  The natural wind environment that volant insects encounter is unsteady and
highly complex, posing significant flight control and stability challenges.
Unsteady airflows can range from structured chains of discrete vortices shed in
the wake of an object to fully developed chaotic turbulence. It is critical to
understand the flight control strategies insects employ to safely navigate in
natural environments. We combined experiments on free flying bumblebees with
high fidelity numerical simulations and lower order modeling to identify the
salient mechanics that mediate insect flight in unsteady winds. We trained
bumblebees to fly upwind towards an artificial flower in a wind tunnel under
steady wind and in a von Karman street (23Hz) formed in the wake of a cylinder.
The bees displayed significantly higher movement in the unsteady vortex street
compared to steady winds. Correlation analysis revealed that at lower
frequencies, less than 10 Hz, in both steady and unsteady winds the bees
mediated lateral movement with body roll, typical casting motion. At higher
frequencies in unsteady winds there was a negative correlation between body
roll and lateral accelerations. Numerical simulations of a bumblebee in similar
conditions permitted the separation of the passive and active components of the
flight trajectories. Comparison between the free-flying and numerical bees
revealed a novel mechanism that enables bees to passively ride out high
frequency perturbations while performing active maneuvers and corrections at
lower frequencies. The capacity of maintaining stability by combining passive
and active modes at different timescales provides a viable means for volant
animals and machines to tackle the control challenges posed by complex
airflows.
