Seasonal variation in size and colour, and daily changes in the distribution of Glossina pallidipes Aust. in the South Busoga forest, Uganda

Abstract

SummaryThe daily distribution of G. pallidipes Aust. was studied on a spiral fly-round covering 180 acres in the South Busoga forest of Uganda in 1954–55. G. brevipalpis Newst. also occurred in the forest, and, rarely, G. palpalis fuscipes Newst. Seasonal changes occur in the size of G. pallidipes, but are less marked than those reported from other places. The size of G. brevipalpis also varies with season. Seasonal colour changes in G. pallidipes indicate that in the driest season about half the puparia experience relative humidity as low as 70 per cent.Catching was done at 349 (later increased to 373) points on the spiral. Mathematically, the catches on a single day, or the sum of many days, conformed to a negative binomial distribution. Although this indicates an uneven distribution, it is pointed out that one negative binomial is compatible with many different distributions on the ground. For this reason the negative binomial is not a satisfactory summary of data which at present can only be shown satisfactorily on a map.Comparison of maps of catches on successive days shows a confusing pattern of vague and ill-defined patches of flies moving about in an unpredictable fashion. A definitive pattern appears only when many days' catches are summed. Such apparently random movement explains the high variance between days' catches previously reported. Sometimes very large numbers of G. pallidipes were found in a small area. As an index of such concentrations, the smallest area which could contain half the day's total catch was determined. Arbitrarily, when half the day's total catch was found in 5 per cent, or less of the total area, a concentration was said to exist. During 16 months' work, concentrations of tsetse, as defined by this criterion, were observed 16 times. Sometimes, but not always, the concentrations followed immediately after the passage of a herd of elephants or buffaloes, and it is concluded that these animals probably caused these concentrations, although the manner in which they may do so remains obscure.