The heat production rates of an intact hornet (Vespa crabro) colony and a honeybee (Apis mellifera) winter cluster were measured by means of direct calorimetry. Two camping cold boxes with Peltier elements as cooling systems were used as simple and inexpensive differential calorimeters. The boxes had a volume of 24 1 and sensitivities of between 6.8 mV W-1 and 14.8 mV W-1, depending on the experimental setup. They were modified in such a way that the foragers had free access to the outdoor environment. The average daily heat production rate of the hornet colony ranged from 1.2 W at the end of the season in October up to 12.5 W at the stage of maximum biomass in August. The heat production rates of a honeybee winter cluster were measured in the following winter at ambient temperatures of 0-5 degrees C. The maximum heat production rate was attained at O degrees C (20.5 W) and the minimum heat production rate at 5 degrees C (11.5 W). These results are compared with data for energy metabolism obtained respirometrically and by artificial heating of empty nests, respectively.