June lo. 1918 Physical Properties Governing Contact Insecticides 525 



nal friction of a liquid." Although the viscosity does not influence the 

 ultimate extent to which a liquid may spread, it does have a very decided 

 influence on the rate of spread. It is possible that a liquid may be 

 so viscous that the rate of spread may be reduced to such an extent as 

 to make it valueless as an insecticide. 



From the foregoing statements it is apparent that when a spray strikes 

 an insect, if there is a chemical affinity between the insecticide and the 

 chitin which forms the outer covering of the insect, wetting will take 

 place and the insecticide will adhere. If the cohesion of the spray is 

 less than the adhesion between the chitin and the spray, then the liquid 

 will spread over the body of the insect. The rate at which this spread- 

 ing will take place is governed by the viscosity of the liquid. If the co- 

 hesion is greater than adhesion, the spray will form into droplets which 

 tend to roll off. The same result is obtained when the spray does not 

 wet the insect. It is apparent therefore that in a contact insecticide it 

 is important not only that the liquid should wet the chitin, but also that 

 the adhesion of the liquid to the chitin should be greater than the cohe- 

 sion of the liquid. 



RELATION BETWEEN SPREADING AND CAPILLARITY 



The rise of a liquid in a capillary tube is governed by the same laws as 

 the spreading of a liquid over the surface of a solid. First, unless the 

 liquid has a specific attraction for the material of which the tube is com- 

 posed (wetting), there can be no capillary rise. Second, unless the 

 adhesion between the liquid and the walls of the tube is greater than the 

 cohesion of the liquid, there can be no capillary rise. This is well shown 

 by Bigelow and Hunter's (i) studies of the rise of water in capillary tubes 

 of difi'erent materials. In their experiments the cohesion of water 

 remained the same, but owing to different degrees of adhesion between 

 the water and the walls of the capillary tubes, variations were noted in 

 the height to which the liquid rose in tubes of different materials. It is 

 evident, therefore, that since the tracheae are lined with chitin, similar to 

 the covering of the body wall, insecticides which will spread over the 

 body will also penetrate the tracheae by capillarity, while those insecti- 

 cides which do not spread over the insect, even though they may come in 

 contact with a spiracle, will not be able to penetrate the tracheae. 



Contact insecticides may therefore be divided roughly into two groups: 

 First, those which wet the insect and, owing to greater adhesion than 

 cohesion, are able to spread over the surface of the body and pass up the 

 tracheae by capillarity. Second, those which wet the insect, but which, 

 owing to a higher cohesion than adhesion, are able neither to spread over 

 the surface nor to gain entrance into the tracheae by capillarity. 



