The dangers of becoming someone's lunch can weigh heavily on a tiny insect, but the diabolical ironclad beetle (Phloeodes diabolicus) handles the pressure with ease. This flightless beetle is able to withstand crushing blows that would pulverize other species. Jesus Rivera from the University of California, Riverside, USA, and colleagues from universities in the USA and Japan used mechanical testing, advanced microscopy and spectroscopy to explore how this remarkable species is able to join together pieces of its exoskeleton to form an impenetrable armour.

First, Rivera and his team compressed the beetles between steel plates to see how tough they were and found that they can resist crushing forces twice as great as those endured by other beetles. Most beetles have exoskeletal structures, called elytra, which cover the wings at rest, but move out of the way to allow flight. However, the researchers found that the elytra of the diabolical ironclad beetle are fused together along the back and at the sides of the body, creating an air-filled space beneath that can cushion impacts. Other terrestrial beetles also have this built-in airbag, so the team wondered whether the impressive crush resistance in this diabolical insect comes from the seams that bond the elytra together.

One way of connecting materials is through interdigitation, where a protruding ‘blade’ on one side of a seam fits into a space on the other side; in cross-section, this looks similar to how jigsaw puzzle pieces fit together. Examining the connections between the elytra and the exoskeleton along the sides of the body, Rivera and his colleagues found interlocking puzzle piece-like structures toward the front of the ironclad beetle's body, but the connecting structures transition toward the back of the body, becoming looser. This arrangement allows the elytra to compress and absorb impacts in the air-filled cavity without bursting at the seams.

Focusing on the seam connecting the two elytra down the beetles’ backs, the researchers found additional interdigitation, but unlike the less resilient seams in other flightless species, this connection included multiple puzzle piece-like blades. Using computer simulations to explore the effects of blade number on the strength of the seam, they found that connections with fewer blades tend to pull apart and fail, but connections with more blades fall apart when the necks of the narrow blades snap. So how is the diabolical ironclad beetle able to form interdigitated connections with multiple slender blades without suffering damage?

The team found that the blade structures are made of multiple layers of material that pull apart under tension, dissipating energy without breaking completely. This microstructure allows the diabolical ironclad beetle to have a strong interdigitated seam between the two elytra without the risk of catastrophic damage to the blades that hold the wing casings together. And, when the researchers created a physical prototype of the connector with a similar shape and layered material, the design outperformed current fastening systems used in extremely demanding applications, like aerospace engineering.

In this study, Rivera and colleagues found that the remarkable crush-resistance of the diabolical ironclad beetle comes from a unique set of structural and material features of the exoskeleton. By fusing together their elytra using interdigitation to provide a uniquely strong connection, these beetles create an airbag that cushions impacts and prevents irreparable damage. Ironically, the authors suggest that the high-performance mechanical fastening which has robbed the beetles of flight has the potential to inspire new advances in aviation design.