Natural Enemies 183 



is to-day, but the adults that traveled against the 

 breeze to Corozal were tracked back to this source. 

 Small fish were useless in this case, they could not 

 reach the larvae. 



In certain parts of the Bas Obispo River during 

 the dry season, shallow water varying in depth 

 from an inch to a foot ran over a bottom covered 

 with stones and gravel. A thin film of oil was 

 generally present, but seemed in no way to interfere 

 with the numerous small fish. We knew that 

 mosquitoes were still there in spite of oil and fish. 

 The film was apparently inadequate to suppress 

 many of the larvae, probably owing to free air 

 spaces on the downstream side of some of the 

 partially exposed stones. 



In the camps affected by this river the malaria 

 fever rate had been higher in the dry than in the 

 wet season. At all other camps in the Canal Zone 

 we had more cases of fever in the wet season. When 

 a heavy application of larvacide was given to this 

 moving water, numerous Culex and Anopheles 

 larvae immediately appeared at the surface. Some 

 of the fish affected jumped out on to the banks and 

 the remainder were killed. Since that time the 

 local fever rate during the dry season has not 

 exceeded that of the wet season, and year after 

 year the adjacent camps have had few mosquitoes 



