Diabolical design is a puzzle

It’s not uncommon for scientists to take cues from nature when it comes to self-protection. In recent months, Cosmos has reported, for example, on armour inspired by a marine mollusc and by mother of pearl.

Now researchers in the US have taken a good look at why the diabolical ironclad beetle earns that name – and the answer is actually a bit of a puzzle.

This is a tough insect. Using compressive steel plates, engineers at the University of California Irvine (UCI) found that it can take an applied force of about 150 newtons – a load at least 39,000 times its body weight – before its exoskeleton begins to fracture.

That’s 50% more force than a car tyre would apply if it ran over the ironclad on a dirt surface, the researchers estimate. Other beetles they tested couldn’t handle half that.

“The ironclad is a terrestrial beetle, so it’s not lightweight and fast but built more like a little tank,” says UCI’s David Kisailus. “That’s its adaptation: it can’t fly away, so it just stays put and lets its specially designed armour take the abuse until the predator gives up.”

Further investigation by researchers from UCI and Purdue University found that the secret lies in the beetle’s two elytra, which meet at a line, called a suture, running the length of the abdomen.

In flying beetles, elytra protect wings and facilitate flight. In the DBI, which doesn’t have wings, the elytra and connective suture help distribute an applied force more evenly throughout the body.

“The suture kind of acts like a jigsaw puzzle,” says Purdue’s Pablo Zavattieri. “It connects various exoskeletal blades – puzzle pieces – in the abdomen under the elytra.”

Computer simulations and 3D models revealed that when under a compressive load the beetle’s suture design offers two ways to dissipate energy and thus circumvent a fatal impact at the neck, where the exoskeleton is most likely to fracture.

First, the interconnecting blades lock to prevent them from pulling out of the suture like puzzle pieces. Second, the suture and blades delaminate, which leads, the researchers say, to a more graceful deformation that mitigates catastrophic failure of the exoskeleton. 

Even if a maximum force is applied, delamination allows the blades to pull out from the suture more gently. If the blades were to interlock too much or too little, the sudden release of energy would cause the beetle’s neck to snap.

UCI researchers have built a carbon fibre composite fastener mimicking the beetle’s suture and testing at Purdue found it is just as strong as a standard aerospace fastener, but much tougher.

“This work shows that we may be able to shift from using strong, brittle materials to ones that can be both strong and tough by dissipating energy as they break,” Zavattieri says. “That’s what nature has enabled the diabolical ironclad beetle to do.”

The study is published in the journal Nature.