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JOURNAL OF ECONOMIC ENTOMOLOGY 
[Vol. 11 
tissues of the larva would cause death by actual contact. Undoubt¬ 
edly this factor of toxicity of the oils would suffice amply for the killing 
of the larvae if sufficient time were allowed for it to act, but when a set 
of larvae were treated with different oils, stained with Sudan III, in 
each case, except with the petrolatum and the crude oil, the larvae were 
dead from an hour to two hours before the oils had penetrated the 
tissue as shown by the presence of stain in the microscopical examina¬ 
tion. The oil enters and spreads throughout the tracheal system with 
great rapidity, but the larvae, in our experience, are always dead long 
before the oil penetrates the tissue. 
Shafer (1911) showed that in terrestrial insects treated with kerosene, 
penetration of the tissue required between three and twelve hours and 
concluded that death from treatment with kerosene resulted long before 
the liquid, as such, had time to penetrate the body. 
Our next experiment gave further basis for this conclusion. 
(6) The Oil Vapors as the Toxic Agents 
Sen in 1914 reported a single experiment in which a few drops of 
kerosene were applied to a cotton plug in a bottle of water containing 
larvae. He does not positively announce their early death as being 
directly caused by the vapor, but intimates that it is possible. 
McFie (1917) repeated this experiment with different species of 
larvae but remarks that its “ action (the vapor) is less constant and 
much slower than that produced by a film of oil.” It seems only rea¬ 
sonable to assume that the results would be much slower in this type 
of exposure than with a film of oil for with the film method the oil and 
its attendant vapors are brought in undiluted and direct contact with 
the respiratory tract, but when the oil vapors pass off from the sat¬ 
urated cotton in the vapor experiment they reach the larvae in an 
indirect manner, highly diluted with air. 
Since with our series of oils the lethal effects of the various grades 
corresponded very closely with the curve of their boiling points, we 
were led to believe that the volatility of the oils was the true index for 
their larvicidal action. With this point in view, we subjected our group 
of oils to the following experiments. 
We placed samples of the series in small straight sided open pans 
and exposed them to a temperature of 28° C.±2°, in a constant circu¬ 
lation of air with the result that at the eftd of 103 hours the oils had 
evaporated to the following extent: 
Kerosene. 98.8% 
Eng. distillate. 85.2% 
High grade stove distillate... 35.4% 
Low grade stove distillate. 10.6% 
Residuum. 6.5% 
Crude. .9% 
