Circadian changes in central excitability—the origin of behavioural rhythms in tsetse flies and other animals?

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Abstract
Diel changes in seven behavioural responses of male tsetse flies, G.morsitans, were measured under controlled conditions in the laboratory. These responses involved different motor and sensory modalities, and were evoked by stimuli that were either exogenously applied at regular intervals or were ever‐present. Six (possibly seven) of the responses were modulated across the photophase (of LD 12: 12) in the V‐pattern typical of biting behaviour in the field: morning and evening responses were greatest, noon least. Two of these rhythms were shown to persist in constant conditions, indicating that the underlying control is truly circadian.Comparison of the form of an optokinetic response rhythm at high and low stimulus intensities, implied central control of the response modulation. This, plus the close similarity in phase of the different rhythms, is interpreted as indicating circadian changes in central excitatory state (arousal) as the underlying neurophysiological basis of behavioural rhythmicity. It is suggested that the sarrie arousal system controls the comparable, longer and shorter term changes in excitability that result from starvation, sex, etc.