Metabolic rates of tsetse flies in the field as measured by the excretion of injected caesium

Teneral tsetse flies, Glossina morsitans morsitans Westw., were injected with labelled caesium ( 137 Cs) <18 h after emergence and released in the Zambezi Valley of Zimbabwe between May 1983 and June 1984, and again in February 1985. Radioactivity in flies recaptured time t days after injection indicated a three‐stage exponential loss of caesium, identical for both sexes. For t≫4 the estimated rate constant (‐0.119 per day) was significantly lower than for 4≫t≫12 (–0.252 per day). By day 15 about 97% of the isotope had been excreted; thereafter the loss rate fell by an order of magnitude. The data for t>4 days were well fitted by the sum of two exponentials but no smooth function was found to fit all three phases. The loss rate from the rapidly metabolized pool increased exponentially with temperature at the same rate as for male tsetse kept in the dark in the laboratory. However, the loss rate in the field was lower at every temperature, suggesting that these flies live at 2–6 o C lower than the average Stevenson screen temperature. Published estimates of hunger cycle and daily flight durations, made on the basis of measured rates of caesium excretion, are invalid because they use the assumption that flies are living in the field at screen temperatures. The data suggest that both sexes have the same metabolic rate up to the age of about 15 days, which implies that the females (being larger and having to nourish a larva in the latter stages of this period) must be less active and/or live at even lower temperatures than the males.