PROPHYLAXIS IN MALARIA 249 



rapidly destroy larvae, as the films formed on the surface 

 prevent the larvae getting air through their respiratory 

 syphons. Unfortunately it also kills off fish and many 

 forms of life, and the water becomes putrid 'and offen- 

 sive. Something may be hoped for from some of the 

 native fish poisons. These usually belong to one or 

 other of the species of Derris. The roots are crushed 

 and thrown into the w r ater, and the milky fluid from the 

 fresh roots, even in minute quantities and much diluted, 

 will destroy the larvae, and for small collections of fluid, 

 cesspits, &c., is highly effective, and though killing off 

 most forms of animal life does not render the water as 

 putrid or offensive as the use of oils. Where fish are 

 present these would be destroyed, and therefore this 

 method is not advisable in larger collections of water. 

 Many fish feed on mosquito larvae, and are one of the 

 important enemies of mosquitoes. Amongst these the 

 Cyprinodontidce are of special importance. The small 

 fish known locally as " millions " in Barbados belong 

 to this family, either Pcecilia or Gambtisia, and are by 

 some believed to be the reason for the absence of 

 anophelines from that island. In India, species of 

 Chela, Haplochilus and Therapon are effective, the last 

 in brackish waters. Even worse enemies are the larvae 

 of some of the Dragon-flies (Agrionidce) ; these breed 

 in similar places to mosquito larvae, and live on them 

 to a large extent. 



In countries where the dangerous mosquitoes are stream 

 and river breeders, drainage is of little value and the use 

 of larvicides impracticable. Much can be done by remov- 

 ing sedges, .reeds, and grasses growing into the streams, 

 keeping the banks clear, and removing obstructions in 

 the river bed or sides, so that fish can get at the larvae 

 and so that a uniform flow of water is maintained in the 

 part of the stream in the vicinity of the settlement, and 

 the larvae are washed further down stream. 



The selection of the site for a settlement is a most im- 

 portant matter. In the case of small stations on unsuitable 



