Seasonal Variations in the Mean Size of Tsetse Flies

The mean size of the males of the three tsetse species Glossina pallidipes , G. morsitans and G. swynnertoni is negatively correlated with the mean 2 p.m. saturation deficit of 2 months before. The correlation is highest (—0·90) for G. pallidipes , the largest species, and lowest for G. swynnertoni , the smallest. The variation coefficient of mean vein length also is highest for G. pallidipes and lowest for G. swynnertoni . The reason for the apparent 2 months' lag in the effect of saturation deficit on size of flies is that the mean male pupal period is 35 days in Shinyanga (23°C.) and the mean age of males measured is 3–4 weeks. If 1 month's lag is tried, the correlation for G. pallidipes drops from —0·90 to —0·32, and with no lag at all it is —0·24. The correlation with maximum temperature of 2 months earlier is insignificantly lower than that with saturation deficit; it is —0·79 for G. pallidipes .