afford a support when the others are busy.
The caddis worm, I was saying, with the piece which it has removed held crosswise to its chest, retreats a little way along its suspended hammock until the spinneret is level with the support furnished by the close tangle of rootlets. With a quick movement, it shifts its burden, gets it as nearly by the middle as it can, so that the two ends stick out equally on either side, and chooses the spot to place it, whereupon the spinneret sets to work at once, while the little fore legs hold the scrap of root motionless in its transversal position. The soldering is effected with a touch of silk in the middle of the bit and along a certain distance to the right and left, as far as the bending of the head permits.
Without delay, other sticks are speared in like manner at a distance, cut off and placed in position. As the immediate neighborhood is stripped, the material is gathered at a yet greater distance and the caddis worm bends even farther from its support, which now holds only its last few segments. It is a curious gymnastic display, that of this soft, hanging spine turning and swaying, while the grapnels feel in every direction for a thread.
All this labor results in a sort of casing of little white cords. The work lacks firmness and regularity. Nevertheless, judging by the builder's methods, I can see that the building would not be devoid of merit if the materials gave it a better chance. The caddis worm estimates the size of its pieces very fairly; it cuts them all to nearly the same length; it always arranges them crosswise on the margin of the case; it fixes them by the middle.
Nor is this all: the manner of working helps the general arrangement considerably. When the bricklayer is building the narrow shaft of a factory chimney, he stands in the center of his turret and turns round and round while gradually laying new rows. The caddis worm acts in the same way. It twists round in its sheath; it adopts without inconvenience whatever position it pleases, so as to bring its spinneret full face with the point to be gummed. There is no straining of the neck to left or right, no throwing back of the head to reach points behind. The animal has constantly before it, within the exact range of its implements, the place at which the bit is to be fixed. When the piece is soldered, the worm turns a little aside, to a length equal to that of the last soldering, and here, along an extent which hardly ever varies, an extent determined by the swing which its head is able to give, it fixes the next piece.
These several conditions ought to result in a geometrically ordered dwelling, having a regular polygon as an opening. Then how comes it that the cylinder of bits of root is so confused, so clumsily fashioned? The reason is this: the worker possesses talent, but the materials do not lend themselves to accurate work. The rootlets supply stumps of very uneven shape and thickness. They include big and small ones, straight and bent, simple and ramified. To combine all these dissimilar pieces into an orderly whole is hardly possible, all the more so as the caddis worm does not appear to attach very much importance to its cylinder, which is a temporary work, hurriedly constructed to afford a speedy shelter. Matters are urgent; and very soft fibers, clipped with a bite of the mandibles, are more quickly gathered and more easily put together than joists, which require the patient work of the saw. The inaccurate cylinder, in short, held in position by numerous guy ropes, is a base upon which a solid and definite structure will rise before long. Soon, the original work will crumble to ruins and disappear, whereas the new one, a permanent structure, will even outlast the owner.
The insects reared in a tumbler show yet another method of building the first dwelling. This time, the caddis worm is given a few very leafy stalks of pond weed (Potamogeton densum) and a bundle of small dry twigs. It perches on a leaf, which the nippers of the mandibles cut half across. The portion left untouched will act as a lanyard and give the necessary steadiness to the early operations.
From an adjoining leaf a section is cut out entirely, an angular and good sized piece. There is plenty of material and no need for economy. The piece is soldered with silk to the strip which was not wholly cut off. The result of three or four similar operations is to surround the Caddis worm with a conical bag, whose wide mouth is scalloped with pointed and very irregular notches. The work of the nippers continues; fresh pieces are fixed, from one to another, inside the funnel, not far from the edge, so that the bag lengthens, tapers and ends by wrapping the animal in a light and floating drapery.
