  Birds, bats and many insects can tuck their wings against their bodies at
rest and deploy them to power flight. Whereas birds and bats use well-developed
pectoral and wing muscles and tendons, how insects control these movements
remains unclear, as mechanisms of wing deployment and retraction vary among
insect species. Beetles (Coleoptera) display one of the most complex wing
mechanisms. For example, in rhinoceros beetles, the wing deployment initiates
by fully opening the elytra and partially releasing the hindwings from the
abdomen. Subsequently, the beetle starts flapping, elevates the hindwings at
the bases, and unfolds the wingtips in an origami-like fashion. Whilst the
origami-like fold have been extensively explored, limited attention has been
given to the hindwing base deployment and retraction, which are believed to be
driven by thoracic muscles. Using high-speed cameras and robotic flapping-wing
models, here we demonstrate that rhinoceros beetles can effortlessly elevate
the hindwings to flight position without the need for muscular activity. We
show that opening the elytra triggers a spring-like partial release of the
hindwings from the body, allowing the clearance needed for subsequent flapping
motion that brings the hindwings into flight position. The results also show
that after flight, beetles can leverage the elytra to push the hindwings back
into the resting position, further strengthening the hypothesis of a passive
deployment mechanism. Finally, we validate the hypothesis with a flapping
microrobot that passively deploys its wings for stable controlled flight and
retracts them neatly upon landing, which offers a simple yet effective approach
to the design of insect-like flying micromachines.
