Heterothermia in Foraging Workers and Drones of the Bumblebee Bombus terricola

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Abstract
While foraging from the dense inflorescences of spiraea (Spiraea latifolia) and goldenrods (Solidago sp.), both workers and drones often allowed thoracic temperature ( ) to fall below the minimum for flight. The bees were physiologically capable of maintaining a high , but the periodic decrease in was strongly correlated with ambient temperature ( ). Decreases of were unrelated to fuel reserves carried in the honey stomach. Drones foraging from the inflorescences were more likely to have low than workers, even though on the average they carried several times greater fuel reserves in their honey stomach. Within workers, old or parasitized (by conopid flies) individuals were more likely to forage with low than young and unparasitized individuals. Workers, unlike drones, showed an increasing tendency to decrease with decreasing body mass. Although the decrease in while foraging ("torpor"), with its associated sluggishness, appears to function as an energy conservation mechanism, it could also be a risk-averting mechanism. By maintaining a high and flight readiness, workers can "gamble" on the chance of finding a new food source, unlike drones who do not have the hive's energy resources to fall back on if they deplete their supply of stored fuel.