INSECTS 7 



by external hairs, than in others. It does not neces- 

 sarily 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. 



Of the poisons in use, those containing arsenic are, 

 perhaps, the most prominent ; these, as is well known, 

 are very deadly to animals, as well as to insects, but 

 in many other cases it is found that substances which 

 act as virulent poisons to vertebrate animals, have 

 little or no effect on insects. This is so with many of 

 the vegetable poisons ; of these there seem to be only 

 a few which can be successfully used as insecticides, 

 the most conspicuous being nicotine (tobacco), quassia, 

 and hellebore, to which may be added pyrethrum, 

 when used as a fumigant. Some recent work, however, 

 has indicated that paraffin oil, when used in a suitable 

 form and of a suitable strength, is more efficacious 

 than the mineral or vegetable poisons mentioned 

 above, though considerable more work is still required 

 to determine the relative merits of paraffin and the 

 compounds containing arsenic. Whether paraffin acts 

 as a poison or as a corrosive agent does not appear 

 to have been established ; probably it acts as both, 

 and it may also act by blocking up the breathing 

 orifices of the insects. Soft-soap is another substance 

 which acts in this way. In the case of red spider and 

 mites, sulphur, or compounds containing sulphur, are 

 often very efficacious, but they seem to be uncertain 

 in their action, and preference is now being given to 

 paraffin emulsions for the destruction of these pests. 



