Measurements of body temperatures of male T. cynosura in the field indicate that temperatures of all three tagmata are elevated above air temperature and that thoracic temperature is unusually high and well-regulated; head temperature may be regulated weakly. Hemolymph circulation between thorax and abdomen probably is augmented at high temperature to enhance heat loss, but the effect is apparently insufficient to account for the decline in heat retained by the thorax. The proportion of time spent hovering is reduced at high temperature, perhaps contributing to reduction of metabolism in these circumstances. The insects are also more likely to patrol in shaded areas at high temperature, but the ability to thermoregulate is evidently independent of solar radiation; in the laboratory, however, body temperature is strongly dependent on radiant load. The capacity for endothermic warm-up is well-developed; the head is warmed as well as the thorax, but to a much smaller extent. Despite excellent thermoregulation, territorial activity is restricted by low ambient temperature.