th poisonous gases, which are got rid of
on the return of the blood to the gills. The letter S in this figure
marks the stump of the leg, which, for the sake of clearness, has been
cut off.

In ourselves, the work of breathing, or of purifying the blood, is done
by means of the lungs. The lungs are large, spongy organs in the chest,
and are continually supplied with fresh air, which passes in through the
nose and mouth and down the wind-pipe, by what we call the act of
breathing.

Insects take in oxygen in a way quite different from that of the
crayfish or mankind. In some larval insects, which live in water, as in
some worms, the body is so thin that no special breathing organs are
necessary; others breathe by means of gills like those of the crayfish,
but arranged differently--sometimes along each side, and sometimes at
the tail end of the body. But in the ordinary adult insect the work of
breathing is carried on by means of a system of tubes, known as
'tracheae,' which run all over the body. Into these tubes the air is
drawn through a number of holes on the surface of the body, called
'spiracles,' or breathing pores. The tracheae or tubes are everywhere
bathed by the blood, which is thus constantly 'aerated,' or kept fresh.

One very remarkable thing about these tubes is the way they are kept
open. A horny, spirally-twisted thread runs through them, and thus they
are prevented from closing up by pressure, or by the bending of the body
or limbs. In fig. 2, this thread is marked C. This plan of keeping open
the passage in a tube likely to be blocked by sudden bending, has been
imitated by mankind, in making rubber gas tubing, for example. As a
plain rubber tube is easily bent, the gas would be in constant danger of
being cut off. To prevent this, Nature's patent is usually imitated, and
a coil of wire is placed along the inside of the tube. Thus, a sharp
bend, such as would instantly obstruct the passage of the gas, is
prevented.

The openings at the end of the breathing tubes, on the surface of the
insect's body, are known, as we have said, as 'spiracles,' or
'stigmata.' They can be closed at will by special muscles, and, to
prevent dust from getting into the tube, the rim of each spiracle has a
more or less complicated fringe or strainer. In fig. 3 the spiracle is
shown open, the opening being marked by the letter O. When closed the
fringes interlock like clasped fingers.

Fig. 1 shows the position of the breat