LOGICAL BASIS OF MOSQUITO-REDUCTION 99 



sents with accuracy the effects of thorough suppression of propaga- 

 tion within a circular area. 



At the circle (a) and beyond it the mosquito-density will be the 

 normal density which existed before the operations were commenced. 

 At (6), the circle bounding the drainage operations, the density will 

 always be about half the normal density. At the circle (c) and within 

 it, the density will be small, inappreciable, or zero. The distance from 

 (a) to (6) may be taken as being the same as that from (6) to (c); 



DIAGRAM III. Effect of drainage of a circular area. b = boundary of drained 

 area. Mosquito-density begins to diminish at the circle a; becomes one half 

 at the boundary 6; and is small, inappreciable, or zero at the circle c. 



and, as the mosquitoes penetrating from (6) to (c) must be drawn 

 from the zone between (a) and (6), the average result will be the same 

 as if no immigration at all takes place. We do not possess sufficient 

 data to enable us to calculate the actual distance between (a), (&), 

 and (c) this will depend in a certain measure on the activity of 

 the species of insect concerned and on the existence or absence of 

 special local attractions; but this fact does not discredit the general 

 principles involved. 



One case has not yet been considered, namely, that in which 

 there exists only a single feeding-place in the whole tract of country 

 such, for instance, as a single house or group of houses situated 

 in the midst of deserted swamps. In such a case the insects may be 

 compelled to come from considerable distances from as far as 

 their senses are capable of guiding them in search of food ; and 

 drainage operations carried on with a view to relieving such a house 

 may, for all we know, have to be extended over miles. But such 

 cases are not of great consequence, because drainage is seldom the 

 appropriate measure for isolated dwellings, which can generally 

 be protected at far less cost by means of gauze screens. Moreover, 

 it is very doubtful whether feeding-places for mosquitoes are ever 

 so solitary as the case assumes. Where there is one dwelling there 