Thus clad for the time being, either in the fine silk of the pond weed or in the linsey-woolsey supplied by the roots of the watercress, the caddis worm begins to think of building a more solid sheath. The present casing will serve as a foundation for the stronger building. But the necessary materials are seldom near at hand: you have to go and fetch them, you have to move your position, an effort which has been avoided until now. With this object, the caddis worm cuts its moorings, that is to say, the rootlets which keep the cylinder fixed, or else the half-severed leaf of pond weed on which the cone-shaped bag has come into being.
The worm is now free. The smallness of the artificial pond, the tumbler, soon brings it into touch with what it is seeking. This is a little faggot of dry twigs, which I have selected of equal length and of slight thickness. Displaying greater care than it did when treating the slender roots, the carpenter measures out the requisite length on the joist. The distance to which it has to extend its body in order to reach the point where the break will be made tells it pretty accurately what length of stick it wants.
The piece is patiently sawn off with the mandibles; it is next taken in the fore legs and held crosswise below the neck. The backward movement which brings the caddis worm home also brings the bit of twig to the edge of the tube. Thereupon, the methods employed in working with the scraps of root are renewed in precisely the same manner. The sticks are scaffolded to the regulation height, all alike in length, amply soldered in the middle and free at either end.
With the picked materials provided, the carpenter has turned out a work of some elegance. The joists are all arranged crosswise, because this way is the handiest for carrying the sticks and putting them in position; they are fixed by the middle, because the two arms that hold the stick while the spinneret does its work require an equal grasp on either side; each soldering covers a length which is seen to be practically invariable, because it is equal to the width described by the head in bending first to this side and then to that when the silk is emitted; the whole assumes a polygonal shape, not far removed from a rectilinear pentagon, because, between laying one piece and the next, the caddis worm turns by the width of an arc corresponding with the length of a soldering. The regularity of the method produces the regularity of the work; but it is essential, of course, that the materials should lend themselves to precise coordination.
In its natural pond, the caddis worm does not often have at its disposal the picked joists which I give it in the tumbler. It comes across something of everything; and that something of everything it employs as it finds it. Bits of wood, large seeds, empty shells, stubble stalks, shapeless fragments are used in the building for better or for worse, just as they occur, without being trimmed by the saw; and this jumble, the result of chance, results in a shockingly faulty structure.
The caddis worm does not forget its talents; but it lacks choice pieces. Give it a proper timber yard and it at once reverts to correct architecture, of which it carries the plans within itself. With small, dead pond snails, all of the same size, it fashions a splendid patchwork scabbard; with a cluster of slender roots, reduced by rotting to their stiff, straight, woody axis, it manufactures pretty specimens of wicker work which could serve as models to our basket makers.
Let us watch it at work when it is unable to use its favorite joist. There is no point in giving it clumsy building stones; that would only bring us back to the uncouth sheaths. Its propensity to make use of soaked seeds, those of the iris, for instance, suggests that I might try grains. I select rice, which, because of its hardness, will be tantamount to wood and, because of its clean whiteness and its oval shape, will lend itself to artistic masonry.
Obviously, my denuded caddis worms cannot start their work with bricks of this kind. Where would they fix their first layer? They must have a foundation, quick and easy to build. This is once more supplied by a temporary cylinder of watercress roots. On this support follow the grains of rice, which, grouped one atop the other, straight or slanting, end by giving a magnificent turret of ivory. Next to the sheaths made of tiny snail shells, this is the prettiest thing with which the caddis worm's industry has furnished me. A fine sense of order has returned, because the materials, regular and of identical character, have cooperated with the correct method of the worker.
The two demonstrations are enough. Sticks and grains of rice make it plain that the caddis worm is not the bungler that one would expect from the monstrous buildings in the pond. Those Cyclopean piles, those mad conglomerations, are the inevitable results of chance finds, which are used for the best because there is no choice. The water carpenter has an art of its own, has method and rules of symmetry. When well served by fortune, it is quite able to turn out good work; when ill-served, it acts like others: the work which it turns out is bad. Poverty makes for ugliness.
