The behaviour of Glossina morsitans and Glossina swynnertoni through the trophic cycle is described. It is suggested that the favored hosts of these tsetse flies are likely to be feeding at the times of day when the flies are active, and therefore that most feeding by the tsetse flies occurs while the hosts are also feeding. It is also argued that the slow, intermittent movements of grazing, browsing or foraging mammals allow large concentrations of "following" flies to build up around the hosts.
