Moths change their behavior, but not their aerodynamics to feed in windy environments

How do moths maneuver in windy environments?

Hawkmoths naturally hover and feed from flowers in nature. Insects have developed an assortment of unsteady aerodynamic mechanisms to generate the high-lift necessary for hovering. While feeding, hawkmoths rely on precise wing kinematics to not only remain aloft, but also track the motion of flowers as they sway in the wind. Previous work revealed that these insects compensate for an unsteady environment by varying wing positions and angles, but how these changes affect lift mechanisms, such as the leading-edge vortex, is largely unknown. The leading-edge vortex (LEV) is formed by flow separation at the leading edge of each wing. On the underside (ventral), air is pushed below the animal, but on the upper (dorsal) side the flow rolls up into a vortex, which remains attached through each half-wingstroke. Since this mechanism depends on the direction of flow as it impacts the wing, it’s possible that an environment with vortices rotating in multiple directions may disrupt the formation and stability of the LEV. While there are many open questions regarding either insect maneuverability or effects of unsteady environments, studying the intersection of these allows for a closer approximation to behavior in the natural world.

We investigate insect maneuverability in an unsteady wake by having hawkmoths, Manduca sexta, track a 3D-printed robotic flower in a wind tunnel and compare to results from previous experiments in a still air flight chamber. The flower motion is a sum of sinusoids, each with a unique frequency (0.2-19.9 Hz), so the frequency response for tracking can be described by calculating response gain (over- or undershoot) and phase delay (lagging or leading). Although interacting with the flower wake decreases tracking performance, these effects are very small at the frequencies matching natural flower oscillation and vortex shedding. Since our robotic flower sheds vortices within 2-5 Hz, moths exhibit large shifts in tracking performance and strategy within this frequency band. In still air, moths respond to these frequencies by undershooting the flower motion, but while tracking in wind moths instead consistently overshoot until they stop tracking above ~9 Hz. Combining results from tracking in still air and stationary feeding in wind gives a predicted (linear) response for tracking in wind, but this approach fails to reproduce the true response for all except the lowest frequencies (0.2-1.1 Hz). This suggests that moths may be able to superimpose tracking maneuvers and responses to wind at low frequencies and for natural flowers, but employ nonlinear responses when a conflict arises between their environment (wind) and task (tracking).

Despite the unsteady character of the flower wake and the nonlinearity in wake interaction, smoke visualization of the moth feeding from a stationary flower reveals that the leading-edge vortex retains features consistent with previous smoke visualization in steady air conditions. Specifically, the leading-edge vortex maintains the same size and structure as seen before, remaining bound over the mid-wing position throughout the downstroke and over the thorax during the upstroke, even though the flow impacting the wings comes from many directions. So, while maneuverability and performance suffer due to wake interactions, the dominant lift mechanism is robust to these effects. Long story short, moths adjust their wing kinematics to achieve complex maneuvers, but not so much to cause a shift in their aerodynamic mechanisms.