The control of circadian activity rhythms in tsetse flies: environment or physiological clock?

ABSTRACT. The typical V-shaped pattern of biting activity that Glossina morsitans show in nature has been assumed to be due to environmental control (especially by temperature), but is now known also to be under control from an endogenous circadian clock. The relative importance of these two components was investigated (a) under constant conditions in actographs, and (b) by a re-analysis of published field data. The actograph study reveals: (1) the length of flight bursts is the same at all phases of the rhythm; (2) the shape of the circadian V-pattern changes with its amplitude, so that the less active a fly is, the more of its activity it performs in the morning; (3) both arms of the V respond to dawn as their principal zeitgeber; (4) high (35°C) and low (19°C) constant temperature reduce spontaneous activity level by c. 80%, and depress the evening arm of the V disproportionately. The analysis of the field data reveals: (1) the V occurs in nature independently of temperature; (2) biting activity is correlated positively with temperature in the morning and evening, but negatively through the middle of the day; (3) light directly influences the daily rhythm mainly by its effects at low intensity (the 'photonegative reaction' being a response to high temperature); (4) humidity has little influence on activity level. It is concluded that roughly 80% of the amplitude of the V-pattern in the field is due to endogenous circadian control, and only 20% to direct control by temperature.