H. M. Lkfroy 295 



but does not wet the whole insect and so the spiracles, or part of them, 

 are free. 



The positions of the spiracles, for instance in a Scale Insect, a Psylla, 

 an Aleurodid, and an Aphid, are very different indeed. Their structure 

 is different, they have difTerent arrangements for closing them. In 

 Psylla and Aleurodes the adult is very readily killed with insecticides, 

 partly probably because it has very inefficient spiracles and requires 

 much air, whereas the young form is harder to kill because it has a very 

 efficient closing and protecting apparatus and requires less air. 



What I suggest is that the way to tackle the question is first to 

 investigate the tracheal system of the insect. Second, to investigate 

 the wetting action of the insecticide on the plant, unless the insect 

 is fully exposed. Thirdly, to investigate the wetting action on the 

 insect, or the dissolving of its waxes or coating; and fourthly the 

 actual toxic ingredient that would be added to the insecticide to act 

 on the insect when it reached it. 



Are any of these factors really taken into account in the recom- 

 mending of insecticides ? I think they should be, and there is here a 

 large field for research. 



In preparing this paper I have not attempted in any way to sum- 

 marise the present practice of insecticides outside this country; this 

 would be a very large subject, complicated by the now enormous use 

 of insecticides sold under fancy names by firms who may or may not have 

 any real knowledge of entomology. But I would like to include ab- 

 stracts of some papers published in America. 



"How contact insecticides kill," G. D. Shafer {Michigan Sta. Tech. Bui. xi, pp. 65, 

 pis. 2, figs. 7). This bulletin consists of two parts, the first of which (pp. 8-53) deals 

 with the effects of certain gases and insecticides upon the activity and respiration of 

 insects, and the second (pp. 53-64) with some properties of lime-sulphur wash that 

 make it effective in killing scale insects, especially San Jose scale. Abstracts of these 

 have been previously noted {E.S.R., xxv, p. 665). The investigations conducted 

 and here reported in detail have been summarised by the author in the following 

 general conclusions : 



"Usually contact insecticides do not depend upon one property or means alone 

 for their effectiveness, yet as a rule some one property is chiefly concerned. Alkaline 

 washes, corrosive subUmate solution, and other fluids, which are capable either of 

 dissolving or of precipitating certaui constituents of insect tissues, are able to pene- 

 trate the chitin of insects into the tissues slowly. The weaker the surface tension of 

 the fluid, apparently, and the thinner the chitin with which it is in contact the 

 more rapid the penetration. Gases and vapors may penetrate the chitin of 

 insects, especially through the tracheae into the tissues far more rapidly than 

 liquids. 



