348 DISCUSSION. 



mosquito larva; in any vessel or collection of water within his household 

 or compound. 



In large, native, tropical towns this might offer some difficulties, but 

 in a town with such excellent sanitary methods and administration as 

 yesterday we had demonstrated to us in Manila, the addition of a "larva 

 column" to the householder's sanitary sheet might he a feasible matter. 



Doctor Francis Clark, medical officer of health, delegate from the 

 Government of Hongkong. — Doctor Dunbar has told us that by clearing 

 and filling within a radius of 200 feet around the buildings at Olongapo 

 malaria disappeared, but I am of the opinion that this will only be for 

 a time. In Hongkong we have, as you know, to deal with mountain 

 streams, and it is no easy matter to train these streams by confining the 

 water to one smooth channel throughout so as to prevent the development 

 of mosquito larva;. Just to show you what occurred in one district: In 

 the western section of the city are many European houses, and near 

 these houses are several streams. The nearest stream was examined for 

 Anopheles larva? and many found in it. This stream was within about 

 30 to 50 yards of the nearest houses. The next stream, further west, was 

 at this time found to contain no Anopheles larva;; and so with the others, 

 still further west. The first stream was trained and confined to a smooth 

 channel in which mosquito eggs and larva; could find no lodgment. The 

 result of this work was that malaria disappeared from this district. 

 This condition lasted two or three years and then malaria again broke 

 out. I went, together with Doctor Thompson, our mosquito expert, and 

 thoroughly examined all these streams, both trained and untrained. The 

 trained streams were clear of larva;, there being no possibility of their 

 lodging there, owing to the fact that these trained streams are all carefully 

 watched, defects remedied and the channel scrubbed once a week with 

 hard brushes to prevent the growth of alga; at their margins, as we have 

 found that the latter will serve for the development of mosquito eggs. 



In the first untrained stream, however, Anopheles larva; had now 

 appeared. This seems to indicate that Anopheles like the neighborhood 

 of houses, and when they are driven out from the immediate neighborhood 

 of a dwelling they will, in the course of two or three years, appear in 

 the nearest stream and many find their way thence to the houses, if 

 the latter are within flying distance. What the limit will be I can not 

 tell, but we hope 400 yards will be sufficient. We must push our efforts 

 certainly a quarter of a mile from the nearest dwellings, and even then 

 it is necessary, to watch the nearest water courses for the reappearance 

 of Anopheles larva;. 



Mr. Banks. — I would like to say that we can not give too much 

 attention to the habits of the different species of mosquitoes. I have 

 always found Myzomyia rossii Giles and other Anophelince breeding in 

 running water which had some current in it and in which green alga; 

 were to be found growing in the surface, as you will see in the specimens 



