104 SOME MINUTE ANIMAL PARASITES 



The second line of attack is by chemical action, 

 when the larvae are killed by such substances as 

 paraffin or petroleum, or the breeding-places are 

 made so repulsive by the same, that oviposition 

 cannot occur. The main difficulty in connexion 

 with the use of such larvicides as paraffin and 

 petroleum is that in the tropical and subtropical 

 countries, where mosquitoes most abound, the heat 

 of the sun soon causes the oil to evaporate. 

 Undoubtedly it is most efficient while it is on the 

 surface, and the larvae are killed rapidly. But about 

 three days after the first application the oil will have 

 evaporated, and as a result larvae again make their 

 appearance. Consequently, the application of such 

 substances as petrol is inapplicable to large areas 

 such as the swamp-lands of the Southern States or 

 of India. Nevertheless, the method has been used 

 with marked success along the course of the Suez 

 Canal and in the towns on it both Port Said 

 and Ismailia and, more recently, along part of the 

 shores of the Panama Canal, though some malaria is 

 still present there. 



Still, it is advisable that breeding-places of mos- 

 quitoes should be destroyed and not treated, if 

 immunity from attacks is desired. Water is neces- 

 sary for household use, but water-barrels and 

 cisterns should be made mosquito-proof with wire 

 gauze covers, and should be inspected frequently. 

 The examination should not be casual, but detailed, 

 for the larvae are very sensitive, and on the least 

 disturbance disappear from the surface and hide in 

 the bottom of the cistern or water-butt. These 



