Energetics of free and tethered flight in bumblebees (Bombus terrestris, Linnaeus 1758)

Many insect species are reluctant to fly freely in wind tunnels, limiting direct free-flight energetics measurements to just two species. More commonly tethered-flight energetics have been investigated, though the effects of tethering on metabolic rate are unclear. Here, we report mass-specific gross metabolic rate (assessed as the rate of carbon dioxide production; [Formula: see text]) across a speed range (0-4.1 m s^-1) in bumblebee (Bombus terrestris) workers during tethered and free flight in a closed-circuit wind tunnel. [Formula: see text] followed a U-shaped relationship with airspeed during both free (p = 0.009) and tethered flight (p < 0.001). Bees were anaesthetized with isoflurane during tethering, which had no subsequent effect on their metabolic rate (p > 0.05), avoiding issues reported during immobilization with CO₂ or cold exposure. Tethered [Formula: see text] was 45% lower than during free flight (p < 0.001), but the minimum power speed and the trajectory of the metabolic power-speed relationship (p > 0.8) were similar. Overall flight efficiency ranged from 7.3 to 14.7% and did not vary with airspeed. These findings confirm a U-shaped metabolic power-speed relationship in insects, and suggest that tethered flight may approximate free-flight energetics. However, a shift in the maximum range speed to slower speeds during tethered flight warrants caution against using this variable to predict behaviour.