David Kisailus / UCI
You can accidentally stomp on the fiendish iron beetle and it won't even flinch. Go one step further - drive your car over it - and that won't cause the animal any problems either. Its exoskeleton is one of the toughest in the animal kingdom. And scientists now think they know why.
In a study published Wednesday in the journal Nature, researchers uncovered the secrets of the fiendish iron beetle's amazing compressive strength and demonstrated how new ultra-tough materials can harness the beetle's biology.
At first glance, the beetle looks impressive: a dark, bumpy exoskeleton that looks a bit like a charred stone. But beneath its humble exterior lurk some structural wonders built by evolution. Many species of beetles can fly and their wings are enclosed in elytra, a protective, tough shell. Flying is a great defense mechanism for beetles, allowing them to escape predators, but the iron coat has no wings and routinely plays dead. It relies on its exoskeleton to keep it safe.
"The iron beetle is an earth beetle, so it's not light and fast, but rather built like a small tank," said David Kisailus, professor of materials science and engineering at the University of California at Irvine and co-author of the study in a paper. The beetle's exoskeleton is so hard that it even poses some problems for entomologists hoping to show them - it is difficult to put a needle through the iron casing.
To examine the tiny tanks, a member of the research team, Jesus Rivera, captured beetles and brought them back to the laboratory. First, the researchers discovered that the beetle's exoskeleton can withstand around 150 Newtons of force - 39,000 times its body weight. Three other species of land beetle were only half as tough.
But why is this particular exoskeleton so much stronger? The research team examined the beetle using a 3D imaging technique called microcomputed tomography, which works like an X-ray for the entire organism. They focused on the Ironclad's Elytra.
It may seem unusual if the Ironclad has Elytra. After all, it's a ground beetle that can't fly. But it evolved from a beetle that once could, and its elytra is critical to the strength of its exoskeleton. They have come together in the most remarkable of ways in a tortuous, swirling seam.
The researchers describe it as pieces of a puzzle that connect with each other. Lock two pieces together and the likely point of failure is at the "neck" of the puzzle piece. However, examining the seam under a powerful microscope and using computer simulations, the team could not find any catastrophic failure. The suture appeared to hold, transferring tension to the entire region instead of breaking open. This is important - it protects the beetle's neck
In addition, the chemical composition of the iron shell's elytra is slightly different from that of a flying beetle. It appears to have a higher concentration of protein which could increase the insect's toughness.
The researchers went a step further and investigated how this exoskeleton geometry could enable the development of harder materials. They took the lessons from the seam of the beetle and created some carbon fiber puzzle pieces to test mechanical strength in a real world application - fasteners used in aerospace engineering. The puzzle pieces that mimicked the iron armor performed the best.
"This work shows that we can potentially move from using strong, brittle materials to those that are both strong and tough by dumping energy when they break," said Pablo Zavattieri, civil engineer at Purdue University and co- Author of the study.
