Printed in Great Britain THERMOREGULATION IN SMALL FLIES (SYRPHUS SP.): BASKING AND SHIVERING

1. Flies of the genus Syrphus aggregated at specific sites in the field ('leeks'). Flies at leeks were always capable of 'instant ' take-of, even at ambient temperatures of 10 °C or less. 2. The flies regulated their thoracic temperature by a combination of basking and shivering. During hovering flight in sunshine thoracic tempera-ture rose 12-14 °C above the ambient temperature. 3. The flies engaged in frequent brief chases while at the leeks. 4. At an air temperature> 18 °C the flies at the leek remained in hover-ing flight most of the time. 5. The vibration frequencies of the thorax during shivering and flight ranged from about iooto 200 Hz at 10-27 °C, though at a given temperature and spike frequency the vibration rate during warm-up was higher than the wing-beat frequency (assumed to be the same as thoracic vibration frequency) during flight. 6. During shivering, but not in flight, there is a tendency for the indirect flight muscles to be activated in synchrony