THE CADDIS WORM

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 bot omed, 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 [all pond snails] and even the Pisidium [a bivalve], 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 [all land snails], 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.