CONDITIONS INFLUENCING PREVALENCE 179 



young lettuce, which is spoken of by the natives as the 

 jalkumi. 



In the tanks so planted, the water was alive with young 

 leeches and nematodes, but in none of them could be 

 discovered a single Mosquito larva, while the others 

 swarmed with them. Introduced into a tank already con- 

 taining Mosquito larvae, however, the plants appeared to 

 exercise no hostile influence whatever on their develop- 

 ment, and for this reason, I conclude that the plants act 

 mechanically in the same way as an artificial cover. The 

 jalkumi floats on the surface of the water and so forms 

 a most effective screen ; but another plant resembling 

 the Canadian duckwood, which grows completely sub- 

 merged appeared almost as effectual, though in this case 

 the surface of the water was certainly not hidden in the 

 ordinary sense of the word, though the green coloration 

 may have masked its presence to the defective and short 

 ranged vision of a Mosquito. Whether the presence of 

 such plants would be equally effective in places where no 

 alternative pools are accessible is of course open to doubt, 

 and the vagrant nature of my employment as Sanitary 

 Commissioner prevented my being able to follow the point 

 Tip, but the matter is certainly worthy of investigation, as 

 if confirmed, it would afford an extremely simple and 

 inexpensive means of diminishing the number of available 

 nurseries for larvae. 



A variety of plants, such as the castor oil and eucalyptus 

 have enjoyed the reputation of being protective against either 

 Mosquitoes and malaria, and it is possible that the scent of 

 certain strong smelling species may be obnoxious to the 

 insects; but the observations of Celli (C. M., p. 143) show 

 that so far from being destructive to them such plants may 

 form an excellent refuge for Mosquitoes. Our experience 

 in India is similar, as may be judged by the following- 

 extract from the Pioneer (April 4, 1901) : — 



To the Editor. 



" Sib, — Some short time back, there appeared, in the 

 correspondence columns of your paper, I think, a recom- 



