PLANTS, FUNGI AND INSECTS 15 



larval stage the method of attack is generally based on poisoning 

 its food, rather than on destroying its body by corrosive sub- 

 stances; with aphides (green-fly or black-fly), poisons are often 

 ineffective, for the insect simply punctures the leaves and extracts 

 the juices : in that case an insecticide must be used which acts 

 externally on the body of the insect, either by corroding it, or 

 by stopping, or otherwise interfering with, the breathing of the 

 insect through the apertures in its body provided for that 

 purpose. Nicotine appears to act by producing paralysis. 



It will readily be seen from this how essential it is to have a 

 knowledge of the life-history of the insect. But more than this 

 is required ; we ought to know the exact action which the various 

 insecticides have on it at various times in its life-history, and in 

 many, it may be said in most cases, our knowledge on this point 

 is very deficient, and can only be increased gradually by very 

 extended and laborious investigations. Every individual case 

 has its own complications ; thus, the larval stage of an insect is 

 generally divided into various sub-stages, the grub moulting 

 (often five times) and changing its nature, and in some of these 

 stages it may be more sensitive to poisons, or less protected by 

 external hairs, than in others. It does not necessarily follow 

 that the earlier stages of the insect are those in which it is most 

 sensitive, but it would appear that, as a rule, poisons are most 

 effective in the earlier stages, and corrosive substances most 

 effective in the later stages. 



