juneio, 1918 Physical Properties Governing Contact Insecticides 535 



dead tissues, appeared at these points. These results explain why the 

 volatility of organic compounds should be related to their toxicity {12). 

 The more nearly the atmosphere is saturated with the vapor, the more 

 likelihood there is of a condensation in the tracheae. Even though there 

 is no condensation, the same forces are at work. One of these forces is 

 the tendency for the vapor to condense on coming in contact with the 

 chitinous walls of the tracheae. The other force is the tendency of the 

 compound to reevaporate from the tracheal walls. In the least volatile 

 this tendency to reevaporate is generally diminished, while in the more 

 volatile compounds, in order to reduce this tendency to reevaporate, very 

 large quantities of the chemical must be present in the air. It is thus 

 apparent that an insect may be killed with comparatively small doses 

 of slightly volatile compounds, while it may require a much heavier dose 

 of a more volatile material. The volatility of organic compounds is 

 therefore, in general, an index of their ability to penetrate into the body 

 of the insect, and inasmuch as the- compound which can not penetrate 

 will be unable to kill, it is apparent that the volatility is correlated with 

 the toxicity. One notable exception mentioned in a previous paper {12) 

 is that of chlorpicrin, which in very minute quantities is able to kill the 

 insect. In our experiments in tracing the penetration of fumigants it was 

 noticed that this material was able to p&netrate the walls of the tracheae 

 and kill the insect very quickly. This may be due to one of two factors: 

 First, the extreme toxicity of chlorpicrin, or second, an abnormal power 

 of penetration. 



The following experiment throws light on this question. Acetic acid 

 and benzene are of about the same volatility as chlorpicrin. On assuming, 

 therefore, for the sake of the experiment, that their powers of penetration 

 are equal, it was determined to test their comparative toxicity to insect 

 tissues. Three living wax-moth larvae were opened on the ventral side 

 and spread out. One was treated directly with acetic acid, the second 

 with benzene, and the third with chlorpicrin for a period of one-half 

 minute. The chemicals were then quickly removed, and the tissues were 

 washed with water and then treated with trypan blue. The larva treated 

 with acetic acid showed a very intense blue-staining throughout the 

 tissues. The larva treated with benzene showed but slight staining. The 

 larva treated with chlorpicrin showed more staining than benzene but 

 much less than that of acetic acid. Of the three compounds acetic acid 

 was by far the most poisonous to the tissues. In actual fumigation, dip- 

 ping, or spraying, the death of the insect occurs most quickly with 

 chlorpicrin. This material, therefore, must owe its abnormal toxicity to 

 its ability to be absorbed by the chitin and passed into the body. The 

 reason for this high power of penetration of chitin by chlorpicrin will be 

 the object of further study. 



