COMBES, S.A.*; DUDLEY, R.; Univ. of California, Berkeley: Limits to maximum flight velocity: Pitch control and roll instability in orchid bees during fast, forward flight

Maximum velocity is a critical component of flight performance, but observing or eliciting maximum flight speeds is difficult in most flying animals. Flight velocity is thought to be limited by either energetic constraints, such as maximum power output, or by aerodynamic constraints, such as maximum stroke amplitude or wingbeat frequency. We measured maximum flight velocity in several species of euglossine bees by placing aromatic oils, which bees are highly motivated to collect, in the mouth of an outdoor, open-ended wind tunnel in Panama. Bees were filmed with two high-speed cameras, and wind tunnel velocity was increased until bees were unable to maintain a stable flight position. We also collected the bees and measured parasite drag using a one-axis force beam mounted in front of the outdoor wind tunnel. We measured maximum flight speeds up to 7 m/s (over 500 body lengths/s), and found that flight speed is highly correlated with body and stroke plane angle. Surprisingly, bees extend their enlarged hindlegs below their body as they fly faster, although this increases parasite drag by 25% or more. However, this increased drag force located below and behind the center of mass produces a torque that tilts the body and stroke plane angle forward, allowing bees to increase their forward speed. At the highest flight velocities, orchid bees with fully extended hindlegs were unable to maintain stable flight, due to rolling moments caused by asymmetric forces on the two legs. These results suggest that pitch control and flight stability may play a role in limiting maximum flight velocity in some insects.