  Due to aerodynamic instabilities, stabilizing flapping flight requires
ever-present fast corrective actions. Here we investigate how flies control
body roll angle, their most susceptible degree of freedom. We glue a magnet to
each fly, apply a short magnetic pulse that rolls it in mid-air, and film the
corrective maneuver. Flies correct perturbations of up to $100^{\circ}$ within
$30\pm7\mathrm{ms}$ by applying a stroke-amplitude asymmetry that is well
described by a linear PI controller. The response latency is
$\sim5\mathrm{ms}$, making the roll correction reflex one of the fastest in the
animal kingdom.
