Anopheles Propagation Areas 6i 



"Pot-holes," or circular depressions, often form 

 in the stream bed. The current may be swift only 

 at the center and larvae may be present at the 

 edges where the debris collects. A sharp down- 

 pour will often collect most of the debris, algae, eggs, 

 larvae, and pupae and carry them on downstream. 

 They may, and often do, collect in quieter places 

 in the larger stream or lagoon below. Where 

 streams flow toward a settlement and empty into 

 quieter waters near it, the adult mosquitoes in 

 houses often increase suddenly, due to the new 

 supply of pupae and larvae thus transported toward 

 it. 



It must not be assumed that algse in ditches 

 when treated with oil or larvacide will cease to 

 exist. A week or less may start and complete 

 a new growth. Often crude oil and algas unite 

 and form a mat that floats on the surface and in 

 a few days a part or all will settle to the bottom 

 of the stream; or may collect in irregular depres- 

 sions along the bank. Such conditions offer 

 hiding places for larvae and make inspection 

 both slower and more difficult. In shallow streams 

 having coarse gravel or stones on the bottom and 

 along the wetted perimeter, when debris, grass, or 

 well-developed algae are absent, larvae hide under 

 the small stones, and though none may be detected 



