100 PREVENTIVE MEDICINE 



are generally many, scattered at various distances over the country; 

 and the insects are known to feed on cattle, birds, and other ani- 

 mals. For towns, where anti-mosquito measures are most demanded, 

 our first assumed condition of uniform attractiveness must, as a 

 rule, be the one in force; and in such cases the centripetal law will 

 hold. 



The effect of wind requires examination. Theoretically, if the 

 insects are supposed always to remain on the wing, wind blowing 

 on a generating-pool will merely have the effect of drifting the whole 

 brood to a certain extent in one direction without changing the 

 relative positions of the insects to each other. The result would be 

 the same as indicated in Diagram I, except that the generating- 

 pool would now be eccentric. If a proportion of the insects take 

 shelter, the circles of Diagram I would become ellipses with the 

 generating-pool as a focus. In such a case the wind, and especially 

 devious winds, would have a distributive tendency; but it must be 

 remembered that if the insects are scattered further apart their 

 numbers at a given point must be reduced. A wind which blows 

 mosquitoes into an area must blow others out of it. The net result 

 of devious winds on a circular drained area would be that the mos- 

 quito-density is not so much reduced at the centre, but is reduced 

 to a greater distance outside the boundary circle so that the 

 average reduction remains the same. With a wind blowing continu- 

 ously from one direction, the indication would be to extend the 

 drainage further in that direction. Obviously, wind may scatter 

 mosquitoes ; but it cannot create them, nor prevent the total average 

 reduction due to anti-propagation measures, as some people seem 

 to think. It is, however, very doubtful whether wind does really 

 drive or scatter mosquitoes to any great degree. In my experience 

 they are extremely tenacious of locality. Thus Anopheles were 

 seldom seen on Tower Hill, a low open hill in the middle of Freetown, 

 Sierra Leone, although numerous generating-pools existed a few 

 hundred yards from the top, all around the foot of it, and the winds 

 were often very strong. If a continuous wind can drive mosquitoes 

 before it, then during the southwest monsoon in India they should 

 be driven away from the west coast and massed towards the east 

 coast; but I have never heard that they are at all less numerous 

 on the west coast. I have often seen very numerous mosquitoes on 

 bare coasts exposed to strong sea-breezes, as at Madras. As a rule, 

 they seem to take shelter in the presence of a strong breeze. Instances 

 of their being driven far by winds are frequently quoted, but in 

 my opinion they were more probably bred, in many such cases, in 

 unobserved pools close at hand. The wind-hypothesis is frequently 

 used by municipal officials as an excuse for doing nothing it is 

 convenient to blame a marsh miles distant for propagating the mos- 



