PROPHYLAXIS IN MALARIA 251 



Sugar-cane is also satisfactory, for though the drainage 

 required is not so deep, the surface of the ground is so 

 thoroughly worked up that water soaks into the soil and 

 no pools are left for more than a short period, even 

 during a rainy season. Maize also, for similar reasons, 

 is a satisfactory crop. Rice, on the other hand, is an 

 unsuitable crop in a malarious tropical country, as 

 during the early stages of cultivation the earlh is covered 

 with water. Wet rice cultivation should not be allowed 

 within one mile of any settlement, as many anophelines, 

 some harmless and others good carriers of malaria, may 

 breed in the paddy fields. 



In many forms of cultivation irrigation is employed, 

 either as in the paddy fields where the ground is flooded 

 and remains flooded, which is very dangerous, or by 

 conveying water through a series of superficial trenches 

 to keep the subsoil moist. In this form of irrigation, 

 which is used for gardens, vegetables, &c., the risk is 

 slight if the irrigation be intermittent, and the drains kept 

 clear and so graded that water does not remain in any 

 part of the trench for more than one or two days. 



It sometimes happens, however, that the water is drawn 

 from a mosquito-breeding place, and then pupa? may be 

 conveyed by the water to the gardens in or close to a 

 settlement and hatch out in the irrigation trenches. 

 No system of irrigation is safe unless well looked after 

 and in which the supply of water is not so under control 

 that it can be rendered intermittent. Care also must 

 be taken that the overflow is conveyed into a definite 

 channel, natural or artificial, and not allowed to spread 

 over the surface of adjoining land, as though the culti- 

 vated area may be free from anophelines the surrounding 

 district may be made worse. And the adult mosquitoes 

 can readily travel with the prevailing wind for at least a 

 quarter of a mile, and much more if there is scrub or long 

 grass to give them shelter. 



The anophelines usually considered to be carriers of 

 malaria are as follows : 



