192 The Control of Mosquitoes 



more useful, and produce conditions that cease to 

 attract the mosquito. 



Most species of Anopheles will not deposit their 

 eggs in bodies of water that would be unfavorable 

 to their complete development. They shun locali- 

 ties that expose them to natural enemies, lack of 

 food, drying up of water, etc. The difficulties in 

 clearing bodies of water on the Isthmus are the rap- 

 idity with which certain grasses grow up through 

 the water and along shore, and the rapid formation 

 of algae in shallow water exposed to the sun. 

 Where the bottom of a pond is soft some of 

 the grasses may be pulled up by the roots by 

 means of long-handled potato hooks, and give 

 little future trouble. On the Isthmus various 

 attempts have been made to prevent the growth 

 of vegetation, but no satisfactory economical and 

 permanent method has yet been devised. Pre- 

 parations containing arsenical compounds were 

 tested both in the dry and the wet seasons, in 

 ditches, ponds, and depressions on low fiat lands 

 that became dry. The results were never perma- 

 nent, although in some cases vegetation was 

 retarded for several months. Certain grasses 

 that run fiat along the ground and throw out roots 

 at each joint grew over ground recently treated 

 with arsenical compounds without forming roots, 



