2^0 J. \V. SCOTT MACFIK. 



By a poisonous action. The oil miiiht exercise a poi.souous action on tbe larvae 

 either by being dissolved in the water and acting on them in solution, or by entering 

 the tracheal tubes in the siphons when the larvae came to the surface to breathe. 



Kerosene in its various forms is only very slightly soluble in water, and as the total 

 amount used in " oiling " is always extremely small as compared with the total bulk 

 of the water over which it is a})plied, the t[uantity dissolved might be su])posed to be 

 negligible. Moreover, as the oil remains at the surface, the solution would be strongest 

 at the surface and one might expect that the larvae rising to breathe would become 

 aware of the poison in time to recede from it before it liecame ])owerful enough to 

 injure tbem. 



In the course of my experiments it sometimes happened that bubbles of air 

 separated on the sides of the vessel in which the larvae were confined. In this case 

 it was found that larvae of Stegomyia fasciata might live for certainly several days 

 under a film of kerosene presumably obtaining the necessary oxygen from the air 

 l.nibbles. As in these experiments the volume of water was relatively small the 

 larvae would almost certainly have been affected had kerosene acted appreciably 

 in solution. The larva of Mansonioides africanus is particularly suitable for experi- 

 ments of this nature, as it does not require to come to the surface at all to breathe 

 and can therefore be kept under a deep layer of kerosene for a long time, during which 

 it would, however, be exposed to the action of any part of the oil which might be 

 dissolved. In one such experiment a larva was kept in a jar of water to the surface 

 of which crude kerosene had been added in an amount corresponding to 100 cc. per 

 square metre. For three days it appeared to be unaffected, and on the fourth day it 

 pupated. 



From these observations it may be concluded that in " oiling " the action of 

 kerosene is independent of its solubility in the water to which it is applied. 



The vapour given off by the oil might be expected to act on the larvae when inhaled 

 into the tracheae during respiration. Sen (1914) has recorded a single experiment 

 in which a few drops of kerosene placed on a cotton plug in the stopper of a jar half- 

 filled with water in Avhich mosquito larvae (C. microannidaius 1) were living appeared 

 to have a fatal effect. " Apart from the presence of vapour," he says, " there was no 

 apparent reason why the larvae . . . should not luive lived as long as those in the 

 control, as no definite surface film was formed under the conditions of the experiment."" 



I have not been able to repeat .Sen's experiment with the species of larvae he 

 employed, but I have satisfied myself that in the case of Stegomyia fasciata and 

 Culex faiujans kerosene applied in this manner has an effect. Its action was most 

 marked on the species with the longer siphon, but was less constant and much slower 

 than that produced by a film of oil. Sen speaks of the vapour as having an " anaes- 

 thetic " efi'ect. In my experiments the larvae undoubtedly often became sluggish 

 when they were not actually killed, and tended to remain adhering to the surface of 

 the water, being abnormally difficult to disturb ; while some which appeared to 

 have been killed recovered when transferred to shallow vessels away from the 

 kerosene. In actual practice the vapour must be very rapidly distributed by currents 

 of air, and it may be doubted if it is ever sntficitMitly com-tMitiatod long enough for its 

 action to have any importance. 



