INSECTS AFFECTING HEALTH OF MAN OR ANIMALS. 15 
effective, while under some circumstances a much broader one may 
be required. Often a small amount of ditching will effectively drain 
a relatively large breeding area. 
Crude oil, not too heavy to spread evenly upon the water surface, 
will be found the most effective under service conditions. The object 
is the formation of a fairly heavy film, as nearly as possible unbroken, 
over the entire area treated. The larve and pupe, unable to reach 
the ai>, are soon smothered. 
If it is found that the oil available is of too heavy a quality to spread 
readily, the addition of kerosene or a 
little caustic soda will usually render it 
“lighter.” A miscible larvicide, of which 
there are several upon the market, has 
advantages for some situations, as it 
| permeates the body of water to which 
) it is applied and does not drift with the 
_ wind, a fault the crude oil possesses. A 
_ knapsack spray pump is the best means 
of applying the oil, but in its absence 
_ an ordinary garden watering pot with 
_ spraying nozzle is good, while a pail and 
tin cup will be useful in an emergency. 
In the matter of receptacles of water 
containing the larve of house or yellow- 
fever mosquitoes, the simple expedient 
of emptying the contents is sufficient, 
but care should be taken to see that 
they are thoroughly drained, or even 
rinsed if of the nature of water barrels 
or water jars which are to be refilled, as 
larvee will often be held in the drops of 
water clinging to the sides. Old tins 
and bottles, etc., should be buried, or at 
least made incapable of holding water. Fig. 14.—Larva of the yellow-fever mos- 
pWater barrels, tanks, etc., should. be — quito in resting position. Much en- 
effectively screened against mosquitoes, ‘@"8°d- Howard.) 
this screening to be protected by heavy wire netting. A wooden 
_ spigot may be placed near the bottom of the former to render unnec- 
essary the constant removal of the screen. 
In a dry or semi-arid region, in which water is often scarce and 
valuable, the contents of infested water barrels may be strained 
through cloth and replaced. When for the same reason the contents 
of breeding pools is required for various uses, no remedy can be sug- 
gested, and protection of human beings by nets, etc., must be resorted 
To exclude the yellow-fever mosquito, wire scr eoning must have 
at least 18 meshes to the inch. 7 
Wy 
