﻿PLACES OF ANOPHELES IN SAINT LtJCIA, BRITISH WEST INDTES. 261 



them and washed them out, but at the same time it has carried to them obstructing 

 sticks and uprooted weeds. Besides, the water pouring off the land contains much 

 suspended organic matter, such as the decomposing bodies of insects and a variety 

 of vegetable substances, upon which larvae can flourish. 



The water in the pools is always opaque during the rains and it requires the 

 cessation of rain for about forty-eight hours before the pools become clear. This 

 opacity, best promoted by continuous light rains, represents a source of food and 

 protects the larvae from their natural enemies which may be lurking in the pools. 



One of these pools, situated in the course of a ditch along a hill-side road is 

 shown on Plate VII. (fig. 1). 



(b.) Spreading pools. Rains will cause a pool to spread over the surrounding 

 area, and although few if any larvae could exist in the pool itself, the newly 

 formed portion, protected by floating matter and the surrounding vegetation, will 

 frequently contain numerous larvae. 



(c.) Obstructed waters. These may be caused by man and his domestic 

 animals or by the agencies of nature. 



A typical example of this caused by man is shown on Plate VI (fig. 1). A 

 piece of marshy land, through which ran several shallow permanent ditches, was 

 being drained. For eighteen months previously I had every week carefully 

 examined this swamp, and Anopheline larvae were always found in numbers, after 

 the rains, all over the marshy land, but very few were present in the ditches. 

 It was decided to level the marshy land, and as this was being done the bush 

 which was cut down was allowed to fall into the ditches, and temporarily filled 

 them up with obstructions and decomposing matter, upon which algae quickly 

 grew. Though there were at the time many fish and dragon-fly larvae in the 

 ditches, they could not compete with the sudden great increase in the food-supply 

 thus brought about, so that Anopheline larvae soon became present in extra- 

 ordinary numbers, and this was followed by a noticeable increase in the number 

 of cases of malaria. The photograph shows on the right-hand side the bush 

 which was obstructing a gutter. The small area of clear water contained no 

 larvae, but they were very numerous around it and anywhere where there was 

 protection. 



Cattle often trample down the edges of gutters and pools, causing small isolated, 

 muddy and obstructed puddles, which are suitable for Anopheles ; but this 

 does not refer to such small puddles as those formed by the impression of 

 a hoof. 



Again the rains carry down much wood, bushes and uprooted vegetation, which 

 may fill up pools and turn them into favourable breeding-grounds ; or gutters and 

 edges of canals may become overgrown with vegetation which protects the larvae 

 from their natural enemies and the elements. 



(d.) Backwaters and pools of rivers, ravines or streams. These are of import- 

 ance because, in dry weather, larvae can usually be found in such situations, when 

 an extended search will reveal few if any other breeding-grounds. They are 

 formed by the shrinkage of the main watercourse, which may also leave a number 

 of small isolated pools in protected situations. Again, dry weather causes small 

 areas to become overgrown or obstructed from the main stream. 



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