THE LIMITATIONS OF KEBOSENE AS A LARVICIDE. 281 



\ third mauuor in which kerosene oil may act as a poison is by entering the tracheal 

 tubes through the siphon and either blocking them or spreading mto the finer branchea 

 and acting directlv on the tissues of the larvae. 



The larvae of Cukxfatigans, which have long siphons (about four times as long a^i 

 the diameter of the base), succumb rapidly to the action of oil Larvae m the fourth 

 phase the last stage of their development, are almost invariably killed withui halt an 

 hour bv " oiling," but if thev are prevented from coming into actual contact with the 

 film of oil thev survive longer. In one experiment six C.fatigans larvae m the fourth 

 phase were isolated singlv in six identical glass tubes each containing 8 cc. of distilled 

 water. A small plug of' wool was placed in each tube just below the surface of the 

 water and then to three of the tubes a layer (one cm. deep) of melted paraffin was 

 applied and to three a similar laver of crude kerosene. The larvae confined under 

 the laver of paraffin were found to be dead after 3, 3|. and 6 hours respectively, and 

 those under the kerosene after 31, 61, and ?! hours. Allowing for individual variations 

 in the power of resistance, it appeared that the larvae, prevented as they were from 

 reaching the surface bv the plug of wool, lived about equally long whether they were 

 under a seal of paraffin or kerosene-that is, the kerosene in this case had no specific 

 action but likethe paraffin, deprived the larvae of access to air and so drowned them. 

 Anything therefore, such as the presence of organic matter, which tends to use up the 

 available" oxvgen in the tubes should accelerate the death of the lar^'ae under such 

 conditions. " This was found to be the case ; for when a natural C. fahgans medium 

 was used instead of distilled water, the larvae sun-ived under paraffin only 65, 70, and 

 80 minutes ; and under kerosene 65 and 70 minutes. The third lar^-a confined under 

 a laver of kerosene in this experiment succeeded in ^^Tlggllng through the wool plug 

 and reached the surface for a moment. It at once returned below the wool and did 

 not again get above it, but this larva died after 35 minutes. 



These experiments prove that the action of kerosene on the larvae of Cvkxfatiguns 



is not dependent on the solution of anv part of the oil in the water, and is not entirely 



due to deprivation of oxvgen, but is the result of some direct action of the oil on the 



larvae Were it merelv a blocking of the tracheae in the siphon, the lal•^'ae should be 



able to live as long as thev do under a laver of kerosene from which they are separated 



by a light plua of wool. It is probable therefore that some of the oil enters the siphon 



tracheae and Is drawn into the body of the laivae, where it exerts a poisonous action. 



In the case of Stegomyia fasciaia, however, the action seemed sometimes to be 



different In a mass experiment a sample of these larv^ae in the flmd in which they 



had been found was divided into two equal parts and poured into two similar bottle^ 



so that the fluids reached just into the narrow necks. The one bottle was then sealed 



with a laver of paraffin one cm. deep, and the other with a similar layer of crude 



kerosene ^ The lar^-ae survived about the same length of time in each, all being dead 



after 6 hours. The pupae m both were all dead after U hours. SimUarly in more 



detailed experiments the larvae of S.fasciata appeared to be able to resist the action 



of kerosene : and one experiment (see Table I) has already been mentioned in which 



these larvae lived for over two days under an unbroken film of oil. The oil did not 



appear to have any direct action on them. 



The larvae of Stegomyia fasckita differ from those of Cukx/atigam in having short 

 stout siphons, which they may often be seen to thrust against the film of oil, as if 

 (C325) 



