Biomechanics and ontogeny of gliding in wingless stick insect nymphs (<i>Extatosoma tiaratum</i>)

Many wingless arboreal arthropods can glide back to tree trunks following free falls. However, little is known about the behaviors and aerodynamics underlying such aerial performance, and how this may be influenced by body size. Here, we studied gliding performance by nymphs of the stick insect Extatosoma tiaratum, focusing on the dynamics of J-shaped trajectories and how gliding capability changes during ontogeny. After being dropped 40 cm horizontally from a visual target, the first-instar nymphs landed on the target within 1.1 s. After reaching terminal speed (at ∼0.25 s), they initiated gliding with significant horizontal force, during which the overall lift-to-drag ratio increased from 0.16 to 0.48. This transition from parachuting to gliding is characterized by a damped oscillation in body pitch, initiated with a rapid nose-down pitching, and led to a higher-lift configuration with reduced body angle of attack. Among instars, increasing wing loading during ontogeny led to greater terminal speed, reduced agility during glide initiation and increased glide angle. Our study demonstrates that a sequence of controlled behaviors, from pre-glide descent to glide initiation and forward gliding, underlies their gliding aerodynamics, which in aggregate form the basis for J-shaped aerial trajectories. Selection for improved gliding performance in wingless arthropods may foster the evolution of more rapid maneuvers and of dedicated morphological traits (such as winglets) that contribute to an overall reduction in wing loading, either across ontogeny or during the evolution of larger body size.