Evolution of and structures involved in wing folding in featherwing beetles (Coleoptera: Ptiliidae)

The ability to fold the wings is an important phenomenon in insect evolution and a feature that attracts the attention of engineers who develop biomimetic technologies. Beetles of the family Ptiliidae (featherwing beetles) are unique among microinsects in their ability to fold their bristled wings under the elytra and unfold them before flight. The folding and unfolding of bristled wings and of the structures involved in these processes varies among ptiliids, but only one species, Acrotrichis sericans, has been analyzed in detail. In this study, we analyze in detail the wing folding pattern and the mechanism of the folding and unfolding of the wings in species of different lineages of Ptiliidae, using scanning electron, сonfocal laser scanning, and optical microscopy, and compare the wing-folding patterns of Ptiliidae with those of the sister group, Hydraenidae, to reconstruct the evolution of the involved structures. We confirm that the two subfamilies of Ptiliidae have two distinct patterns of wing folding: Nossidiinae has retained the ancestral (‘agyrtid’) asymmetrical pattern with overlapping wings and with folds at different angles to the wing axis, while Ptiliinae, which includes the smallest of all known beetles, has evolved a symmetrical pattern with non-overlapping wings and folds perpendicular to the wing axis, with one additional oblique fold in the genus Ptenidium. Ptiliids have a longer alacrista, which helps to lock the elytra at rest, and a more complex set of structures involved in wing folding on abdominal tergites. These genus-specific structures, which include setae and wing-folding patches on some of the tergites and the palisade fringe of setae on the posterior margin of tergite 7, help the insect to tuck the wing under the elytron and fold it after flight. The symmetrical wing-folding pattern is simpler than the wing folding patterns of most larger beetles. The obtained data on the mechanisms and patterns of the folding and unfolding of the wings in Ptiliidae elucidate the evolution of wing folding as an adaptation protecting the wings at rest. Structures involved in wing folding can be used as distinguishing characters in taxonomy. The wing-folding mechanisms of Ptiliidae may eventually be used for developing miniature biomimetic robots.