Beyond winglets: evolution of flight-related morphology in stick insects (Phasmatodea)

The first winged insects evolved from a wingless ancestor, but details of the transition from wingless to winged morphology remains unclear. Studying extant pterygotes undergoing secondary flight loss may help to understand such a transition in reverse temporal sequence. Stick insects (Order Phasmatodea) are characterized with slender body shape and exhibit various intermediates between transitions between fully-winged and wingless forms. Here, we studied the flight-related parameters of wings and body-leg system in different phasmids over a wing loading range from 1.4 – 2300 Nm−2 to address how these traits may correlate with flight evolution. Phasmid wings exhibited low aspect ratios (1.2 to 2.7) that was correlated with wing size. With reduced size, wing venation exhibited structural reconfiguration and the wingbase shifted nearer to the center of body mass, which position was highly conserved despite a wide range of variation in body shape. In addition, the masses of the wings and flight musculature are best predicted by relative wing size, and the area of the body-leg system was predominately predicted by specific allometric scaling. Our results demonstrate complex reconfiguration of the flight apparatus and body-leg system during wing loss in phasmids, albeit with some conserved features that influence flight performance.