ACTIVITY RHYTHMS OF TSETSE-FLIES (GLOSSINA SPP) (DIPTERA, GLOSSINIDAE) AT LOW AND HIGH-TEMPERATURES IN NATURE

Glossina morsitans morsitans Westwood and G. pallidipes Austen were caught hourly on electric nets from dawn to dusk in the Zambesi Valley, Zimbabwe, in cold and hot weather. When middle of the day temperatures reached only 24-degrees-C, activity of G. morsitans was at a low level from dawn to early afternoon but then rose to a dusk peak (most marked in males); G. pallidipes behaved similarly, but showed almost no activity before noon. When dawn temperatures rose above ca. 24-degrees-C (and afternoon temperatures above 38-degrees-C) the evening peak disappeared, to be replaced by a peak at dawn (and with almost no activity occurring through the rest of the day). This reversal of the more usual field activity pattern of late afternoon maxima reveals that the positive correlation between activity level and temperature at dusk (Brady & Crump, 1978) breaks down above ca. 33-degrees-C, although the earlier conclusion that the typical U-shaped activity pattern of tsetse flies is mainly driven by endogenous timing is still valid for moderate temperatures.