Anopheles Propagation Areas 75 



numerous where the fallen leaves were most 

 plentiful. These leaves were about eighteen 

 inches wide and four feet or more in length. At 

 Beach Island, the wet area, a mangrove swamp, 

 was less brackish and well shaded by trees. It 

 had a depth of from two to twelve inches and was 

 filled with stumps of young trees relatively close 

 together. No algse and very little debris were 

 present. The young larvas were more numerous 

 than those further developed. The older larvae 

 kept closer to the stumps. Probably the fish 

 preferred them ; this would account for the relative 

 absence of large larvae. In the swampy area in the 

 Rio Grande valley the percentage of salt water 

 varied with the tide and rainfall. Anopheles 

 alhimanus was the prevailing species. The deep 

 water contained many mangrove trees and drift 

 from upstream, while the more shallow water 

 was well covered with grass, dead leaves, and 

 plants that thrive in brackish water in the tropics. 

 Larvas could always be found in untreated portions 

 of this area where there were sufficient hiding- 

 places. The area was about a mile in length. 



GATUN PROPAGATION AREA IN I9I3 



The largest influx of Anopheles that occurred in 

 the Canal Zone during the canal construction 



