MOSQUITO CONTROL 277 



grow on the edge of the bank. Tree roots must be removed from the 

 stream. Any kind of vegetation growing in the bed of a stream favors 

 mosquito breeding as it affords some protection against natural enemies, 

 and prevents adequate artificial control. The stream bed must be clear 

 of vegetation. The banks must be straiglitcned and without overhanging 

 ledges. There should be no obstruction to the free flow of the stream. 

 If it meanders, a new and straight course ought to be constructed and 

 tlie old course filled. Springs which furnish good water should be boxed 

 and protected. Le Prince and Oronstein A^ery ably describe in their 

 book tlie method of clearing streams and propagation areas in jungles 

 in the tropics. 



Clearing of Weed-Filled Bays and Lakes 



Large bodies of water in which dense growths of grass and weeds 

 occur furnisli great problems in many localities, and in tropical coun- 

 tries especially, where feasible, it is often desirable to furnish the mos- 

 quito squad with two motor boats and submarine saws or other imple- 

 ments for cutting and removing vegetation. If this cut vegetation remains 

 it aggravates the situation. Large lily leaves, which when alive furnish 

 no place for breeding, will often, when dry, form cups for water in 

 which mosquitoes breed prolifically. 



Drainage 



The construction of drainage systems should be done preferably by a 

 sanitary engineer who understands the mosquito phases of the problem. 

 The main ditches should be constructed first and later the laterals added. 

 Sometimes where weed growth is rapid it is desirable to have a double 

 parallel series of ditches, one only operating at a time except during heavy 

 rains, with the idea that the idle ditch can be cleaned and shaped up. 

 It is essential that the floor level of the ditch affords no opportunities 

 for puddles to form after the greater part of the water has passed off. 

 In permanent ditching it is sometimes feasible and advisable to line the 

 ditch with concrete or at least to line the bottom. Weep-holes should be 

 made at sufficient intervals to carry into the drain water whicli gathers 

 on the outside of it. Branch ditches should enter the main ditch at an 

 acute angle or on a curve. At the junction of ditches there should be a 

 splash wall to confine the water A\athin the ditch. Pot-holes formed in 

 dirt ditches should be filled up after rains with gravel or stone and tamped 

 hard (see Le Prince and Orenstein, pp. 137-144). 



In certain soils where seepage water outcrops abundantly on hill- 

 sides, it is sometimes practicable to install an intercepting tile drainage 



