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The O2, CO2, and H2O exchange of single flying male bumblebees (Bombus lucorum and B . terrest,ris) were measured simultaneously. A respiratory quotient RQ = 1 was found for all activities investigated (torpor -flight). The dependence of respiratory CO2 production in flight on body-weight was measured: for a 2.20-mg male bumble-bee it amounts to 24.5 mg C02/h ( = 56.4 ml 02/ g h). The corresponding evaporative water loss amounts to 6 mg H2O/h. Males transferred to a climatic test chamber and conditioned to artificial flower feeders started to fly, after a few days of acclimatization, in typical scent-marked flight-paths. The daily pattern of flight activity was re-corded: the mean total time in flight amounts to 244 min, and the corresponding daily flight length is about 17 km. At 20° C and 50% relative humidity (RH) a daily uptake of 180 μI (220 mg) of 50% sugar solution was measured, equal to the mean bod weight of the male bumblebees. Since the body weight remains constant on consecutive days a 24-h energy-and water-budget could be calculated. The energy-budget is balanced; the activities observed can be fueled with the sugar available. About 70% of the energy is used for the 4 h of flight activity. With the daily nectar volume 110 mg of water is ingested; in the oxidation of 110 mg sugar, 66 mg of metabolic water is produced and 40 mg water is dissipated by the evaporative water-loss. Thus, to have a balanced water-budget, 136 mg of water must be voided in 24 h, which equals the total body-water of the bumblebees. Nectar is a nutrient of high water con-tent which not only provides the sugar necessary for activity but also, in most circumstances, an excess of water. The effect of this high water load in limiting daily activity is discussed and compared with the water-and osmoregulation of hummingbirds. The strategy of maximizing energy for a male bumblebee must be one of minimizing water load.

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