It is advantageous for flying insects to be functional across a range of temperatures when competing for mates; breadth of performance can arise from broad tolerance or thermoregulation. Tactics of flying insects typically include moving hot hemolymph from the thorax to cooler body segments, evaporative water loss, and changes in metabolic heat production. For dimorphic Centris caesalpiniae male bees, large morphs (metanders) dig for females at ground-level, flying infrequently, whereas the small morph hovers and flies. At a mate-aggregation site, air temperature varied from 17 °C to 37 °C across and within days; operative temperatures of metanders reached 50 °C on the ground when mating activity ceased, suggesting that high temperatures and radiation constrain the mating period, and that C. caesalpiniae’s upper thermal tolerance is high, but not exceptional. These bees exhibited modest thermoregulation of thorax and head temperatures, with segment temperatures increasing 0.5-0.6 °C per °C air temperature both on the ground and in flight. During flight, evaporative water loss and heat movement to the abdomen contributed to thermoregulation. On the ground, bees moved hemolymph to the head. These results suggest bees thermoregulate the thorax to improve flight performance. Metabolic water production was about half of water loss rates and metanders lose about 17% of their body water per hour during flight. No one has observed metanders feeding or drinking; several hours of flight would presumably cause a lethal loss of body water. While we currently lack information on the duration of activity of individuals, it is possible that thermoregulatory water loss may constrain the activity of metanders. Supported by 2019-2020 Fulbright Student Program and USDA 2017-68004-26322.