36 



KARL PEARSON 



(a) From egg to egg (i.e. from laying of eggs, hatching, larval and pupal 

 stages, to laying of eggs again) takes roughly about a fortnight in hot countries 

 with most mosquitoes. In England, gnats may have only one generation or two 

 in a summer, but in the tropics they may go on breeding throughout the year. 

 In cool countries the egg to egg cycle may be prolonged to a month or two. 

 In certain very hot and dry countries, breeding may be checked entirely except 

 during the rainy season. I have accordingly taken 20, 10 and 5 breedings to 

 the year to represent roughly these conditions. 



(/3) Major Ross -distinguishes between "minor vicissitudes," which an insect 

 makes when it hovers round its victim or mate, and " major vicissitudes " which 

 it makes when it passes from feeding place to pool for egg laying. These cor- 

 respond to my "flitters" and "flights." He considers that they go back to 

 water every four or five days, so that a " major vicissitude '' occurs every two 

 days or so. We might therefore take, excluding flitters, the average number 

 of flights to be six or seven. Of course this is the roughest approximation, but still 

 not an unreasonable estimate of what probably takes place in the mosquito's life. 



(y) As to the magnitude of I we have less definite data. Mosquitoes of a 

 rare kind have been said to have been found two or three miles from their breeding 

 place. Major Ross thinks that Anopheles will exceptionally, when no houses 

 are near, probably travel -^ mile for their food, or perhaps further, but he supposes 

 the average distance scarcely to exceed ^ mile, and it may, as houses and suitable 

 pools often abound not more than 50 yards apart, be not greater, perhaps, than 

 100 yards. 



I have accordingly taken 100 yards and 500 yards as likely values for I, and 

 considering 1 per cent, of the boundary value of the mosquitoes' density as a 

 limit to their existence and 5 per cent, as objectionable, we have the following 

 table : 



Table VII. Distances from the Boundary of a cleared but not sterile area at 

 which 1 per cent, and 5 per cent, of the boundary density of Mosquitoes will 

 be found in the course of a Year. 



