ironclad

David Kisailus/UCI

You can accidentally stomp on the diabolical ironclad beetle and it won’t even flinch. Go one further — drive over it in your car — and that won’t cause the critter any trouble at all, either. Its exoskeleton is one of the toughest in the animal kingdom. And scientists now believe they know why.

In a study, published in the journal Nature on Wednesday, researchers have unraveled the secrets of the diabolical ironclad beetle’s astounding crush-resistance and demonstrate how new ultra-tough materials may take advantage of the beetle’s biology. 

At a glance, the beetle appears impressive: a dark, bumpy exoskeleton that looks a little like a charred rock. But lurking beneath its humdrum exterior lie a few structural marvels, built by evolution. Many species of beetles can fly and their wings are encased within elytra, a protective, tough shell. Flying is a great defensive mechanism for beetles, allowing them to escape predators, but the ironclad doesn’t have wings and routinely plays dead, relying on its exoskeleton to keep it safe.

“The ironclad is a terrestrial beetle, so it’s not lightweight and fast but built more like a little tank,” said David Kisailus, a professor of materials science and engineering at the University of California, Irvine and co-author on the study, in a release. The beetle’s exoskeleton is so tough it has even presented some issues for entomologists hoping to display them — it’s difficult to put a pin through the ironclad.

Cross-section of ironclad beetle's elytra.

The two elytra of the diabolical ironclad beetle fuse together in a winding suture (circled) 


Jesus Rivera/UCI

To study the tiny tanks, a member of the research team, Jesus Rivera, captured beetles and brought them back to the lab. First, researchers discovered the beetle’s exoskeleton could withstand around 150 newtons of force — 39,000 times its body weight. Three other species of terrestrial beetle were only half as resilient.

But why is this particular exoskeleton so much stronger? The research team looked at the beetle using a 3D imaging technique called microcomputed tomography, which works like an X-ray for the whole organism. They focused in on the ironclad’s elytra.

It may seem unusual for the ironclad to have elytra. After all, it’s a ground-dwelling beetle that can’t fly. But it has evolved from a beetle that, at one time, could, and its elytra are critical to its exoskeleton’s strength. They’ve fused together in the most remarkable way creating a winding, twirling suture. 

The researchers describe it like pieces of a jigsaw puzzle, connecting together. Lock two pieces together and the likely point of failure is at the “neck” of the jigsaw piece. But studying the suture under a high-powered microscope and using computer simulations, the team didn’t see any catastrophic failure. The suture seemed to hold up, transferring the stress across the entire region, rather than cracking open. That’s important — it protects the beetle’s neck

In addition, the chemical composition of the ironclad’s elytra is slightly different from that of a flying beetle. It appears to have a higher concentration of protein mixed in, which could increase the insect’s toughness.

The researchers took it further and looked into how this exoskeleton geometry might enable development of tougher materials. They took the lessons learned from the beetle’s suture and created some carbon fiber jigsaw pieces to test the mechanical strength in a real world application — fasteners used in aerospace engineering. The jigsaw pieces that mimicked the ironclad performed the best.

“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,” said Pablo Zavattieri, a civil engineer at Purdue University and co-author on the study.

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