Origin of the insect wing and wing articulation from the arthropodan leg

Abstract

The most primitive known pterygote terga, wing articulation, wings, and upper leg segments with exites, occur in gigantic Upper Carboniferous Paleodictyoptera, Homoiopteridae. Fossil features are used as clues for reinterpreting some structures connected with flight in modern Pterygota. Brief comparisons with Paleozoic Diaphanopterodea, Permothemistida, Ephemeroptera, Protodonata, and with living Ephemeroptera, Odonata, and Neoptera are given. The wing articulation of all Pterygota is derived from a common ancestral ground plan based upon features present in fossils. The ancestral wings were articulated by a closely packed band of multiple sclerites which were hinged to eight lateral tergal lobes, and aligned with eight pairs of wing veins. The axillaria of Neoptera and axillary plates of Paleoptera are composite sclerites, which originated by fusion of several sclerites of the original band. Articular patterns of Paleoptera and Neoptera evolved differently and show (i) the presence or absence of a gap at the cubital level, (ii) the presence or absence of a turning–pivoting composite third axillary sclerite (3Ax), and (iii) a different composition of all composite sclerites. Gliding and wing folding adaptations within the articular band are discussed. A new fossil-based interpretation of veinal stems, veinal sectors, and of their fluting near the wing base is offered. An underlying symmetry of thoracic tergal sulci, articular sclerites, and wing venation seems to point to a nearly symmetrical, nonflying pro-wing engaged in up-and-down movement. Evidence of articulation in Paleozoic nymphal wings and evolution of metamorphic instars are examined. Pitfalls of paleoentomological work are discussed. Criteria for major divisions of Pterygota are reassessed. It is hypothesized that the wing originated from the first segment (epicoxa) of the euarthropodan upper leg and its exite. An epicoxal podomere became incorporated into the body wall and broke up into an articular ring of dorsal and ventral sclerites, and an epicoxal exite flattened and became a pro-wing. The pro-wing originally operated on a row of pivots from the epicoxa and subcoxa (pleuron) and became mobilized by epicoxal leg musculature.