The tiger beetle Cicindela hybrida is a diurnal predator inhabiting open sandy areas. The activity pattern on a sunny day in May with a mean maximum surface temperature of 40°C is described (Fig. 2). The inactive period is spent in burrows in the sand and it is suggested that a threshold of 19°C releases the daily round of activity (Fig. 5). The animals appear on the sand between 7.00 and 10.00 and the number reaches a maximum at 34-42°C (surface temp.). The duration of the activity period is indirectly determined by the number of available prey which is maximal at 36°C and decreases both at higher and lower temperatures. Mean onset and termination of activity occur at about 28°C (surface temp.) when the expected capture rate is 3 per hour. In actograph experiments, onset was controlled by the illumination and occurred several hours before "dawn" (Fig. 9). 2. The preferred body temperature is at about 35°C and thermoregulatory behaviour patterns are used in order to approach this. At low ambient temperatures, body temperature is increased by sun-basking, while at high temperatures stilting is used to prevent it from increasing above 35°C. If the temperature becomes intolerably high, the animals dig into the sand (Figs. 8, 16). Measurements of body temperatures in the field show that tiger beetles have partial regulation (Fig. 10) and it is suggested that this is because the cost of thermoregulation is low, but prey is not abundant enough to allow a full exploitation of this, and predation (hunger) interferes with the thermoregulation. 3. An estimation of the daily water loss under field conditions is presented based on measurements of the water loss in the laboratory at different temperatures and relative humidities. These values are weighted by the times actually spent at different combinations in the field (Figs. 17, 19). The loss amounts to about 10% per day of initial body weight, and it is concluded that tiger beetles have not evolved any special adaptations with respect to this factor.
