244 BACOT : MOSQUITOES AND THE DANGER OF MALARIA.
the action of their mouth-bristles, which function in a manner
similar to the cilia of a rotifer, setting up currents in the water
and thus sweeping small particles into their mouths. Although
the jaws of most species are apparently well adapted for rending
and tearing, their chief use seems to be to aid in the removal of
the film which forms on decaying organic matter.
The gut-contents of larva of the Culicina consists chiefly of
small fragments of organic matter—generally, in my experience,
of an indigestible nature; but there is good reason to think
that these are only the dross of the meal, the real meal being
bacteria, yeasts, diatoms, small protozoa, etc. As regards the
larvae of the yellow fever mosquito, Stegomyia fasciata, I have
definite proof that they thrive well on bacteria and yeasts alone,
The gross particles of matter which are swallowed do not come
into direct contact with the digestive cells of the stomach, as
they are confined within a delicate chitinous tube, similar to
that described by Miall as present in the larvae of Chironomus.
When dissecting the larvae, this tube, resembling a sausage,
can be removed from the alimentary canal, although without
due care it is easily ruptured.
I have had no experience in dissecting larvae of Anopheles,
so cannot say if this tube is present in them, but probably it is.
Larvae of the cannibal species, such as those of Megarhinus,
have heads and jaws specially modified for seizing and holding
their fellow mosquito-larvae while they suck their body juices.
Probably, in their case, the alimentary system is so far modified
as to be without the chitinous tube, for which, as they live
upon the body-juices of other larvae, there would be no par-
ticular need.
The length of the larval period of species inhabiting warm
climates seems to be mainly conditioned by food and temperature.
This is also the case with some of the species found in temper-
ate countries ; but, with others, these factors are not obviously
the controlling ones, ingrained instincts leading the larvae to
mark time in the autumn, apparently in order that the appearance
of the adult insects may coincide with the spring season. The
yellow fever mosquito may develop from egg to adult within a
week, or the larval period may be extended for seventy-five
days.
Larvae change their skins (or, as it is called, "moult")