Birds, bats and many insects can tuck their wings against their bodies when at rest and deploy them to power flight. Whereas birds and bats use well-developed pectoral and wing muscles, how insects control their wing deployment and retraction remains unclear because this varies among insect species. Here we demonstrate that rhinoceros beetles can effortlessly deploy their hindwings without necessitating muscular activity. We validated the hypothesis using a flapping microrobot that passively deployed its wings for stable, controlled flight and retracted them neatly upon landing, demonstrating a simple, yet effective, approach to the design of insect-like flying micromachines.
The wings of flying insects are vulnerable and fragile structures that are crucial for evading predators, foraging, migrating or mating. Most insects are therefore capable of folding and resting their wings against the sides of the abdomen to reduce wing damage risk and interference in terrestrial locomotion through narrow spaces. Mechanisms of wing deployment and retraction vary among insect species due to the absence of intrinsic wing muscles and to differ-ences in wing shape, structure and function12,13. In particular, beetles (Coleoptera) possess one of the most complex mechanisms among the various insect species5,8, having two distinct pairs of wings—one a pair of membranous and fragile wings (the hindwings) and the other a pair of hardened forewings (the elytra), used mostly to protect the hindwings when at rest (Fig.1a,b). Hindwings are an origami-like fold-able structure that allows them to neatly stow between the body and the elytra and deploy to power flapping flight. The complete hind-wing deployment procedure consists of elevating the wing base and unfolding the wing tip. Studies of the hindwing have primarily focused on the origami-like fold of the wing tip and have proposed the use of elastic elements4,14, thoracic muscles5,6, a hydraulic mechanism7 or flapping forces8 to drive the unfolding. However, the mechanism employed by beetles to elevate the hindwing bases to flight position, and bring them back to rest against the body, remains little under-stood. The most common explanation is that beetles, as well as other insects in the Neoptera, such as wasps, bees and flies, use direct flight muscles attached to the basalar sclerite and the third axillary sclerite of the wing base to drive these movements5,9–11. However, no experimental evidence has been presented showing muscle activity during hindwing deployment and retraction. A previous study has shown that the beetle Mecynorrhina torquata deploys and retracts its hindwings even when the third axillary muscle is inactivated or has been removed15, but how these movements are initiated has not been explained.
Passive hindwing deployment
To gain insight into how a beetle elevates its hindwings at the bases, we used synchronized high-speed cameras to record the wing deployment kinematics of the rhinoceros beetle, Allomyrina dichotoma (Fig.1c–g and ‘Wing kinematics experiments’ in Methods). We observed that the beetle initiates a flapping flight with a two-phase wing deployment. In the first phase, the beetle fully elevates the elytra, followed by a partial release of the hindwings to an angle of about 48.5 ± 0.7° (n = 7) from the abdomen while maintaining the wing tip in the folded configuration (Fig.1c and Supplementary Video1). Because the tips of the left and right hindwings overlap when stowed against the abdomen (Fig.1a), the hindwing cannot be released instantly upon opening of the ipsilat-eral elytron if the contralateral elytron and hindwing remain folded, consequently affecting the hindwing’s release time and speed (Fig.1d,e