Olfactory sensitivity in tsetse flies: a daily rhythm

The diurnal tsetse Glossina morsitans morsitans bites especially in earlymorning and late afternoon; around midday feeding is at a low. Inlaboratory apparatus that measures the amount of locomotion under constantconditions over the photophase, the flies display a similar patterning ofactivity levels. The profile of daily rhythms for G. morsitans reported inthe literature includes a number of motor and sensory motor systems thatfluctuate cophasically. Lacking is a study on the patterning of the senses'response levels. In this paper we present the first instance of a dailymodulation in the sense of smell. We stimulated the antennae withconcentration series of host-derived odours and measured the spiking rateof cells at different times during the photophase. Theconcentration-response curves suggest that the sensitivity of antennalolfactory cells flows in parallel with the other daily rhythms. This wasalso reflected in electroantennograms (EAGs). The electroantennography wasextended to G. fuscipes fuscipes, whose level of spontaneous locomotoractivity--instead of following a U- shaped pattern--rises gradually overthe photophase. Again, the EAGs appeared to parallel the species' locomotoractivity. What we believe happens is that the organism tones down thesensitivity of its odour receptors during periods of anticipated inactivityfor reasons of economy.