134 The Control of Mosquitoes 



Under normal conditions the subsiirface drainage 

 systems cost practically nothing to maintain. 

 Many of the drains installed seven years ago have 

 received no attention whatever since they were laid, 

 and are doing their work now as well as when new. 

 Anopheles are not produced in subsurface drains. 



A few essential points to be kept in mind for 

 intercepting tile drainage for Anopheles eradication 

 are as follows: No water should be allowed to 

 enter the upper end of the tile drain or of its 

 branches. The grade of the trench bottom should 

 be true; tiles must not be located on soft mud, 

 where they may sink. Where the tile line comes 

 near the surface, due to topographical variations, 

 proper bridge crossings must be made, so that 

 wagons wiU not pass over and crush the tile. 

 Greasy water and house waste must not be 

 allowed to discharge into any part of the tile line 

 system. If drainage from roadside ditches or 

 excavated areas is turned onto the cover stone, a 

 tile line will probably become useless. 



The profile of a proposed line will indicate the 

 amount of material to be excavated, and the depth 

 of the trench. It may often be advisable to use 

 one or more branch lines to include all the seepage 

 water, as the cost of excavation and cover stone 

 may be less than that of a single deep ditch. 



