294 Insecticides 



I take it to mean that the insect is actually in contact with the insecti- 

 cide all over without any air-film between. It means, for instance, 

 that the liquid is in contact with the spiracles. 



(3) Means that it also has a toxic action. Now assuming that we 

 have an insecticide that spreads, that wets and that penetrates to the 

 insect, what happens then we do not know. 



With a view to getting some accurate ideas I made a large series 

 of experiments with meal-worms with these results : 



1. No action is got by applying any liquid for a short time to the 

 skin, but many act at once if applied to a single spiracle. 



2. Interference with the mechanical functions of at least five pairs 

 of spiracles is necessary to produce any symptoms due solely to mechan- 

 ical interference. 



3. Closing all spiracles produces a condition we may call 'rigour,' 

 in which a liquid will enter the spiracles. 



4. It is impossible to get a liquid to enter one spiracle unless this 

 condition is produced. 



5. Dipping a meal-worm in a hquid produces the condition in which 

 the liquid enters. We can therefore compare the effects of hquids by 

 this means. 



6. Of liquids tested the following results were got: 



Killed all Killed some Killed none 



Clove oil Quinoline Picric acid in water 



Xylol Carbolic acid Alcohol 70 % 



Turpentine Formol 4 % Nicotin(> 1 % aq. 



Nitro-benzene Pyridine Acetone 



Chloroform Acetic acid Ether 



Amyl acetate Eucalyptus oil Chloral hydrate aq. 



Cymene Methyl salicylate Water 



Pseudocumine Aniline 



Acetic ester 



Now results with meal-worms are not of much direct use and we 

 must, for the moment, eliminate the peculiar 'rigour' results. But 

 they show us, I think, that a much greater range of liquids than are 

 generally used have a toxic action and that we must attach far more 

 weight to the details of the structure of the spiracles and tracheal 

 system. 



It would look even as if only through the spiracles could one reach 

 most insects with an insecticide, and that the insecticide that did not 

 actually fill the spiracles was not acting. I suggest this is why contact 

 poisons fail against large insects as a general rule; the liquid may wet 



