1900.] Capt. L. Rogers — Malaria North of Calcutta. i97 



a small area of the Maniktolla Municipality. The results which were 

 obtained are as interesting as they were unexpected. I much regret 

 that I have not been able to confirm Major Ross' observations as to the 

 very localised and small number of the breeding places of anopheles 

 mosquitos. On the contrary I found them very commonly in large 

 tanks, and that too it spite of most of them swarming with fish. In 

 fact in the dry season when small pools are few in number the tanks 

 are the common breeding place. Further, although I found them in 

 two out of three very small pools (from two to five yards in diameter), 

 and with but some two inches of water in them, these pools also con- 

 tained small fish, which during some time that I watched them did not 

 touch one of the anopheles larv83 which floated temptingly past their 

 noses, although they eat several small beetles, which they appeared to 

 prefer. If then, the larvae can survive in spite of fish in such tiny pools 

 is it any cause for wonder that they live in tanks. 



Tiie question then arises as to whether there is any relationship 

 between the number of anopheles and the amount of fever, in order 

 to test which I resolved to make a monthly examination of some thirty 

 tanks, together with any pools near them in a small area of Manik- 

 tolla. These observations will have to be continued for a year, bat 

 it may be mentioned here that during the dry season, when malaria 

 fevers were at a minimum, from one-third to two-thirds of the tanks 

 have been" found to harbour the anopheles larvae, and that too at a 

 time when three visits to the local dispensary, after having given 

 notice that all fever cases were to be kept for me to see, I failed to 

 obtain a single case. Yet some of the tanks were estimated to have 

 confained over one million larvae, so thickly were they lying near the 

 leeward bank in particular. In short it would have taken a very 

 large number of the small pools to harbour as many larvae as one of 

 these tanks, so that in this area the tanks form the principal breeding 

 ground in the dry season at any rate. It will be very interesting to 

 see what happens in the rainy season, but I may mention that after the 

 recent heavy rain the larvae nearly disappeared from the tanks, and 

 were enormously reduced in numbers in spite of several new small 

 infected pools having appeared, so that further observations promise 

 to be of interest. 



The importance of the above observations lies in the impossibility 

 of destroying all the anopheles larvae in even a very small area in 

 Bengal, for the thirty tanks mentioned above all lay within an area of 

 one-sixteenth of a square mile, and formed but a small fraction of 

 those of the very small municipality of Maniktolla, so that unless some 

 very much more potent method of destroying mosquitos is discovered I 



