Functional Morphology of the Flight Apparatus of Ephemera vulgata (Ephemeroptera: Ephemeridae) and Implications for the Evolution of Flight in Insects

The mesothoracic flight apparatus of preserved and freshly killed specimens of Ephemera vulgata is examined in detail by means of functional morphological examinations. The basic drive mechanism of the forewing of Ephemera vulgata constitutes a four-bar kinematic chain, which encompasses the whole mesothorax. Based on this reassessment, the operating principles of the different subsystems of the flight apparatus of the mayfly were newly analysed. As in the Neoptera, the indirect dorsolongitudinal downstroke and dorsoventral upstroke muscles rotate the wing around an axis which runs through the main wing joint of the fulcrum. In contrast to the Neoptera, the wing is driven via the posterior notal wing process and fourth axillary. A strong direct subalar muscle is able to move the subalare (together with the pleural ridge) inwards and to affect the wingstroke via a bistable click mechanism in this unusual way. The axillary sclerites, two frontal first axillaries and one caudal fourth axillary, permit the radioanal plate to rotate forward–backward on the fulcrum. This motion system, which is superimposed on the kinematic chain mechanism, permits alterations of the wingstroke plane. A short muscle of the radioanal plate allows adjusting the passive pronation of the wing during the downstroke and achieving an increase of the aerodynamical angle of attack. The step-by-step derivability of basal flight mechanisms of the main groups of Pterygota indicates a paranotal origin of the wings. The possibility that the Ephemeroptera and Neoptera constitute sister groups is discussed.