Anopheles Propagation Areas 63 



into the ground. The locations of the portions of 

 the stream that act in this manner may change 

 from year to year, and the observer is never sure 

 that a dry stream is not producing mosquito 

 larvas until he has followed it throughout its 

 entire length. 



A minimum of mosquito larvae will be present in 

 a stream when it has been properly trained, that 

 is, reduced to the minimum width of uniform cross 

 section and freed from stones, grass, and debris 

 that would interfere with the flow or velocity and 

 furnish hiding-places for larvae. 



RIVERS 



The larger rivers, where the banks are steep, 

 are not sources of larvae. As a rule they are less 

 troublesome in the wet season than in the short 

 dry one. If the banks are not much above the 

 average water level, where flat lands are adjacent 

 and floods occur, we have conditions similar to 

 those in lagoons. At times of great floods, twigs, 

 branches, and even whole trees are carried down- 

 stream and when they come to rest may act as 

 collectors and hold grass, vines, and finely divided 

 debris. Such places become sources of mosquitoes 

 that larvae-eating fish or top minnows often cannot 

 catch. Obstructions of this nature at times change 



