disappear."

[Illustration: FIG. 79.--Top view is of larva of Anopheles. Bottom view
is of larva of Culex.]

Another factor in the development of the mosquito from the egg to
full-grown mosquitohood is that in the larvae stage air must be supplied,
curiously enough, through the tail which projects slightly above the
surface of the water as the larvae hang head downwards (see Fig. 79). If
the surface of the water is covered with some impervious material, the
mosquito larvae will be suffocated, and it has been found that oil lends
itself most readily to this desirable purpose, applied at the rate of
one ounce per fifteen square feet of water surface. The oil spreads out
over the surface in a very thin film, but persistent enough to keep off
the air supply from the mosquito larvae. This method, about which much
has been written and said, is perhaps the one most commonly employed,
and its results have been most satisfactory. In the vicinity of the city
of Newark, New Jersey, for instance, is an area of about 3500 acres, 8
miles long and about 3 miles wide, practically all marshland. In 1903
ditches were dug throughout this marsh in such a way that the surface
water was drained off, drying the ground so that hay can now be cut
where formerly rubber boots were necessary to get onto the ground at
all. The consequence has been that the mosquitoes have practically
disappeared from this region, formerly frightfully infested, and the
cost of the 70 miles of small ditches dug has been amply repaid by the
freedom from malaria as well as from the nuisance of the ordinary
mosquito.

Other campaigns have been waged, using kerosene or crude petroleum for
the coating of ponds or pools. Wherever clear water exists the kerosene
treatment is probably best. Where marshland is found, through which the
kerosene penetrates with difficulty, drainage is a more useful method.

The size of the pools required for the development of the mosquito is
very small. Thousands of mosquitoes may be formed in the amount of water
contained in an old tomato can, and barrels half full of rain water or
pools of water in the vicinity of an old pump or in the barnyard will
afford golden opportunities for mosquitoes looking for a place to lay
their eggs. While the ordinary culex requires from one to two weeks only
for the complete transition from egg to mosquito, so that a pool filled
with rain water and not dried up within that period will be sufficient
to deve