H. M. T.EFROY 297 



by immersing test insects in definite strengths of the insecticide, and noting the time 

 required to produce death. The common bedbug {Cimex ledularis) appears to 

 be the most satisfactory test insect. As yet the mode of action, the way in which 

 the contact insecticides cause the death of the insects, has not been determined. 

 Apparently, the fewer the number of spiracles, the smaller their size, and the better 

 they are guarded by hairs or valves, the more resistant is the insect to the contact 

 insecticides. 



"Chemical standardisation of this class of insecticides is with our present know- 

 ledge impossible. With two substances, having essentially the same composition, 

 the insecticidal values may vary enormously. Even the same substance, prepared 

 with what are apparently unimportant chemical variations, gives widely different 

 insecticidal values." 



Relative efficiency of contact insecticides. 



" A further contribution to our knowledge of insecticides," C. T. McCUntock, H. (/. 

 Hamilton and F. B. Lowe {Jour. Amer. Pub. Health Assoc, i (1911), No. 4, pp. 227- 

 238, pi. 1): "It is possible to standardise substances which are subject to sophis- 

 tication or deterioration by comparing the efficient dilution of their vapours with 

 that of a product of known purity. This is particularly appUcable to solutions of 

 nicotin and to powdered chrysanthemum flowers. As yet there is nothing from which 

 to conclude what action the vapours have on the insects. If it were merely irrative, 

 formaldehyde would be valuable and the vapours of the burning insect powder 

 without value. If the action were similar to anesthesia, chloroform should have 

 been of greater value. If the action were pui'ely that of poisoning one would have 

 expected the highly poisonous hydi'ocyanic-acid gas to be of exceptional value for 

 all species of insects." 



These show that the way in which contact poisons act is not by 

 any means clear, and that a great deal of research in insect physiology 

 is required before we do know anything about it. 



In conclusion, I wish to emphasise the practical bearing of this 

 subject. I may have shed no light on it, but I do think that we owe it 



