352 Report of the Department of Entomology. 



ing atmosphere. Thus the percentage of moisture may vary, which 

 might alter the rate of metabolism, but this could not seriously affect 

 the respiration of the caterpillars under discussion until they had 

 been in the changed atmosphere long enough to absorb perceptible 

 water, which would not occur in the interval of fumigation. The 

 caterpillars used in these experiments were in a hibernating condition 

 and hence probably low in body moisture. If the insect gained or lost 

 moisture, a part of this interchange would take place through the 

 walls of the tracheae. This might be very important in altering the 

 effect of the hydrocyanic acid gas, for the poison enters the system 

 mainly by dissolving in the moisture of the tracheal wall. It is pre- 

 sumed that the tracheal wall of an insect with a low-water content 

 would be in a dried condition as compared with that of a normal 

 insect. Surround such an individual with dry air and the inner sur- 

 face of the tracheal tubes would remain dry, a condition that would 

 not favor a rapid solution of the gas. However, surround the same 

 insect with a moist atmosphere and the dried wall would at once 

 begin to absorb water at its outer surface, which would facilitate the 

 formation of prussic acid. Thus a slight increase in the moisture of 

 the air might add much to the speed and effectiveness of the gas. 



Conclusion. — It appears from the above analysis that the unusual 

 resistance of these caterpillars to fumigation is due to a condition of 

 hibernation in which the moisture content of the body is low and the 

 insects are comparatively inactive. It is possible that the spiracles 

 are partially closed, which would also lessen the effect of the gas. 

 A rise in temperature is correlated with greater effectiveness, but 

 this increase in the deadly properties is more apparent between fifty 

 and seventy degrees than at lower temperatures. A dry air is not 

 favorable to fumigation with cyanide when the insects are in a hiber- 

 nating state, which suggests that the low moisture content which 

 usually accompanies this condition is one of the factors that help 

 modify the effect of the gas. 



