LOCOMOTOR PERFORMANCE OF INSECTS WITH RUDIMENTARY WINGS

THE evolution of flight in insects triggered an unparalleled radiation and diversification such that flying insects comprise approximately t wo-thirds of all species(1), yet a gap in the fossil record obscures t he origins of wings and flight(2). Among modern insects, stoneflies ar e morphologically primitive for several flight-related traits, which m akes their locomotor behaviour and physiology of particular interest(3 ). Here we show that Allocapnia vivipara stoneflies use a non-flying f orm of aerodynamic locomotion which may exemplify a precursor to fligh t. They raise their wings in response to wind, thereby sailing across water surfaces, but they are incapable of flapping. Sailing performanc e improves steadily with increasing wing size, and even the smallest w ings significantly increase sailing velocity compared to wingless indi viduals. Performance during aerial gliding is less affected by wing si ze, which suggests that sailing is a more plausible setting for wing e volution. These results support the hypothesis that insect wings evolv ed from articulated gill plates of aquatic ancestors through an interm ediate semi-aquatic stage(4).