398 



than this adhesion, the spray will form into droplets, which tend to 

 roll off, the same result being obtained when the spray does not wet 

 the insect. The same properties determine the degree of penetration 

 of a hquid into the tracheae by capillarity. 



Hence contact insecticides may be divided into two groups : — 

 (1) those that wet, but which do not spread, owing to their adhesion 

 being less than their cohesion ; and (2) those that wet and spread 

 over the surface and are able to gain entrance to the tracheae by 

 capillarity, since their cohesion is less than their adhesion. 



Experiments seem to indicate that the composition of chitin is 

 such that it is easily wetted by oil and oil solvents, or its surface may 

 be coated with an oily or fatty substance, and therefore contact 

 insecticides which are either soluble in ether or chloroform, or are 

 fat solvents, are able to spread over the insect and enter the tracheae. 

 It has also been shown that compounds with a viscosity as high as, 

 or higher than castor oil, spread so sloMy that they may be classed 

 as poor insecticides ; while compounds more volatile than xylene 

 evaporate too quickly for effective work. 



In the case of emulsions, the power to penetrate the tracheae 

 depends on the character of the emulsifier and not on the emulsified 

 oil. Those made with gelatin or saponin tend to form droplets and roll 

 off ; those made with Castile or soft soap adhere, spread and penetrate 

 the tracheae, carrying the emulsified oil with them ; but those 

 made with ivory soap, though they adhere, are too viscous to spread 

 rapidly and ultimately break down, though the oil remaining may 

 spread over the insect and enter the spiracles. The watery part of 

 such emulsions evaporates in from 5 to 30 minutes, the length of time 

 depending on the temperature and humidity of the atmosphere, the 

 time however being sufficient for a volatile oil to evaporate before 

 reaching the tracheae. 



Experimental evidence shows that solutions of soaps containing a 

 large proportion of alkali are more toxic than those which are practically 

 neutral. It also proves that the vapour of volatile oils and acids 

 can penetrate the walls of the tracheae more rapidly than the liquid. 

 On the other hand, aqueous solutions such as nicotine do not penetrate 

 the tracheae at all except in the form of vapour. When nicotine 

 sulphate comes in contact with the body of an insect, it is slowly 

 decomposed, with the formation of nicotine which enters as a vapour. 

 Such a decomposition doubtless explains the fact that leaves sprayed 

 with nicotine sulphate, even when dry, are repellent and poisonous 

 to insect larvae, even though not taken internally. 



In the case of very volatile compounds, there is a tendency to 

 re- evaporation from the tracheal walls, hence the atmosphere must 

 be saturated, and consequently a much larger amount of such a 

 compormd is required to kill an insect than of a slightly volatile 

 compoimd, and thus volatility comes to be an index of the ability 

 of a compound to gain entrance into the insect, and is therefore 

 closely correlated with toxicity. 



A notable exception to this is the compound chloropicrin, which 

 penetrates the walls of the tracheae and kills very quickly, its action 

 being due, either to its extreme toxicity, or to an abnormal power 

 of penetration. 



