246 TROPICAL MEDICINE AND HYGIENE 



M. rossi, which practically does not carry malaria. Asiatic 

 towns, therefore, are relatively free from malaria, whilst 

 in African and South American towns a much more 

 complete drainage would be required to have any material 

 effect on the amount of malaria. 



N. willnwri, M. culicifacies, and A T . karivari, all stream 

 breeders in open country, are good carriers. 



M.funesta in Africa and many of the important Asiatic 

 malaria-carrying mosquitoes (e.g., M. culicifacies) are only 

 found in clear water which is usually in motion, and in 

 grass-grown edges of springs, streams and rivers. They 

 cannot thrive in stagnant waters. They may be found 

 in marshes, but only when the water is kept fresh by 

 springs, or streams, and during seasons when there is 

 a heavy rainfall. 



The problems to be dealt with will vary according 

 to the breeding-places of the dangerous mosquitoes. 

 Where, as in a settlement, there is artificial interference 

 with the natural drainage, as in making roads, fords 

 and bridges, the problem is more complicated. In such 

 places frequently hollow r s have been left in house building 

 or road making, and there are trenches at the roadsides 

 or for local drainage which must also be dealt with. In 

 the East such places are of little importance in the 

 towns, whilst in Africa they are of vital importance. 



As a general principle, superficial drainage must be so 

 complete that even during the rains the shallow pools 

 formed only last for a day or two, or at any rate not for 

 more than a week, and drains must be so graded that 

 with heavy rains they are well flushed and no pools are 

 left of any depth in the intervals. 



In all places where there is a hill at the back of a settle- 

 ment a well-devised intercepting dram must be arranged 

 along the base of the hill so that flooding from the hill 

 sides will be avoided. This hill water is the main cause 

 of the constant high level of subsoil water; the rain 

 actually falling on the area of the settlement is com- 

 paratively unimportant. In low-lying land, as in many 



