Rolling instabilities and maximum flight speed in foraging orchid bees

Tropical orchid bees cover vast distances at high flight speeds foraging for scattered resources. Although this long-distance foraging is central to feeding, mating and nest-building behaviors, very little is known about the physiological and mechanical challenges that foraging bees face. To understand how bees achieve and maintain high flight speeds during foraging, we studied fast, forward flight in wild orchid bees (Euglossa imperialis) flying in front of an outdoor, open-ended wind tunnel in Panama. We attracted bees to the mouth of the tunnel with aromatic oils and filmed bees flying at a range of forward velocities with high-speed video cameras. Orchid bees tilted their bodies down to increase flight speed, and flapped their wings more quickly through a larger amplitude to compensate for body drag at high speeds. Bees reached maximum flight speeds up to 7.25 m/s (over 600 body lengths/s) and were ultimately limited by rolling instabilities, which increased in severity until bees were no longer able to right themselves. To counteract these rolling instabilities, bees extended their enlarged hindlegs below their bodies at high flight speeds, despite the fact that this behavior increases body drag and power expenditure by approximately 30%. We found that individual bees vary widely in their maximum flight speed, suggesting that the ability to maintain mechanical stability at high flight speeds may affect foraging range, survival and reproductive success.