Wing folding and the functional morphology of the wing base in Coleoptera

The wing unfolding of Pachnoda marginata was examined using digital video (50 half-fps) and high speed video sequences (1000 fps), and the skeleto-muscular apparatus of the metathorax was described. Left and right hind wing are able to promote independently of each other. The hind wings do not unfold instantly when the elytra are lifted and may also reach the flight position (and beat) while still folded. Wing promotion is exhaustible and the time needed for unfolding varies considerably. These observations strongly suggest a muscular control. Wing unfolding is probably triggered by contraction of M. pleura alaris and a resulting proximad movement of the 3rd axillary sclerite, pulling the Media posterior backwards, while the Radius anterior is held by the basalar muscle as the antagonist. Our findings are in clear contrast to the earlier assumption that the hind wings of Coleoptera either unfold or fold due to intrinsic elasticity. The specific wing folding and unfolding mechanisms are autapomorphic character states of Coleoptera. They were maintained during evolution even though considerable variations of skeletal thoracic structures, musculature and venation occurred. (Additional material is available from the Zoology web page: http://www.urbanfischer.de/journals/zoology).