Transverse folding and evolution of the hind wings in beetles (Insecta, Coleoptera)

Sharp intensification of the protective function of the fore wing in Coleoptera has made their flight apparatus posteromotoric and initiated the development of an apparatus for folding the hind wings beneath the elytra. This could hardly take place without a higher deformability of veins relative to each other, which diminished the strength properties of the wing support. The effect has been stressed by the presence of folds. The wing support and folding pattern evolved as interrelated: the former evolved to be more flexible, with no or minimum loss of rigidity; the latter evolved to be less harmful for the supporting structures. The only function performed, as well as simple structure and low specialization of folds made the folding pattern highly labile in the course of evolution. It evolved superior to wing venation, thus defining many transformations of the latter. This evolutionary conservatism of wing venation stemmed from the fact that many veins performed two conflicting functions. For this conflict to be resolved an adaptive compromise was needed, which brought the wing to orthogenetic development. The main evolutionary trends in wing venation and the folding pattern were those towards simplification and higher complexity, respectively. The coleopteran wing has passed two main evolutionary stages. The major evolutionary factors were wing posteromotorism at the first stage of evolution, with a trend toward miniaturization at the second stage. Each stage resulted in the development of a particular wing type, archostematan and “cantharoid”, respectively. They differ fundamentally. In the wing of the former type, the folding and flight apparatus constitute a steady coadaptive ensemble, because their supporting systems overlap considerably. This wing remained stable through evolution, and its structural variations were only minor deviations from the ground plan. The development of the “cantharoid” wing type was an upgrade of the morphofunctional organization. The region of maximum transverse deformations initiated by folding was extruded bayond the basal, supporting, part of the remigium, which contributed much to rigid properties of the chief supporting axes. The two wing apparatus have become more autonomous after acquiring the above structure. This expanded the range of possible specialization and gave rise to a great variety of derived wing types.