﻿PLACHJS OF ANOPHELES IN SATNT LUCIA, BRITISH WEST INDIES. 259 



so as to leave nothing to obscure the water ; in a few minutes the larvae will be 

 seen returning to the surface, there is now a good chance of forming a conception 

 of their numbers. 



(3) The colour of the larvae varies with the nature of the situation in which 

 they are found ; ranging from a very light semi-transparent colour to nearly 

 black ; stirring up the mud at the bottom of the pool or adding a little chalky 

 water will often show them up very distinctly. 



(4) After much of the suspended material has settled, a net made of book- 

 muslin may be swept in all directions over the pool ; when turned inside out into 

 a shallow dish the larvae taken will wriggle free into the water. 



This net, or a net made of mosquito netting, is now employed for thoroughly 

 searching the pool for other denizens, such as small fish, dragon-fly larvae, 

 crustaceans, etc. 



(5) Notes are now taken, showing the position and nature of the pool, eleva- 

 tion above sea-level, etc. The following are examples taken from my 

 notebook : — 



(a) March 16th, 1911. Small pool at the edge of Marchand River, about 

 100 yards from the cemetery, apparently caused by the receding level of the 

 river, due to a few days' dry weather. Pool about one yard square ; it 

 contains algae, and small vegetation overhangs it ; the bottom is sandy ; con- 

 siderable shade is derived from neighbouring mango trees. Fourteen larvae, 

 no pupae counted ; also contains several small fresh-water shrimps, and very 

 small dragon-fly larvae. 



March 20th. Most of larvae have disappeared. They could not have 

 hatched out in this time. 



(b) August 21st, 1911. Pool in continuity of road-side gutter, Cul-de-Sac 

 road, elevation 640 feet above sea-level, average gradient of gutter about 

 1 in 20. Most of the water ceases to pass along the gutter, consequently it 

 requires much rain to wash out the pools formed in it. There was con- 

 siderable vegetation growing in and around the pool, which is also full of 

 recently cut " bush." Larvae numerous. 



(6) For the purpose of forming an approximate conception of the larvae which 

 eventually become imagines in various situations, types of all forms of breeding- 

 grounds must be screened with a muslin tent (PI. IX, fig. 2). The method is 

 to count as nearly as possible the number of larvae or pupae present in a definite 

 area (usually six feet by four feet). The screen is placed over this area and the 

 mosquitos which hatch out are counted. Again a definite number of larvae 

 can be introduced into various places and the number of ultimate survivors noted. 



Great care must be employed in setting the net ; it should be well weighted 

 down all round, or if it only includes a portion of the water, the edges of 

 the net must be submerged. On several occasions I have found Attid spiders 

 in the net, which had succeeded in crawling in under the free edge. Their 

 presence makes the results valueless, for they will soon devour a number of 

 mosquitos. 



It is remarkable how few mosquitos succeed in breeding out from certain 

 types of ground, whereas in others the struggle for existence is at times very small. 



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