OLFACTORY SENSITIVITY IN TSETSE-FLIES - A DAILY RHYTHM

The diurnal tsetse Glossina morsitans morsitans bites especially in ea rly morning and late afternoon; around midday feeding is at a low. In laboratory apparatus that measures the amount of locomotion under cons tant conditions over the photophase, the flies display a similar patte rning of activity levels. The profile of daily rhythms for G. morsitan s reported in the literature includes a number of motor and sensory mo tor systems that fluctuate cophasically. Lacking is a study on the pat terning of the senses' response levels. In this paper we present the f irst instance of a daily modulation in the sense of smell. We stimulat ed the antennae with concentration series of host-derived odours and m easured the spiking rate of cells at different times during the photop hase. The concentration-response curves suggest that the sensitivity o f antennal olfactory cells flows in parallel with the other daily rhyt hms. This was also reflected in electroantennograms (EAGs). The electr oantennography was extended to G. fuscipes fuscipes, whose level of sp ontaneous locomotor activity-instead of following a U-shaped pattern-r ises gradually over the photophase. Again, the EAGs appeared to parall el the species' locomotor activity. What we believe happens is that th e organism tones down the sensitivity of its odour receptors during pe riods of anticipated inactivity for reasons of economy.