June lo, 1918 Physical Properties Gouerning Contact Insecticides 533 



TabliS III. — Penetration of materials as indicated by Sudan III , with trypan-hlue 



method — Continued 



ESSENTIAL OILS 



FIXED OILS AND THEIR ACIDS 



LUBRICATING OILS 



Lubricating oil 6. 

 Lubricating oil i . 



5 to 20 hours. 

 ....do 



Slight blue stain along red-stained tra- 

 chese. 

 Do. 



The insecticides may be divided into four different groups: (i) The 

 nonvolatile insecticides which as liquids penetrate the tracheae. (2) The 

 volatile insecticides which are able to penetrate the tracheae as liquids. 

 (3) The volatile insecticides which only penetrate the tracheae in vapor 

 form. (4) The nonvolatile insecticides which decompose on contact 

 with the insect, producing a vapor which is capable of entering the 

 tracheae. The results in Table III show that relatively nonvolatile oils 

 pass through the walls of the tracheae only where the liquid has pene- 

 trated. 



Soap solutions are particularly interesting, since when a soap is dis- 

 solved in water it is hydrolyzed, some of the soap molecules reacting 

 with the water to form sodium hydrate and the free fatty acid. This 

 free fatty acid unites with the fatty acid of that portion of the soap 

 which has not been hydrolyzed, forming a sol {10). Since the chitin is 

 a semipermeable membrane, the sol is able to pass through but very 

 slowly into the body of the insect. The sodium hydrate in the solu- 

 tion is no doubt the portion which accounts for death. Experimental 



