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No abstract

Some male butterflies defend specific mating sites, e.g. sandy patches (Hipparchia semele) or plants (Ochlodes venata). When perching within its territory, a male orients the body axis and tilts its wings and body in order to control the body area exposed to the sun, and thereby keeps its body temperature (T ) as close to a preferred level as possible. In accordance with a model presented here, these behaviours can be separated into three successive phases. At low temperatures, the males maximized the heat load by exposing the maximum body area (sun-basking). This raised T above the temperature of a non-regulating animal by c. 3° C. At an intermediate range of temperatures, T was kept constant at the preferred level by means of a gradual change of body orientation and posture (graded phase). At high temperatures, the heat load was minimized by exposing the minimum body area. This lowered T below that of a non-regulating animal by c. 2.5° C. H. semele went through all three phases, but O. venata only reached the basking phase due to a more moderate microclimate. Three types of thermoregulation in ectothermic animals and their functions are discussed. Thermoregulation in territorial male butterflies serves to prepare the animal for efficient flight performance if another male should try to take over the territory, or a predator attacks. The males also made frequent short flights, spontaneously or elicited by other insects. Their duration was independent of temperature, and they may function as a sexual signal.

1. 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.

ABSTRACT. The daily onset of flight activity in nocturnal moths from Denmark and Florida was investigated in natural and experimental reductions of illumination. The daily cycle of illumination is described, especially the variation of the rate of change during the evening. Different species commence activity (‘onset’) at different mean light intensities. These means occur around the termination of twilight, at between 0 and –2.50 log lux. In a given species the illumination at mean onset is constant, and the dispersion of onsets in the population is constant with respect to the illumination. As a consequence, the dispersion with respect to time increases, the longer the duration of twilight. The temporal dispersion, therefore, depends upon season and latitude. A model describing the number of onsets in relation to the decrease of illumination is presented. At the physiological level it is proposed that a preparatory process precedes the actual onset of activity. This is observed as a latency between the light change and the activity onset. It is presumed to represent a change, from an inhibiting to a disinhibiting output from a controlling CNS centre, brought about by the reduction in illumination increasing the sensitivity of the centre to a ‘sensitizer’. This might represent the Zeitgeber effect of illumination reduction. The results indicate that during a gradual decrease of illumination, the latency process is initiated by a certain low illumination and that it proceeds further, the lower the illumination. The faster the rate of decrease, the faster is the rate of the process, so that it is completed at about the same time as a threshold illumination for overt activity is reached, irrespective of the rate of decrease. However, at very fast reductions of illumination, the physiological process cannot keep pace with the reduction, and activity onset occurs at a lower illumination than normal.