198 
TENTH REPORT. 
The conditions which produce the most efficient contact insecticide have 
been found to differ for oils even of the same origin. Of those derived from 
coal tar, two of them which differ in apparently only unimportant character- 
istics may differ greatly in their effects on insects, quantitatively if not 
qualitatively. 
To standardize such preparations therefore, requires the determining 
of not how much of a certain constituent there is present, for similar oils 
may counteract as well as dilute the effect of this ingredient; but to deter- 
mine whether the proper conditions have been obtained by noting its action 
on the insects themselves. And as it must be a method allowing of indefinite 
repetition, the number of variable factors must be reduced to a minimum 
The insect must be detached from its host; it must be possible to limit 
to a second the time it is exposed to the action of the solution and to remove 
the protecting film of air which prevents actual contact with the insecticide. 
These conditions have been secured by the use of tubes open at both ends, 
in which the insects may be placed and imprisoned by covering the ends 
with a porous cloth (India Mull). Then by using a hook which will fasten 
into the cloth, the tube with the insects may be quickly plunged into the 
solution. By rapid agitation, the protective air globules which surround 
the spiracles and prevent the drowning of insects can be removed so that 
intimate contact with the insecticide results. At the end of the period of 
time, usually one minute, during which time the insect is submerged, it is 
quickly removed with the hook and the clinging solution shaken off, the 
capillary action of the cloth tending to draw off all excessive moisture. 
It is then transferred to bibulous paper and covered with a clock glass, 
which allows sufficient air and easy observation. Different insects vary 
greatly in the rapidity with which they recover from the action of an insecti- 
cide too greatly diluted to be effective, and also in the degree of dilution 
necessary to kill. Different insecticides are peculiar in the fact that the 
immediate and ultimate effects on the same kind of insects are so different. 
In the course of these experiments numbers of the readily available insects 
have been used, including house flies, black ants, hog lice, sheep ticks, cattle 
ticks, dog fleas and bed bugs (Cimex lectularius). Each has its peculiar disad- 
vantage as a test insect, even aside from the natural difficulty of catching it 
and applying the material. Flies, ants and lice are short lived in captivity, 
the controls often dying as soon as or before those that were dipped; Southern 
cattle ticks have not been studied in all their stages of development; the 
large females under observation being extremely resistant to some very 
efficient insecticides: sheep ticks have the disadvantage of being very slow 
to recover from the effects of preparations which stupifv but do not kill; 
fleas are too lively to work with to any advantage; while bed bugs, after one 
becomes accustomed to the odor, and skillful in allowing no escapes, have 
some points of decided advantage. 
They can be kept in captivity for a considerable period of time, and be 
almost as resistant as when first captured; they are the most resistant of 
all the common insects; they recover promptly when the insecticide is too 
weak to be effective, and lastly, one need not depend on the household pest 
for supplies. Contrary to the accepted belief that this insect is found to 
any extent only in human habitations, the true Cimex Lectularius makes 
its home with, and preys upon, the guinea pig; and an unfailing supply may 
usually be found in the cracks and crevices of the pens. 
One of the difficulties which would naturally suggest itself in the attempt 
to standardize a series of insecticides by the method outlined is the variation 
