  Flapping insect flight is a complex and beautiful phenomenon that relies on
fast, active control mechanisms to counter aerodynamic instability. To directly
investigate how freely-flying D. melanogaster control their body pitch angle
against such instability, we perturb them using impulsive mechanical torques
and film their corrective maneuvers with high-speed video. Combining
experimental observations and numerical simulation, we find that flies correct
for pitch deflections of up to 40 degrees in 29 +/- 8 ms by bilaterally
modulating their wings' front-most stroke angle in a manner well-described by a
linear proportional-integral (PI) controller. Flies initiate this corrective
process after only 10 +/- 2 ms, indicating that pitch stabilization involves a
fast reflex response. Remarkably, flies can also correct for very
large-amplitude pitch perturbations--greater than 150 degrees--providing a
regime in which to probe the limits of the linear-response framework. Together
with previous studies regarding yaw and roll control, our results on pitch show
that flies' stabilization of each of these body angles is consistent with PI
control.
