THE CADDIS-WORM
[The caddis-worm is the grub of the caddis-fly, which is like a small
moth and is often seen flitting over our streams and ponds. There are
about one hundred and fifty species of this fly in America.]
Whom shall I lodge in my glass trough, kept always wholesome by the
action of the water-weeds? I shall keep Caddis-worms, those insects
which clothe themselves with little sticks and other materials. They
are among the most ingenious of the self-clothing insects.
The particular species of Caddis-worm I have chosen is found in
muddy-bottomed, stagnant pools crammed with small reeds. It is the
little grub that carries through the still waters a bundle of tiny
fragments fallen from the reeds. Its sheath, a traveling house, is an
elaborate piece of work, made of many different materials.
The young worms, the beginners, start with a sort of deep basket in
wicker-work, made of small, stiff roots, long steeped and peeled under
water. The grub that has made a find of these fibers saws them with
its jaws and cuts them into little straight sticks, which it fixes
one by one to the edge of its basket, always crosswise. This pile of
spikes is a fine protection, but hard to steer through the tangle of
water-plants. Sooner or later the worm forsakes it, and builds 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.
It does not always use wood, however. If there are plenty of small,
dead Pond-snails in the pond, all of the same size, the Caddis-worm
makes 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 like baskets. With grains
of rice, which I gave the grubs in my glass pond as an experiment, they
built themselves magnificent towers of ivory. Next to the sheaths of
snail-shells, this was the prettiest thing I ever saw the Caddis-worms
make.
THE PIRATES’ ATTACK
What is the use of these houses which the Caddis-worms carry about
with them? I catch a glimpse of the reason for making them. My glass
pond was at first occupied by a dozen Water-beetles, whose diving
performances are so curious to watch. One day, meaning no harm and for
want of a better place to put them, I fling among them a couple of
handfuls of Caddis-worms. Blunderer that I am, what have I done! The
pirate Water-beetles, hiding in the rugged corners of the rockwork,
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 carpenter Caddis-worms. Each Beetle grabs a sheath by the middle
and tries to rip it open by tearing off shells and sticks. While this
is going on, the Caddis-worm, close-pressed, appears at the mouth
of the sheath, slips out, and quickly escapes under the eyes of the
Water-beetle, who appears to notice nothing.
The brutal ripper of sheaths does not see the little worm, like a 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.
[Illustration]
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 rockwork. If things were happening in a larger, outdoor pond, it
is clear that, with their clever way of removing themselves, most of
the worms 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 foiled all over
again by the very same trick!
AN INSECT SUBMARINE
Caddis-worms are able to remain on the level of the water indefinitely
with no other support than their house; they can rest in unsinkable
flotillas and can even shift their place by working the rudder.
How do they do it? Do their sticks make a sort of raft? Can the shells
contain a few bubbles of air and serve as floats? Let us see.
I remove a number of Caddis-worms from their sheaths and put the
sheaths in the water. Not one of them floats, neither those made of
shells nor those of woody materials. The Worm also, when removed from
its tube, is unable to float.
This is how the Worm manages. When at rest, at the bottom of the pond,
it fills the whole of the tube of its sheath. When it wishes to reach
the top of the pond, it climbs up the reeds, dragging its house of
sticks with it; then it sticks the front of its body out of the sheath,
leaving a vacant space in the rear, like the vacuum in a pump when one
draws out the piston. This promptly fills with air, enabling the Worm
to float, sheath and all, just as the air in a life-preserver holds a
person up in the water. The Caddis-worm does not need to cling to the
grasses any longer. It can move about on the surface of the pond, in
the glad sunlight.
To be sure, it is not very talented as a boatman. But it can turn
round, tack about and shift its place slightly by using the front part
of its body, which is out of the tube, as a rudder and paddle; and that
is all it wishes to do. When it has had enough of the sun, and thinks
it time to return to the quiet of the mud-bed at the bottom, it draws
itself back into its sheath, expelling the air, and at once begins to
sink.
We have our submarines—the Caddis-worms have theirs. They can come out
of the water, they can dip down and even stop at mid-depth by releasing
gradually the surplus air. And this apparatus, so perfectly balanced,
so skillful, requires no knowledge on the part of its maker. It comes
into being of itself, in accordance with the plans of the universal
harmony of things.
[Illustration]