Anopheles Propagation Areas 73 



remain favorable to that species. We do not 

 think this to be true. Certain changes take place 

 the nature of which is not yet well understood. 

 The fact remains that within a short period one 

 species will sometimes disappear and be replaced 

 soon afterwards by a large number of larvae of 

 another species. There are changes in the food- 

 stuffs present, formation of algae, plant growth, 

 etc., and also other factors and changes that are 

 at the present time unknown. Changes of species 

 have been observed in relatively large bodies 

 of water, such as lagoons, wide parts of rivers 

 containing much aquatic growth, and in large 

 ponds. In 1913 when the lake at Gatun was 

 formed, conditions appeared to be quite favorable 

 to the development of Anopheles malefactor and it 

 became the prevailing species at Gamboa from 

 November, 1913, to February, 1914, while previously 

 the Anopheles albimanus was more common in 

 houses. Since the last mentioned date the Anoph- 

 eles albimanus has replaced it and later constituted 

 about ninety per cent, of all Anopheles at Gamboa. 

 A similar change is taking place at Cano Camp, 

 southwest of Gatun, on the shore of the lake. At 

 that point the jungle is flooded with water and 

 most of the trees and brush are now dead. Much 

 of this growth was alive when the camp was first 