There is another matter wherein the caddis worm deserves our attention. With a perseverance which repeated trials do not tire, it makes itself a new tube when I strip it. This is opposed to the habits of the generality of insects, which do not recommence the thing once done, but simply continue it according to the usual rules, taking no account of the ruined or vanished portions. The caddis worm is a striking exception: it starts again. Whence does it derive this capacity?
I begin by learning that, given a sudden alarm, it readily leaves its scabbard. When I go fishing for caddis worms, I put them in tin boxes, containing no other moisture than that wherewith my catches are soaked. I heap them up loosely, to avoid any grievous tumult and to fill the space at my disposal as best I may. I take no further precaution. This is enough to keep the caddis worms in good condition during the two or three hours which I devote to fishing and to walking home.
On my return, I find that a number of them have left their houses. They are swarming naked among the empty scabbards and those still occupied by their inhabitants. It is a pitiful sight to see these evicted ones dragging their bare abdomens and their frail respiratory threads over the bristling sticks. There is no great harm done, however; and I empty the whole lot into the glass pond.
Not one resumes possession of an unoccupied sheath. Perhaps it would take them too long to find one of the exact size. They think it better to abandon the old clouts and to manufacture cases new from top to bottom. The process is a rapid one. By the next day, with the materials wherein the glass trough abounds — bundles of twigs and tufts of watercress — all the denuded worms have made themselves at least a temporary home in the form of a tube of rootlets.
The lack of water, combined with the excitement of the crowding in the boxes, has upset my captives greatly; and, scenting a grave peril, they have made off hurriedly, doffing the cumbersome jacket, which is difficult to carry. They have stripped themselves so as to flee with greater ease. The alarm cannot have been due to me: there are not many simpletons like myself who are interested in the affairs of the pond; and the caddis worm has not been cautioned against their tricks. The sudden desertion of the crib has certainly some other reason than man's molestations.
I catch a glimpse of this reason, the real one. The glass pond was originally occupied by a dozen Dytisci, or water beetles, whose diving performances are so curious to watch. One day, meaning no harm and for want of a better receptacle, I fling among them a couple of handfuls of caddis worms. Blunderer that I am, what have I done! The corsairs, hiding in the rugged corners of the rock work, at once perceive the windfall. They rise to the surface with great strokes of their oars; they hasten and fling themselves upon the crowd of carpenters. Each pirate grabs a sheath by the middle and strives to rip it open by tearing off shells and sticks. While this ferocious enucleation continues with the object of reaching the dainty morsel contained within, the caddis worm, close pressed, appears at the mouth of the sheath, slips out and quickly decamps under the eyes of the Dytiscus, who appears to notice nothing.
I have said before that the trade of killing can dispense with intelligence. The brutal ripper of sheaths does not see the little white sausage that slips between his legs, passes under his fangs and madly flees. He continues to tear away the outer case and to tug at the silken lining. When the breach is made, he is quite crestfallen at not finding what he expected.
Poor fool! Your victim went out under your nose and you never saw it. The worm has sunk to the bottom and taken refuge in the mysteries of the rock work. If things were happening in the large expanse of a pond, it is clear that, with their system of expeditious removals, most of the lodgers would escape scot-free. Fleeing to a distance and recovering from the sharp alarm, they would build themselves a new scabbard and all would be over until the next attack, which would be baffled afresh by the selfsame trick.
In my narrow trough, things take a more tragic turn. When the sheaths are done for, when the caddis worms that are too slow in making off have been eaten up, the Water beetles return to the rockery at the bottom. Here, sooner or later, there are lamentable happenings. The naked fugitives are discovered and, succulent morsels that they are, are forthwith torn to pieces and devoured. Within twenty-four hours, not one of my band of caddis worms is left alive. In order to continue my studies, I had to lodge the water beetles elsewhere.
