" THE CATTLE GRUBS OR OX WARBLES 93 
of the larve zn situ. In such cases the observations were continued | 
for several weeks to determine the effect on the host of the destruction 
of the grubs by this method. In order to make accurate observa- 
tions on the killing power of the various materials used, most of 
these were administered after the major part of the grubs had 
reached the fifth or last stage. In most cases a sufficient number of 
erubs were in the younger stages so that the effect of the various 
materials upon them could be determined. The tests indicated that, 
although the younger stages may be more susceptible to the action 
Fic. 35.—Injecting Hypoderma cysts with chemicals by means of an oil can 
of the insecticides, in those stages they are probably more protected 
against the materials, since the apertures in the skin are usually 
much smaller. 
Most of the tests in which the results were carefully checked are 
given in Tables 11 and 12. It will be observed from a study of the 
results that a number of the materials will produce a mortality above 
80 per cent. The writers are of the opinion that materials which 
will not accomplish the destruction of at least 80 per cent of the grubs 
present should not be considered as practicable. On the other hand, 
some of these materials which gave a very high percentage of de- 
struction are not adapted to use in general practice on account of the 
cost. 
