drawn again with the blood if the mosquito is
allowed to feed long enough. There may be some truth in this, but for
most of us a bite means a hurt anyway and few will be content to sit
perfectly still and watch the little pest gradually fill up on blood.

It is not known just what the action of the saliva is, its composition
or reaction on the tissues. It is generally supposed to prevent
coagulation of the blood that is to be drawn through the narrow tube of
the labrum. Others think that its presence causes a greater flow of
blood to the wound. But the sad part of it is, for us at least, that it
hurts and may cause malaria and possibly other diseases.

HOW MOSQUITOES BREATHE

Mosquitoes and other insects do not have any nostrils nor do they
breathe through any openings on the head. Along the sides of the thorax
and abdomen is a series of very minute openings known as the spiracles.
Through these the air passes into a system of air-tubes, the tracheae.
There are two main trunks or divisions of the tracheae just inside the
body-wall and a number of shorter connecting trunks. From these larger
vessels arise a great number of smaller ones which branch and subdivide
again and again until all the tissues are supplied by these minute
little air-tubes that carry the oxygen to all parts of the body and
carry off the waste carbon dioxid. These air-tubes are emptied and
filled by the contractions of the walls of the abdomen. When the
body-wall contracts the air is forced out of the thin-walled trachea
through the spiracles; when the pressure is removed they are refilled by
the fresh air rushing in.

THE BLOOD

After a mosquito has been feeding on a man or some other animal it is
often so distended that the blood shows rich and red through the thin
sides of the walls of the abdomen. This, however, is the blood of the
victim and not of the mosquito. The blood of insects is not red but pale
yellowish or greenish. It is not confined in definite vessels, but fills
all the space inside the body cavity that is not occupied by some of the
tissues or organs. It bathes the walls of the alimentary canal and
gathers there the nourishment which it carries to all parts of the body.
It does not carry oxygen or collect the carbon dioxid as does the blood
of higher animals. That work, as we have just seen, is done by the
air-tubes. Above the alimentary canal, extending almost the whole length
of the abdomen and thorax, is a thin-walled pu