Heat production rates of Eying hornet drones, workers and queens have been measured by means of direct calorimetry. A novel kind of calorimeter including a flight roundabout, to which insects can be fixed, has been constructed for this purpose. Heat production rates of flying drones and workers showed a weak negative correlation with ambient temperature. Mean specific heat production rates for workers were 159+/-38 mW g(-1) at 20 degrees C and 103+/-44 mW g(-1) at 30 degrees C, and for drones 119+/-27 mW g(-1) at 20 degrees C and 70+/-23 mW g(-1) at 35 degrees C, The same weak negative correlation with ambient temperature could be shown for flight speed, whereas wing-beat frequencies were independent of temperature with values between 85 and 97 Hz. Methodical limitations are discussed in the application of direct calorimetry for flight investigations as well as in some biological implications of our results,