458 MOSQUITOES 



termination, not necessarily in a whole continent or a whole 

 country, but in local places. Only comparatively recently has 

 the local extermination or even reduction of mosquitoes ceased 

 to be looked upon as too vast an operation to be undertaken. 

 Because ponds or marshes were known to exist, perhaps miles 

 away, the value of destruction of such breeding places as rain 

 barrels, tin cans full of water, cesspools and troughs was looked 

 upon as a mere drop in the bucket. Knowing as we do now that 

 in most cases every annoying mosquito which attacks us was 

 born and bred within 200 yards of where we meet her, the 

 local extermination of mosquitoes has taken on a very different 

 aspect. It is difficult for the uninitiated to realize that the 

 mosquitoes which make life miserable for him did not travel from 

 distant marshes and ponds but were probably bred in his own 

 backyard or in his own living room. 



Wonderful results have been obtained by the destruction of 

 larvae in their breeding places. This is accomplished either by 

 pouring into the water some substance which will form an emul- 

 sion, and will destroy the larvae when very dilute, or by pouring or 

 spraying some oil on the water which will spread out and form 

 a thin film over the whole surface. When the larvae rise to obtain 

 air through their breathing, tubes or pores, the latter become 

 plugged by a tiny bit of oil, and the larvae drown. It has recently 

 been pointed out by Lima, in Brazil, that the drowning is has- 

 tened by the coating of the body of the larvae by the oil, especially 

 in Anopheles, so that air cannot be absorbed through the body 

 wall. 



The oil film is the method most commonly employed, espe- 

 cially for use on a small scale. Except for wind-swept bodies of 

 water, ordinary petroleum is as cheap and efficient as any oU 

 that can be obtained. The oil film is so thin and light, however, 

 that it is blown aside by a high wind, and a considerable portion 

 of the water left uncovered. Different grades of oil can be used, 

 varying with conditions. The thick heavy grades do not readily 

 form a uniform film, especially if obstructed by water weeds, 

 whereas the very thin oils evaporate rapidly, and the film is easily 

 broken. Howard, Dyar and Knab recommend a grade known 

 as " light fuel oil " for ordinary use. These authors state that 

 about one ounce of petroleum to 15 square feet of water surface 

 gives satisfactory results, and produces a film which lasts for 



