THE NATURAL HISTORY OF BOMBAY MALARIA. 419 



bright green seeds. Where it is growing' in large amount so as to 

 form a continuous layer on the top of the water no mosquito larvae 

 can live and pools and tanks covered in this way will be found 

 quite free of larvae. Anopheles larvae placed in a bowl of water 

 covered with a layer of Wolffia arhiza immediately assume an 

 almost vertical position, hanging head downwards in a manner 

 quite unnatural to them. After a short interval they sink to the 

 bottom dead. The larger the larvae the more quickly they die, as 

 young larvae can pass their tails between the plant grains and so 

 get access to the air. Oulex larvae and Stegomyia larvae are also 

 destroyed, but the latter live for several hours in the weed-covered 

 water. Where the continuity of the layer of Wolffia arhiza is 

 broken by the presence of grass or reeds or large leaved weeds, 

 mosquito larvae are enabled to exist without difficulty, and it would 

 appear therefore that before this plant could be used with success 

 as a protection for tanks, the latter would have to be cleared of other 

 growths. The round tank at the Mint which is usually covered with 

 a good growth of Wolffia arhiza is free of mosquito larvae, and so 

 is the Nakhoda tank and several pools near Parel. 



In this paper I have endeavoured to deal with certain points in 

 the Natural History of Bombay Malaria which I hope may prove 

 of interest. I have said very little about malaria as it affects the 

 population of Bombay, but I may mention before concluding that 

 the infection may be found in every part of the city and island and 

 that in certain areas it is so prevalent that over fifty per cent, of 

 the population harbour one or other form of the parasite. 



In the north of the island and round about the Flats and low- 

 lying parts malaria is uncommon, contrary to what people have 

 believed for many years ; but in the older parts of the city, 

 especially where there are many wells, malaria is present to an extra- 

 ordinary extent. The New Dock Works have been responsible 

 for an increase in malaria in the Frere Road area, but not so much 

 on account of the excavation work there as because numerous pools 

 have formed on the land around the excavations. But the New 

 Docks did not bring malaria to Bombay, and there is evidence that 

 it has been present in many localities for a very long time ; and 

 had there been no mosquito infected wells, cisterns and tanks in 

 21 



