No wings required: Leg-assisted flight control in Phantom Crane flies and robots

Phantom crane flies are a widespread species in Eastern North America. Naturalists have long speculated that these insects employ a distinctive flight strategy: by spreading their hollow, elongated legs, they allow themselves to be passively carried by air currents without relying on active wing flapping. We analyze and compare the 3D aerial kinematics of individuals during flapping ascent flights in still air, and when exposed to an upward flow. In still air, crane flies flap their wings to move upward, generating shedding vortices during each stroke. In contrast, when in an upward flow, they continue oscillating their halteres but cease flapping their wings, with their legs nearly fully extended and pointing upward. A 3D-printed reconstruction of the insect was mounted in a towing tank filled with mineral oil to maintain dynamic similarity and measure drag forces associated with leg spreading. Because postural adjustments of individual legs appear to influence pitch, roll, and yaw, we are now developing a bio-inspired robot to investigate how this control is achieved. This research aims to inform the design of a new generation of micro aerial vehicles capable of transitioning from active flapping to passive, leg-mediated flight, enabling them to exploit air currents and reduce energy expenditure.