‘As long ago as 1901 it was found that, even under what seemed to be the most favourable conditions, certain individual mosquitoes in a successful batch either failed altogether to become infected or became infected very lightly. In 1908, Darling again called attention to the point, and in 1921, Wenyon laid stress on it in recording the results of his feeding experiments in Macedonia.' (486)

'Theoretically the species of anopheles which suck the largest volume of blood at a meal should be those most often found to be infected and to carry the greatest numbers of zygotes. But in the field this is not the usual finding, and even in experimental work it seldom or never happens that in single feeding experiments the number of zygotes found can be correlated at all with the amount of blood ingested, or that the increased infections resulting from second [489-490] and third feeds bear any obvious relation to the amounts of blood taken in at those feeds. The subject is of interest in connection with endeavours to forecast the probability and amount of infection in mosquitoes fed upon carriers whose blood contains a known number of ripe gametocytes (or exflagellating male forms) per cubic millimetre. The late Dr. Darling worked systematically at this problem... He assumed that half the gametes would be females capable of developing into zygotes in the mosquito's tissues, and therefore that in the above example he ought to be able to find 544 zygotes in mosquitoes fed once, 1,088 in those fed twice, and 1,632 in those fed thrice. In actual experiments, however, the numbers of zygotes found never approached those figures, and from the fact that only fifty zygotes were found in a mosquito when 1,632 were predicted by the formula, he concluded that 97 per cent. of all the female gametes taken into the stomach of a malaria-carrying mosquito fail to develop into zygotes. If this were the general rule, it would indicate not only that anopheles are hardly more successful malaria-carriers than culicines, but that any differences of carrying-power between different species of anopheles would be so small as to be inappreciable.' (489-490)