﻿258 L. NICHOLLS — SOME OBSERVATIONS ON THE BIONOMICS AND BREEDING- 



(b.) A certain group of barracks, less than 200 yards across, has a small 

 swamp located about 100 yards away on the south side. The men dwelling in the 

 barracks on this side are extremely liable to malaria, which is uncommon among 

 those on the north side. 



There are several other instances of a similar kind in this island. 



About 150 yards is the furthest I have had to search for a breeding-ground 

 from a place in which I have found the adult Anopheline. I have known of 

 primary malarial infection taking place at a greater distance than this, but the 

 probability was that recent dry weather had obliterated the true place where 

 the infecting mosquito was bred. A determined search often brings to light 

 breeding-grounds nearer than is at first expected. On the other hand, I have 

 made notes as to the distances at which Anopheline larvae have been found 

 from the nearest human habitation, the greatest recorded distance being from 

 400 to 500 yards. But St. Lucia is, perhaps, too small and too densely popu- 

 lated an island to yield reliable observations of this kind. 



There are reasons for believing that the few mosquitos which are blown some 

 distance by the wind are almost a negligible factor in malarial infections. 



In some countries, under different conditions and with other species, it is 

 possible that nights of a considerable distance are made : especially would 

 this be so where the inhabitants are a long way from the mosquito breeding- 

 grounds ; but this is never the case in this island with the species under 

 consideration. The greater distance that a female Anopheline has to fly in 

 search of blood, the greater must be the odds against her long survival. 



Methods for ascertaining the distribution of the larvae. 



When I first started to hunt for larvae I was accustomed to examine the pools 

 principally by a definite number of " dips " with a tumbler. It soon became 

 apparent that this alone does not allow a sufficiently close study. As soon as a 

 pool is disturbed, Anopheline larvae usually go to the bottom and often remain 

 there for more than a minute ; the consequence is they may not be immediately 

 obtained in the dipper. Again, many thousands of larvae may be distributed 

 over an area of swampy overgrown meadow-land and but a few be present in a 

 small road-side pool ; dipping will quickly reveal the latter, whereas the former 

 will not be easily obtained, and the relatively unimportant pool may receive more 

 attention than is paid to a piece of water-logged land which is giving rise every 

 day to hundreds of mosquitos. 



The following is the procedure which I have adopted for the last eighteen 

 months : it has been of much value in throwing light upon many obscure 

 points: — 



(1) The pool is first thoroughly and closely inspected, especially at the 

 edges and around suspended matter ; if many larvae are present, one experienced 

 in hunting for them will usually soon see them. They are especially apparent in 

 freshly-formed opaque muddy pools which are favourite breeding-grounds, but in 

 other situations, such as marshy land, a true conception of their numbers is not 

 easily formed. A number of dips should now be made. 



(2) The pool or swamp, or a definite portion of it, is cleared of floating matter 

 and other debris ; grasses and other vegetation are cut awav below the surface 



