Attack by Oiling 153 



time; the Anopheles larvae remain on the surface of 

 the water unless frightened, and are more rapidly 

 killed by the oil film than others. 



There is little, if any, ovipositing by mosquitoes 

 on water heavily treated with oil. 



Kerosene has its merits. Its especial desir- 

 ability is its property of rapidly forming a thin 

 film. It was tried in Panama, but rejected in 

 favor of crude oil. The objections to its use are : 



1 . The film is so thin that very slight disturb- 

 ances of the water surface, by flotsam, vegetation 

 projecting through the surface of the water, ripples 

 caused by wind or current, etc., break the con- 

 tinuity of the film. 



2. Kerosene is expensive. 



3. It is transparent, and is wasted by oilers 

 because it is difficult to see where the film is 

 satisfactory. 



4. Liability to fire, where sparks may drop 

 into it ; for instance, near a railroad. 



Crude oil of asphaltum base is used extensively 

 in Panama. Its great advantage is its low cost, 

 and because of that, its poor spreading qualities and 

 high specific gravity may be overlooked in a warm 

 country. We doubt if the quality of oil used in 

 Panama would serve in a cooler country during 

 spring and fall months. The oil delivered in 



