CH. IX] LARVICIDES I5I 



wells are an important source of Anopheles they often require 

 special devices to prevent them being a source of danger whilst 

 allowing water to be obtained from them, and other breeding 

 places of a special nature may require special measures to deal 

 with them. On the whole the most important consideration 

 in such measures is that the action taken should be of as per- 

 manent a nature as possible. 



The use of larvicides. Many substances have been experi- 

 mented with in respect to their lethal effect upon the larvae 

 of mosquitoes. The larvicide in most general use is some form 

 of kerosene or mixture containing this substance. The oil 

 floats on the surface of the water and thus interferes with the 

 respiration of the larvae and pupae. Certain aniline dyes, and 

 other chemical bodies which dissolve in the water, have very 

 powerful larvicidal properties, and their use has been suggested. 

 The chief difficulty with regard to all larvicides is the temporary 

 nature of the effect they produce, and the necessity for regular 

 and periodical treatment of the breeding places attacked in 

 this way. Ordinarily such substances are now used only where 

 more permanent measures are not possible. 



The introduction of natural enemies. So far the hope that 

 by the introduction of some natural enemy, the number of 

 Anopheles in a country could be at all affected, has not met 

 with much encouragement. Nevertheless, under certain cir- 

 cumstances, the use of suitable small fish has an application. 

 The most effective larvae-eating fish for the most part belong 

 to the family CypriondontidcB, but species of Anabas, Ophio- 

 cephalus and others are also of practical use in this respect. 



Measures directed against the adult mosquito. The fumigation 

 of houses with burning sulphur, pyrethrum and other sub- 

 stances, has been found of use. In Panama the catching of 

 adult mosquitoes by children for a small reward is said to be 

 very effective. Whitewashing and improvement in lighting 

 during the daytime may be called for sometimes. 



The various measures briefly outlined above are but some 

 of the tools in the hand of the sanitarian, to be used as circum- 

 stances require them. Before they can be applied with success 

 it is almost always necessary for the conditions relating to 



