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Abstract: Foraging honeybees are subjected to considerable variations of microclimatic conditions challenging their thermoregulatory ability. Solar heat is a gain in the cold but may be a burden in the heat. We investigated the balancing of endothermic activity with radiative heat gain and physiological functions of water foraging Apis mellifera carnica honeybees in the whole range of ambient temperatures (T a) and solar radiation they are likely to be exposed in their natural environment in Middle Europe. The mean thorax temperature (T th) during foraging stays was regulated at a constantly high level (37.0–38.5°C) in a broad range of T a (3–30°C). At warmer conditions (T a =30–39°C) T th increased to a maximal level of 45.3°C. The endothermic temperature excess (difference of T body − T a of living and dead bees) was used to assess the endogenously generated temperature elevation as a correlate of energy turnover. Up to a T a of ∼30°C bees used solar heat gain for a double purpose: to reduce energetic expenditure and to increase T th by about 1–3°C to improve force production of flight muscles. At higher T a they exhibited cooling efforts to get rid of excess heat. A high T th also allowed regulation of the head temperature high enough to guarantee proper function of the bees’ suction pump even at low T a. This shortened the foraging stays and this way reduced energetic costs. With decreasing T a bees also reduced arrival body weight and crop loading to do both minimize costs and optimize flight performance. [ABSTRACT FROM AUTHOR]