l64 MADRAS FISHERIES BULLETIN VOL. XI, 



defined and all obstructions which form little backwaters, removed ; 

 the sides should be protected by stone slabs throughout the 

 malarial zone. If this is done the odds against the larvas in their 

 struggle lor existence are enormous and very few are likely to 

 survive. Where cross-channels are taken off the main, care should 

 be taken to avoid the formation of pools, and properly constructed 

 culverts should be adopted. The channel can be made entirely 

 harmless, but generally in the case of a garden supply there is a 

 receptacle for storing small quantities of water ; these are some- 

 times just hollows dug out of the earth, but in many cases small 

 concrete tanks are built; all these should be abolished. 



WELLS. 



All wells should be kept clear of debris and weed and stocked 

 with suitable fish. In the town of Cuddapah, for example, 50 per 

 cent of the wells were found infested with larvae of the malaria- 

 carrying mosquito A. step/icnsi. The stocking of these wells with 

 fish larvicides had a very decided and immediate result ; heavily 

 infested wells were found free from larvse three days after the 

 introduction of suitable fish (Haplochilus). 



LARGE TANKS. 



Mosquitos seldom lay eggs in large, open or wind-swept 



stretches of water, but select secluded shallows where the wave 



action is absent. The danger of these tanks generally lies in the 



unevenness of the foreshore and as the water recedes puddles are 



left which are inaccessible to fish larvicides. These depressions, 



where possible, should be drained by trenches into the tank ; if the 



levels tlo not permit of this, they should be filled in. In fact the 



whole of the foreshore in the malarial zone should be conserved 



and all debris or floating matter in the sheltered bays cleared. If 



stocked with suitable fish when the tank first starts to fill, they will 



have time to multiply and be of great value in keeping the larvee 



down. 



PADDY FIELDS. 



The destruction of small fish by basket traps of minute mesh 

 should be prohibited in all paddy fields in malarial districts and if 

 larvae are found after this the fields should be re-stocked with 

 suitable fish. If the above suggestions are carried out the field 

 can be kept comparatively free from larvae. The paddy fields 



