Earlier work in Nyanza Province, Kenya, had led to the conclusion that riverine communities of Glossina palpalis fuscipes Newst. were permanently hungrier than lake-side communities. This was based on analysis of catches (percentage of teneral flies in the whole catch, and percentage of females among non-teneral flies) and on a behaviour criterion (percentage of non-teneral males caught on the party), but subsequent observations in the two types of habitat, in which a different area was used to represent the lake side, showed the lake-side flies to be slightly smaller than the riverine flies and not to differ in fat content, and a further investigation was accordingly made.
Puparia were collected on three dates between January and March 1958 and their surface area measured. A measure of the thoracic surface and fat content was taken of all adults emerging. Non-teneral flies were collected, for comparison, about two weeks later than the puparia.
In all samples, the riverine puparia were significantly larger than those from the lake side, and flies that emerged showed a comparable difference in size, but not in fat content corrected for size. Non-teneral riverine flies were slightly larger than lake-side ones but the difference was less than half that shown by the puparia and the teneral flies obtained from them. Evidence is adduced to show that this discrepancy is due to elimination of the lower size-groups in the lake-side community. Estimates of fat content of non-teneral flies support the conclusion that the lake-side community was under stress as compared with the riverine one, and analysis of available fly-round data from the lake side also suggests stress conditions.
The lake-side results are at variance with those from the site used in the earlier work. Available meteorological data show that the average temperature during February and March is 2°C. higher at the present site than at the former; such difference may directly cause the stresses that appear to be operating in the present instance.
The results suggest that the percentage of non-teneral males caught on the party has a real meaning in terms of nutritional status, and that it is also a very responsive indicator, but that the proportions of tenerals and of female nontenerals may not be simply related to the nutritional state as had hitherto been supposed.