126 The Control of Mosquitoes 



seepage water and springs complicate the problem. 

 Where these conditions are found the use of the 

 herring-bone type of ditch is often unsatisfactory, 

 and the position of the branch ditches may depend 

 to a large extent on the location of the seepage 

 water and the points at which it comes to the 

 surface, "seepage outcrops." 



A man who has had extensive practice in drain- 

 age for anti-malaria purposes can do much of the 

 minor detail work without supervision, but where 

 grades are light and work extensive, it is essential 

 to have levels taken, to profit by all the existing 

 grade. If ground and relative elevations are 

 judged by the eye alone, wrong conclusions may 

 easily be drawn as to the possibilities of drainage, 

 and it is too late, or at least more expensive, to cor- 

 rect errors after the actual work is well advanced. 



It must be kept in mind that some swampy 

 areas, for reasons not yet thoroughly understood, 

 are not sources of Anopheles. In other cases they 

 may be the source of non-malaria conveying 

 species. Where possible, without increasing the 

 cost of the drainage work, it is often advisable to 

 determine the most prolific sources of the malaria- 

 carrying Anopheles, and to give the drainage of 

 these places preference over other work. 



Seasonal changes may affect the production of 