Under natural conditions, the caddis worm has its persecutors, the most formidable of whom appears to be the Water beetle. When we consider that, to thwart the brigand's attacks, it has invented the idea of quitting its scabbard with all speed, its tactics are certainly most appropriate; but, in that case, an exceptional condition becomes obligatory, namely, the capacity for recommencing the work. This most unusual gift of recommencing it possesses in a high measure. I am ready to see its origin in the persecutions of the Dytiscus and other pirates. Necessity is the mother of industry.
Certain caddis worms, of the Sericostoma and Leptocerus species, clothe themselves in grains of sand and do not leave the bed of the stream. On a clear bottom, swept by the current, they walk about from one bank of verdure to the other and do not think of coming to the surface to float and sail in the sunlight. The collectors of sticks and shells are more highly privileged. They can remain on the level of the water indefinitely, with no other support than their skiff, can rest in unsubmersible flotillas and can even shift their place by working the rudder.
To what do they owe this privilege? Are we to look upon the bundle of sticks as a sort of raft whose density is less than that of the water? Can the shells, which are always empty and able to contain a few bubbles of air in their spiral, be floats? Can the big joists, which break in so ugly a fashion the none too great regularity of the work, serve to buoy up the over-heavy raft? In short, is the caddis worm versed in the laws of equilibrium and does it choose its pieces, now lighter and now heavier as the case may be, so as to constitute a whole that is capable of floating? The following facts are a refutation of any such hydrostatic calculations in the animal.
I remove a number of caddis worms from their sheaths and submit these, as they are, to the test of water. Whether formed wholly of fibrous remnants or of mixed materials, not one of them floats. The scabbards made of shells go to the bottom with the swiftness of a bit of gravel; the others sink gently. I experiment with the separate materials one by one. No shell remains on the surface, not even among the Planorbes, which a many-whorled spiral ought, one would think, to keep afloat. The fibrous remnants must be divided into two categories. The first, darkened by time and soaked with moisture, sink to the bottom. These are the most plentiful. The second, considerably fewer in number, of more recent date and less saturated with water, float very well. The general result is immersion, as in the case of the intact scabbards. I may add that the animal, when removed from its tube, is also unable to float.
Then how does the caddis worm manage to remain on the surface without the support of the grasses, considering that itself and its sheath are both heavier than water? Its secret is soon revealed. I place a few high and dry on a sheet of blotting paper, which will absorb the excess of liquid unfavorable to successful observation. Outside its natural environment, the animal moves about violently and restlessly. With its body half out of the scabbard, this time composed entirely of fibrous matter, it clutches with its feet at the supporting plane. Then, contracting itself, it draws the scabbard towards it, half-raising it and sometimes even making it assume a vertical position. Even so do the Bulimi move along, lifting their shell as they complete each crawling step.
After a couple of minutes in the free air, I replace the caddis worm in the water. This time, it floats, but like a cylinder with too much weight below. The sheath remains vertical, with its hinder orifice level with the water. Soon, an air bubble escapes from the orifice. Deprived of this buoy, the skiff at once goes down.
The result is the same with the caddis worms in shell casings. At first, they float, straight up on end, and then dip under and sink, faster than the others, after sending out an air bubble or two through the back window.
That is enough: the secret is out. When cased in wood or in shells, the caddis worms, which are always heavier than water, are able to keep on the surface by means of a temporary air balloon which decreases the density of the whole structure.
This apparatus works in the simplest manner. Consider the rear of the sheath. It is truncated, wide open and supplied with a membranous partition, the work of the spinneret. A round hole occupies the center of this screen. Beyond it lies the interior of the scabbard, which is smoothly lined and wadded with satin, however rough the exterior may be. Armed at the stern with two hooks which bite into the silky lining, the animal is able to move backwards and forwards at will inside the cylinder, to fix its grapnels at whatever point it pleases and thus to keep a hold on the cylinder while the six legs and the fore part are outside.
