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 him which forms on decaying organic matter. 
The gut-contents of larvae of the Cnlicince 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 ”) 
