280 OUR INSECT FRIENDS AND FOES 



surface to take in air. If malaria and yellow 

 fever are to be stamped out, the* mosquitos, 

 which are the transmitting agents, must be 

 exterminated, and this, thanks to our knowledge 

 of the insect's life-history, is being successfully 

 accomplished. So far as is possible all swampy 

 lands in the immediate vicinity of towns and 

 villages are being drained, for the mosquito 

 does not appear to be able to fly easily for any 

 great distance from its breeding place, half a 

 mile apparently being well beyond the normal 

 distance of its flight. Again, all stagnant pools, 

 open troughs, etc., which form favourite breed- 

 ing grounds, have their surface carefully covered 

 with a film of crude oil, which successfully 

 prevents the mosquito larvae and pupae taking 

 in air when they rise to the surface to breathe, 

 the oil choking the stigmatic opening through 

 which the air is drawn, and so the insects are 

 swiftly drowned. The great difficulty, of course, 

 is to keep an absolutely unbroken surface of oil 

 on the water, wind and slight currents causing 

 the film of oil to crack, and so offer clear spaces 

 where the larvae and pupae will quickly con- 

 gregate to breathe. But in spite of the many 

 difficulties which they have to surmount, those 

 who are engaged in this important work are 

 achieving wonderful success, and it is not too 

 much to hope that in a very few years' time, 

 many large areas now the prey of malaria and 

 yellow fever will, through the extermination 

 of the mosquito, have become prosperous and 

 healthy. Already these two diseases have 

 been greatly reduced in Cuba, and to a very 