When at rest, the body remains indoors entirely and the grub occupies the whole of the tube. But let it contract ever so little towards the front, or, better still, let it stick out a part of its body: a vacuum is formed behind this sort of piston, which may be compared with that of a pump. Thanks to the rear window, a valve without a plug, this vacuum at once fills, thus renewing the aerated water around the gills, a soft fleece of hairs distributed over the back and belly.
The piston stroke affects only the work of breathing; it does not alter the density, makes hardly any change in that which is heavier than water. To lighten the weight, the caddis worm must first rise to the surface. With this object, it scales the grasses of one support after the other; it clambers up, sticking to its purpose in spite of the drawback of its faggot dragging through the tangle. When it has reached the goal, it lifts the rear end a little above the water and gives a stroke of the piston. The vacuum thus obtained fills with air. That is enough: skiff and boatman are in a position to float. The now useless support of the grasses is abandoned. The time has come for evolutions on the surface, in the glad sunlight.
The caddis worm possesses no great talent as a navigator. To turn round, to tack about, to shift its place slightly by a backward movement is all that it can do; and even that it does very clumsily. The front part of the body, sticking out of the case, acts as a rudder. Three or four times over, it rises abruptly, bends, comes down again and strikes the water. These paddle strokes, repeated at intervals, carry the unskilled oarsman to fresh latitudes. It becomes a voyage on the right seas when the crossing measures a hand's breadth.
However, tacking on the surface of the water affords the caddis worm no pleasure. It prefers to twitter in one spot, to remain stationary in flotillas. When the time comes to return to the quiet of the mud bed at the bottom, the animal, having had enough of the sun, draws itself wholly into its sheath again and, with a piston stroke, expels the air from the back room. The normal density is restored and it sinks slowly to the bottom.
We see, therefore, that the caddis worm has not to trouble about hydrostatics when building its scabbard. In spite of the incongruity of its work, in which the bulky and less dense portions seem to balance the more solid, concentrated part, it is not called upon to contrive an equipoise between the light and the heavy. It has other artifices whereby to rise to the surface, to float and to dive down again. The ascent is made by the ladder of the water weeds. The average density of the sheath is of no importance, so long as the burden to be dragged is not beyond the animal's strength. Besides, the weight of the load is greatly reduced when moved in the water.
The admission of a bubble of air into the back chamber, which the animal ceases to occupy, allow it, without further to-do, to remain for an indefinite period on the surface. To dive down again, the caddis worm has only to retreat entirely into its sheath. The air is driven out; and the canoe, resuming its mean density, a greater specific density than that of water, goes under at once and descends of its own accord.
There is, therefore, no choice of materials on the builder's part, no nice calculation of equilibrium, save for one condition, that no stony matter be admitted. That apart, everything serves, large and small, joist and shell, seed and billet. Built up at haphazard, all these things make an impregnable wall. One point alone is essential: the weight of the whole must slightly exceed that of the water displaced; if not, there could be no steadiness at the bottom of the pond, without a perpetual anchorage struggling against the pull of the water. In the same manner, quick submersion would be impossible at times when the surface became dangerous and the frightened creature wanted to leave it.
Nor does this important heavier-than-water question call for lucid discernment, seeing that almost the whole of the sheath is constructed at the bottom of the pond, whither all the materials picked up at random, having descended once before, are likely to descend again. In the sheaths, the parts capable of floating are very rare. Without taking their specific levity into account, simply so as not to remain idle, the caddis worm fixed them to its bundle when sporting on the surface of the water.
We have our submarines, in which hydraulic ingenuity displays its highest resources. The caddis worms have theirs, which emerge, float on the surface, dip down and even stop at mid-depth by releasing gradually their surplus air. And this apparatus, so perfectly balanced, so skilful, requires no knowledge on the part of its constructor. It comes into being of itself, in accordance with the plans of the universal harmony of things.
1. All pond snails.
2. A bivalve.
3. All land snails.