194 Transactions. 



into the film is paralysed and seldom comes again to the surface. Dishes 

 of equal size, 2 ft. by 1 ft. 3 in., containing the same quantity of water and 

 the same number of larvae at the same stage, have been treated, one with 

 light oil, the other with crude petroleum in like amount. In all experi- 

 ments — and I have repeated them over a dozen times — the larvae under the 

 film of light oil have been dead or helpless on the bottom within fifteen 

 minutes, while in the petroleum dish some have been active after an hour or 

 more. In view of the possible breaking of films, comparative rapidity of 

 action is a matter of great importance. The experiments referred to have 

 been repeated on the large scale on pools in various parts of the North 

 Island, and the laboratory results have been amply confirmed. 



The film is best produced by spraying the pool, but the oil may be 

 sprinkled from a bottle or other vessel, or a leafy twig may be dipped in it 

 and shaken over the water. In choosing a spraying instrument for light 

 oil it is necessary to choose one without rubber tubing, as some constituents 

 of light oil are solvents of rubber. 



Experiments with regard to the toughening of films to render them less 

 easily broken are now being made. Up to the present I have found nothing 

 better than raw linseed-oil. It should be shaken up well with the light oil 

 before being applied. I am not yet sure that the advantage gained is suffi- 

 cient to justify a strong advocacy of its use ; but it certainly' does make a 

 film more resistant. 



In testing the killing-power of crude carbolic acid I have taken the 

 formula for the emulsion from the report of the Wellcome Laboratory at 

 Khartoum for 1911, p. 109, where directions sent from Panama are quoted : 

 " Crude carbolic acid* containing about 15 per cent, phenol is heated to 

 212° F., finely pulverized resin is added, and the mixture kept boiling until 

 the resin is all dissolved. Caustic soda is then added, and the mixture 

 kept at 212° F., for about ten minutes, or until a perfectly dark emulsion 

 without sediment is obtained. The mixture is thoroughly stirred from the 

 time the resin is added until the end." It is stated that 1 part of this 

 mixture in 5,000 parts of water containing mosquito-larvae will kill all 

 the larvae within five minutes. If it is used in the proportion of 1 to 8,000 

 the larvae are killed in thirty minutes. In my experiments I was unable 

 to obtain results as good as these. I obtained, however, much better results 

 when using an emulsion of light oil. 



The experiments tabulated below are only a few of a very long series, 

 and all have been verified by actual work at normally infested pools in 

 the open. With regard to various entries in the table I may make the 

 following explanation : — 



In the column headed " Twitching " is noted the time at which the 

 larvae were first observed to be all motionless or twitching helplessly at the 

 bottom of the vessel. This is for all practical purposes the time of death, 

 as the larvae do not recover from this condition unless removed to fresh 

 water. Time of actual death is, however, of importance in view of the fact 

 that mosquitoes sometimes breed in slowly flowing water. 



In the column " Apparently dead " is entered the time at which response 

 could not be obtained to weak induction shocks. 



* A fine account of the efficacy of crude carbolic acid and other larvicides is given 

 by Howard, Dyar, and Knab in The Mosquitoes of North and Central America and the 

 West Indies, vol. i, pp. 379 et seq., Carnegie Inst., Washington, 1912. 



