534 Journal of Agricultural Research voi. xiii, no. n 



evidence shows that solutions of soaps containing a large percentage of 

 free alkali are more toxic than those which are practically neutral. 



Volatile oils and acids show a blue-staining along the walls of the 

 trachea where the liquid has penetrated, and also blue blotches along 

 the walls beyond this point. Examination of insects which have been 

 treated for varying lengths of time show that the blue blotches caused 

 by the vapor of the chemical appear more quickly than the blue caused 

 by the penetration of the liquid itself, showing that the vapor is able to 

 penetrate the walls of the tracheae more quickly than the liquid. It 

 might be noted here that these results are somewhat modified by the 

 fact that the vapor is passing through somewhat thinner chitin than that 

 through which the liquid must pass. 



Aqueous solutions such as nicotine do not penetrate the tracheae other 

 than in the form of a vapor. The blue in this case occurs in blotches 

 along the walls of the tracheae. What was evidently a condensation of 

 the vapor appeared in the larger tracheae and the blue staining of the tis- 

 sues was particularly strong at these points. This confirms the observa- 

 tions of Mclndoo (8), who has shown by chemical means the condensa- 

 tion of nicotine within the tracheal tubes. When nicotine sulphate comes 

 in contact with the body of the insect, it is slowly decomposed, with the 

 formation of nicotine which enters as a vapor. Such a decomposition is 

 no doubt the explanation of the results of Lovett (6), who found that 

 leaves sprayed with nicotine sulphate even when dry are repellent and 

 poisonous to insect larvae, even though not taken internally. 



To such aqueous solutions, which normally are not able to enter the 

 tracheae, the addition of soap increases their efficiency, as it enables the 

 liquid to spread over the body and enter the spiracles. The addition of 

 too much soap will somewhat decrease the efficiency of the spray, owing 

 to increased cohesion. Some of the results obtained in the use of nico- 

 tine sulphate and fish-oil-soap sprays by Smith (17) are thus explained. 



In general there was little evidence of penetration of the insecticides 

 through the body wall. This does not necessarily mean that the com- 

 pounds are not able to penetrate the chitin, but that they were unable 

 to do so prior to their entrance by way of the tracheae. Compounds 

 which are readily soluble in water and readily diffuse in aqueous solu- 

 tions were frequently found to have gained entrance through the anus 

 and through the mouth. Alcohol, acetic acid, and sometimes soap 

 showed such penetration. The more viscous compounds require very 

 much longer to enter the insect than the compounds with a low viscosity. 

 Lime-sulphur differs from other contact insecticides in its action, but 

 its action has been fully described by Shafer (16). 



PENETRATION OF FUMIGANTS 



As will be noted in Table III, fumigants gain entrance into the insect 

 by way of the tracheae. In many cases what appeared as a condensation 

 was noticed within the tracheae and in such cases blue staining, indicating 



