Geometry of elytra opening and closing in some beetles (Coleoptera, Polyphaga)

Elytra in beetles move actively, driven by their own muscles, only during transient opening and closing. The kinematics of these movements have been inadequately described, sometimes controversially. Our goal was a quantitative 3-D description of diverse active movements of the elytra, in terms of directions of the axes of elytra rotation. Broad opening and closing was video recorded in beetles, tethered by the mesothorax, and has been analyzed frame by frame. For tracing, small dots or straw arms were glued to the elytra. Opening and closing traces coincided. The trace of the elytron apex was a flat circular arc about the axis of abduction-adduction (AAA). The rising hemiaxis pointed contralaterad. The AAA was tilted forwards in Melolontha hippocastani, Allomyrina dichotoma and Prionus coriarius but backwards in Chalcophora mariana. In Cetonia aurata, the AAA had a low elevation and a strong backward orientation. If another elytra-fixed point was traced in addition to the apex (in M. hippocastani and P. coriarius), then secondary rotation about the sutural edge (supination on opening) occurred. Modeling of abduction-adduction revealed that the elytron rose on opening if the AAA pointed contralaterad. The more the AAA was tilted forward, the more negative was the attack angle of the open elytra. The negative attack angle was partly compensated by positive body pitch and, more effectively, by supination of the costal edge about the sutural edge. The initial stage of opening included elevation of closed elytra (by 10-12 degrees) and partition to the sides, combined with an inward turn (<2-3 degrees). Axis of rotation at this stage presumably coincided with the AAA. Movement of one elytron with respect to the opposite one at the beginning of opening followed the shallow arc convex down. The geometry of this relative movement describes the initial partition of the elytra and release of the sutural lock.