152 The Control of Mosquitoes 



to hold themselves at the surface. Possibly a 

 combination of three causes kills the larvs : 



1. Specific toxicity of the oil to the larvae and 

 pupae. 



2. Minute particles of oil clogging the breath- 

 ing tube. 



3. Reduction of surface tension, making it 

 difficult for the larvae to remain long enough at the 

 surface to make a rupture in the oil film and thus 

 obtain air. 



Oil appears to be toxic to mosquito larvae. We 

 have observed that a number of larvae die shortly 

 after coming in contact with it. Mere deprivation 

 of air does not cause death very rapidly in mos- 

 quito larvae. In submerged cages the larvae and 

 pupae of Anopheles often survive more than an 

 hour. 



Probably the minute particles of oil find their 

 way into the breathing tubes, and cause death by 

 clogging. We have seen larvae take their breathing 

 tubes into their mouths after the contact with 

 the oil and apparently make frantic endeavors to 

 remove some offending substance. 



The decrease in surface tension is also a factor, 

 although its importance varies with the various 

 genera of mosquitoes. The Aedes calopus can 

 remain at the bottom of a water vessel for a long 



