62 The Control of Mosquitoes 



at the water stirface it is always well to remove 

 sufficient stones to be sure that they are not hiding 

 underneath. Frequently they remain hidden for 

 long intervals. When the stone under which the 

 larvae are hiding is moved, they will rapidly travel 

 to the next nearest stone that affords a suitable 

 hiding-place. 



As the dry season advances, the source of hill- 

 side streams moves farther downhill, the upper 

 part remaining dry. In case the stream bed is of 

 irregular grade and contains deep depressions, 

 as the bed becomes dry pools remain and become 

 breeding places. Should the stream be of great 

 length it is so easy to assume that its entire upper 

 length is dry that such places are apt to be over- 

 looked. Eventually the pools become dry, but 

 generally produce one or more crops of mosquitoes 

 before this happens. During the dry season, if a 

 shower falls, the stream bed will probably be dry 

 within a day or two, but it is never certain that 

 these depressions have not collected water which 

 they may retain for a week or more. 



In the upper portion of the stream bed and 

 higher than its dry season source, the sub-surface 

 flow may outcrop at one or more points, and, due 

 to the soil formation, grade, etc., flow along the 

 stream for a short distance and again disappear 



