Attack by Drainage 129 



ditch is dry. In many cases these depressions may 

 be filled with stone, well rammed into place and 

 chinked, and the original grade of the ditch bottom 

 reestablished before much harm is done. There is 

 a tendency to neglect these holes, and to assume 

 that the ditch is dry and does not need inspection. 

 This assumption would result in Anopheles pro- 

 duction in the depressions below the ditch grade 

 line. 



- When ditches contain water continuously, or 

 for periods long enough to bring Anopheles larvae 

 to full development, they must receive regular 

 weekly inspection. During the part of the year 

 when large volumes of water pass off in short 

 periods, the ditches are generally swept free of 

 mosquito larvae. When the average flow is fairly 

 rapid, the same effect is produced. 

 ■ It is when the velocity of the water becomes 

 retarded that conditions become most favorable 

 to mosquito development. In open ditches having 

 a rather fiat grade such conditions often prevail, 

 and vegetation within them may develop rapidly. 

 Both grass and aquatic vegetation assist in re- 

 tarding the stream flow, besides furnishing food 

 and hiding-places for mosquito larvae. The retard- 

 ing of the current causes silt to deposit and this 

 affords sufficient plant food to make vegetation 



