It must be remembered that two negatives in the English language destroy each other and are equivalent to an affirmative. Thus "I don't know nothing about it" is intended to convey, that I am ignorant of the matter under consideration, but i... Read more of DOUBLE NEGATIVE at Speaking Writing.comInformational Site Network Informational
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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

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

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

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

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

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

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

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

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.


Previous: THE POND

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