32 LIFE AND HABITS OF DOMESTIC MOSQUITOS 



discovered by Aaron in America in 1890, is made use 

 of largely to prevent the larvae coming to maturity, 

 and is thus a means of reducing mosquitos. A 

 simple experiment will show how the oil acts. Place 

 a little water containing mosquito larvae into a 

 tumbler, and watch them wriggling. Then put a few 

 drops of any oil on to the surface of the water, and 

 wait until it spreads evenly. Note how each mos- 

 quito larva rises to the surface to breathe, and how 

 it tries to thrust its breathing-tube through the 

 layer of oil but its efforts are in vain. In half an 

 hour all the mosquito larvae will be dead at the 

 bottom of the water. This discovery has been one 

 of the many that have led to the saving of human 

 lives. 



Other artificial larvicides have been employed. 

 An attempt has been made to posion all water con- 

 taining mosquito larvae with sulphate of copper or 

 lime. But the former is very poisonous, and can 

 only be used in cesspools and such-like places, and 

 the latter forms a hard calcareous mass in the water ; 

 both are less efficient and more expensive than oil. 

 Placing iron nails into water cisterns is not much 

 better. Professor Ross has recently suggested the 

 use of cyanide of potassium. It is very poisonous, 

 of course, but it can be made up into soluble tablets 

 and thrown into cesspools or water collections in 

 such a quantity that it will not hurt human beings 

 or animals even if they were to drink it. This use of 

 cyanide of potassium is only in its experimental stage. 

 Petroleum is the larvicide that is commonly used. 



