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, le