64 The Control of Mosquitoes 



the direction of the river bottom and leave new 

 isolated pools as the water recedes. The high 

 water causes much silt containing plant food to 

 collect on the banks. This in turn produces 

 luxuriant vegetation at the water's edge. Fre- 

 quently the vegetation extends into the water 

 and retards the current near the shore sufficiently 

 to create suitable areas along shore in which 

 Anopheles larvae accumulate and remain. The 

 Rio Grande on the Pacific slope of the Isthmus 

 has been expensive to control because of these 

 conditions, and it has been necessary to have 

 workmen in boats continually on the river for 

 mosquito control. 



Rivers may or may not assist mosquito develop- 

 ment in the wet season as above described. They 

 are generally a most prolific source in the dry 

 season. As the pools, ponds, streams, and marsh 

 lands dry, the rivers become more sluggish and 

 are the only available water sources remaining. 

 Naturally the mosquitoes use them. Where 

 the banks are kept clean, free from vegetation, 

 debris, and stones in shallow water, the larvae 

 are seldom f otmd. Mosquitoes instinctively avoid 

 places that fish and other enemies reach easily, and 

 lay their eggs in more favorable places. They 

 select shallow pools with stony bottoms in the river 



