Role of Body Temperature in the Seasonality of Daily Activity in Tenebrionid Beetles of Eastern Washington

On four occasions between spring and fall we studied populations of eight species of tenebrionid beetles in the arid Artemisia steppe of eastern Washington, USA. We obtained simultaneous but independent measures of (1) the times at which beetles were active, (2) body temperature (T b ) of beetles active on the surface, and (3) meteorological parameters (temperatures, radiation, wind seed). Ninety—seven percent of the 255 T b 's obtained between April and October ranged between 10° and 30°C (mean = 20°), and we observed no systematic seasonal shift T b of active beetles. In contrast, environmental temperature was strongly seasonal, and in order for beetles to be active within the same 10°—30° range of T b at all seasons, their activity patterns were not to the same phase of the daily light—dark cycle at all seasons. Rather, the beetles were strongly day—active in early spring and fall, mostly active during twilight and at night in the midst of summers, and active bimodally (morning and evening)in between these seasons. T b 's predicted with a biophysical model using our meteorological data correlate significantly with actually measured T b 's (r 2 = .70) and can be used to predict satisfactorily the times of day when beetles are active. Some direct measures of environmental temperatures predict T b more accurately than the model. Advantages of biophysical modelling are discussed. The tenebrionid beetles we studied have a range of T b 's during activity which is 10°—15° lower than that of many other terrestrial insects, including other insects in the same habitat and other species of tenebrionids found in more extreme desert environments. We suggest that tenebrionids in the eastern Washington steppe have evolved a low range of active T b 's as part of a strategy by which the season of activity is expanded into earlier and later times of year, when both quantity and quality of food may be greater.