19 
SUMMARY AND CONCLUSIONS. 
Formaldehyd gas is a feeble insecticide. Mosquitoes may live in a 
very weak atmosphere of the gas overnight. It will kill them, how- 
ever, if it is brought in direct contact in the strength and time pre- 
scribed for bacterial disinfection. For this j)urpose any of the accepted 
methods for evolving the gas is ai)plicable, but the methods which lib- 
erate a large volume in a short time are more certain than the slower 
ones. 
Direct contact between the insects and the gas is much more difficult 
to obtain in ordinary room disinfection against mosquitoes than against 
germs, because the sense of self-protection helj^s the former to escape 
from the effects of the irritating gas. They hide in the folds of towels, 
bedding, clothing, hangings, fabrics, and out-of-the-way places where 
the formaldehyd gas does not penetrate in sufficient strength to kill 
them. The gas is polymerized and deposited as paraform in the meshes 
of fabrics, which prevents its penetration, and large quantities are lost 
by being absorbed by the organic matter of fabrics, especially woolens. 
In our tests, whenever the insects were given favorable hiding places, 
such as in crumpled paper or in toweling, they quickly took advantage 
of the best place for themselves and thus escaped destruction. 
There is a striking analogy between the strength of the gas and the 
time of exposure necessary to penetrate the fabrics in order to kill 
mosquitoes, and the strength and time necessary to penetrate in order 
to kill the spores of bacteria. 
Mosquitoes have a lively instinct in finding cracks or chinks where 
fresh air may be entering the room, or where the gas is so diluted that 
they escape destruction. They are able to escape through incredibly 
small openings. , Some of the smaller varieties, such as the stegomyia 
fasciata can get through a wire screen having 12 meshes to the inch. 
Therefore, formaldehyd gas can not be trusted to kill all the mosquitoes 
in a room which can not be tightly sealed. 
It is concluded, that to succeed in killing all the mosquitoes in a closed 
space with formaldehyd gas, the following definite requirements are 
essential : A very large volume of the gas must be liberated quickly, 
so that it may diffuse to all portions of the space in sufficient concentra- 
tion. The room must have all the cracks and chinks where the insects 
may breathe the fresh air carefully sealed by pasting strips of paper 
over them. The room must not contain heavy folds of drapery, cloth- 
ing, bedding, or fabrics in heaps, or so disposed that the insects may 
hide away from the full effects of the gas. 
Sulphur dioxid is unexcelled iis an insectide. Very dilute atmos- 
pheres of the gas will quickly kill nios((uitoes. It is quite as efficacious 
for this purpose when dry jis when moist, whereas the dry gas has prac- 
tically no power against bacteria. Contrary to formaldehyd it has sur- 
