Wings Fold Multiple Times Without Wear

Biological Strategy

Beetles

AskNature Team

Manage Mechanical Wear

A living system is subject to mechanical wear when two parts rub against each other or when the living system comes in contact with abrasive components in its environment, such as sand or coral. Some abrasive components are a constant force, such as finger joints moving, while others occur infrequently, such as a sand storm moving across a desert. Living systems protect from mechanical wear using strategies appropriate to the level and frequency of the source, such as having abrasion-resistant surfaces, replaceable parts, or lubricants. For example, human joints like shoulders and knees move against each other all day, every day. To protect from mechanical wear, a lubricant reduces friction between the cartilage and the joint.

Search this Function

Prevent Fatigue

When materials that comprise living systems are subjected to repeated loads, they can weaken over time. This is called fatigue and it can lead to cracks and eventually failure. However, fatigue doesn’t occur just anywhere on a material; the cracks usually start in areas subjected to elevated local stresses. Therefore, living systems often employ shapes, materials, and/or smooth transitions to decrease the potential for fatigue. An example of using shape is found in brown algae, an intertidal seaweed. Rather than being bent by the constant flow forces of waves and currents, its shape causes it to be pulled. This reduces stress by roughly 800 times compared to another species that bends with the flow, ultimately reducing structural fatigue.

Search this Function

Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

Search this Living System

Wings of beetles fold multiple times without wear or fatigue by having resilin in key joints.

Introduction

Unlike other insects like bees or butterflies, beetles can’t simply keep their wings out and at the ready. Their terrestrial activity, tunneling underground or under leaf litter, has been eased by folding their hind wings under their forewings, which have been thickened and hardened into a protective case. The adaptations that have been made to those folding wings hold great promise for efficient mechanical motion for humans.

The Strategy

Beetle wings consist of stiff veins connected by thin membranes. Pre-conditioned joints and folds allow the wings to consistently fold and unfold in an efficient manner. Remarkably, this all happens without internal musculature. The only muscles are back in the thorax of the beetle. Their flexing causes a mechanical distortion to the main vein of the wing, which initiates the preset cascade of unfolding.

These mechanics are in turn made possible by the integration along lines and points where the folding occurs, of an elastic called resilin. The intrinsically disordered molecular structure of resilin imparts its qualities of low stiffness, high extensibility, efficient energy storage, exceptional resilience, and long fatigue lifetime. It not only allows the wing to fold and unfold, it stores energy from wind resistance in flight and prevents damage from wear on the wing.

A science illustrations showing resilin locations in the wing.

Resilin-containing areas in the wings of the sun beetle, Pachnoda marginata

Remarkably, these joints and folds do not appear to be any more stiff or soft than the rest of the wing while in flight. Despite intense pressures from torsion, the wing bends uniformly, showing no signs of weakness in the high-resilin areas.

When it is time for the wings to be folded back up, again the work is done without any internal musculature on the wing itself. Instead, the wings are brushed back into place by movements of the hairy plates on the back of the beetle (called abdominal tergites). This sweeping motion pulls the wing surfaces towards the center of the body. While the wings move, the softness of the high-resilin areas causes them to fold in sequence along the predetermined areas.

While all beetles share these basic characteristics, the particular pattern of veins and folds, the locations of resilin and the quantities of it vary species by species based on overall body size and other factors. Additional inspiration based on the geometry of different wings can be found with further investigation.

The Potential

Today, researchers are making diverse resilin-like substances for human design purposes. Similarly incorporating elastic compounds to the materials of robotic assemblies, and making use of geometrically dynamic folding patterns, humans could reduce the number of moving parts and number of materials needed for our applications. Such structures could also significantly reduce noise and energy loss through wasted motion, integrating robots into human environments with much added grace.

Related Strategy

photograph of red, blue and yellow winged bug on a green plant

Biological Strategy

Earwigs’ Complex Origami Wing-Fold

Earwigs

A complex series of joints enables earwig wings to spring from folded to flight without the use of muscles.

Related Innovation

Innovation: Academia

Collapsible Gripper Inspired by Earwig Wings

ETH Zurich & Purdue University

Gripper from ETH Zurich and Purdue University has a soft elastic joint that enables it to methodically self-unfold.

Last Updated February 26, 2024

References

“Beetles use their fore-wings for a different purpose altogether. These creatures are the heavy armoured tanks of the insect world and they spend a great deal of their time on the ground, barging their way through the vegetable litter, scrabbling in the soil or gnawing into wood. Such activities could easily damage delicate wings. The beetles protect theirs by turning the front pair into stiff thick covers which fit neatly over the top of the abdomen. The wings are stowed neatly beneath, carefully and ingeniously folded.” (Attenborough 1979:79)

Book

Life on Earth: A Natural History

01/01/1970 | David Attenborough

embedly preview

Reference

“This account shows the distribution of elastic elements in hind wings in the scarabaeid Pachnoda marginata and coccinellid Coccinella septempunctata (both Coleoptera). Occurrence of resilin, a rubber–like protein, in some mobile joints together with data on wing unfolding and flight kinematics suggest that resilin in the beetle wing has multiple functions. First, the distribution pattern of resilin in the wing correlates with the particular folding pattern of the wing. Second, our data show that resilin occurs at the places where extra elasticity is needed, for example in wing folds, to prevent material damage during repeated folding and unfolding. Third, resilin provides the wing with elasticity in order to be deformable by aerodynamic forces. This may result in elastic energy storage in the wing.” (Haas et al. 2000:1375)

Journal article

The function of resilin in beetle wings

Proceedings of the Royal Society B: Biological Sciences | 26/07/2002 | F. Haas, S. Gorb, R. Blickhan

embedly preview

AskNature