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1. Desert bees do not show significant differences in most thermal parameters; mean endothermic warm-up rates are similar to those of temperate species, with no special cooling mechanisms, and normal upper critical temperatures (unlike desert ants and beetles). Thermoregulatory abilities may however be improved.

2. They show the whole range of possible water balance problems: small species are acutely water-stressed when foraging, but large bees suffer from excessive generation of metabolic water in flight.

3. Activity patterns are therefore either matinal, crepuscular or bimodal; essentially desert bees avoid heat and adapt to cold desert dawns and dusks. Desert plants must be coevolved to offer appropriate rewards and match the physiological constraints on their pollinators.

4. Endothermy in bees may have evolved primarily in arid zones, and served as a pre-adaptation for subsequent invasion of cool temperate biomes. (C) 1998 Elsevier Science Ltd. All rights reserved.