The field of entomology is built on the humble pin: Biologists venture into grasslands and forests, scoop up insects, euthanize them, and pin them onto the trays that make up natural history collections in museums and universities, thus immortalizing the specimens for future scientists to examine. But the diabolical ironclad beetle—its actual name, though it’s more formally known as Phloeodes diabolicus—will suffer no such indignity. Native to the southwestern US, it’s known as a “pin-bender,” an insect so tough that when biologists try to drive a pin through its black, bumpy shell, the puny metal gives way. It’s so tough that entomologists have to drill a hole through it first, then drive the stake through. Which is an extra indignity, come to think of it.
The diabolical ironclad beetle is so tough, in fact, that if you run one over with a car, it just walks away. It can withstand forces 39,000 times its body weight. To actually crush this beetle requires 150 newtons of force, which, if you don’t speak fluent physics, is 7.5 times stronger than the force you can muster by squeezing something between your thumb and index finger.
For University of California Irvine materials scientist David Kisailus, the diabolical ironclad beetle isn’t just a curiosity—it’s an inspiration. Kisailus and his colleagues are today publishing a paper in the journal Nature decoding at least part of the mystery of how the beetle can manage such feats of strength. Natural selection has invented an ingenious structure that keeps the insect from flattening, a structure that Kisailus has begun to mine for ways of engineering new super-strong materials. “We're pretty stoked, because we think we can go to aircraft, automotive, sporting good industries with this kind of design,” says Kisailus.
So, to begin: What in the wide, wide world of insects is a beetle doing withstanding such forces? Morphologically speaking, it’s the beetle’s elytra—the two hard shells that you see a ladybug open when it unfurls its wings and takes flight—that are acting as its shield. But the diabolical ironclad beetle (henceforth known as the DIB) can’t fly. Over evolutionary time its elytra fused together and to the rest of its exoskeleton, creating a cohesive shell.
“Many large flightless beetles tend to have this characteristic (being really tough), particularly those that do not have strong chemical defences,” writes Matthew Van Dam, a beetle expert at the California Academy of Sciences, in an email to WIRED. (He wasn’t involved in this new work.) “Other studies have found that it is a good defense against predation. So the trait probably evolved as a defense against predators.”
We might first assume that the beetle is integrating some kind of mineral into its exoskeleton to give it extra strength. That wouldn’t be unprecedented: One deep-sea snail, for instance, builds a shell out of iron. But nope, the DIB is fully organic. “What we do know is that it's simple organic materials—there's no mineral, like you'd find in a shell that is really crush-resistant,” says Kisailus. “The beams that hold up your freeways are concrete for a reason: Ceramics are great under compression. And yet there's no mineral in this. It's all organic.”
So there has to be something special going on with the structure of the exoskeleton: The body must be constructed in such a way that absorbs the energy of a crushing blow, sort of like the way a skyscraper is built to sway slightly in an earthquake to avoid snapping in half. And indeed, Kisailus and his colleagues found two key evolutionary innovations that make the DIB so dang tough: lateral supports and a medial suture.
Think of the elytra and the exoskeleton of the insect’s underbelly as being like a cup and its lid, fitting together. Where the two come together around the abdomen, they form lateral supports with "interdigitation," intertwining like fingers. “When it's compressed, those lateral supports, like columns of a bridge, provide some compressive strength,” says Kisailus. “And what we found was that near its vital organs, the amount of overlap between the top half and bottom half of those columns was significant—significant interdigitation. But as you moved away from the vital organs towards the tail section of the beetle, there was less interdigitation. And it actually allows the beetle to compress and have some compliance.”
That is, instead of failing under pressure—like from a car’s tire—the beetle’s shell gives. Think about the way your belly compresses when you take a punch to the gut. In this case, instead of absorbing the blow with stomach fat and muscles, the DIB’s rigid exoskeleton compresses to cushion the impact.
The medial suture is the second critical ingredient. Species of flying beetles fling open their two elytra to unfurl their wings. But as a terrestrial beetle, the DIB has fused the two coverings together. In the image at the top of this story, you can see that the two halves aren’t just fused—they’re intertwined like puzzle pieces.
“Jesus [Rivera], the lead author who's the MacGyver in the group, he showed me this structure from the microscope images,” says Kisailus. “I was like, ‘Hey, that's a jigsaw puzzle.’ So I went out to RiteAid that night and bought a 100-piece jigsaw puzzle, just for big-enough-sized pieces. I brought it to the group meeting the next day, and my students were laughing at me and making fun of me like I was a crazy guy.”
If you think about two puzzle pieces locking together, a sort of bulbous lobe fits into an empty space. Given its shape, you’d expect the lobe to snap if you tried to pull the two pieces apart. Specifically, it should snap where there’s the least material, where the lobe narrows into a kind of neck. But, it turns out, that takes a lot of effort. “If you take two pieces of that jigsaw puzzle, and you try to pull them apart, once they're attached, it's a pretty robust interface,” says Kisailus. “And so that is what provides the beetle with strength.”
For their study, Rivera demonstrated this by compressing a cross section of the DIB’s exoskeleton in a sort of miniature vise and pointing an electron microscope at it. This imaging technique works by bombarding the subject with electrons, so researchers can watch how the particles scatter, producing a picture of the beetle’s structure in fine detail. When they applied force to the cross section, approximating something trying to squish the DIB, the structure held. “Lo and behold, the interface at that suture—the jigsaw puzzle we're talking about—did not fail,” says Kisailus.
Looking through an electron microscope, any cracks in an object should show up as bright white spots. Instead, what Rivera and Kisailus found was that the DIB’s protective shell is actually a kind of laminated composite, layer upon layer of fibrous material. “When you pull on the puzzle piece, instead of ripping apart the neck, those laminated structures start to separate slightly,” Kisailus says. “That's what defines the toughness in this organism—it provides a lot of energy dissipation instead of failing brittlely. It just de-lamanites.” The researchers could actually see a kind of fraying of the material under the microscope. This medial suture works with the lateral supports to keep the beetle's body from collapsing, even under extreme stress.
Now Kisailus and his team could copy the DIB’s tricks. They were thinking of the construction of airplanes (the team received funding from the Air Force for this research), and how it’s so tricky to attach the pieces of composite material that form the fuselage to each other. Engineers do this with rivets and adhesives, but these are vulnerable to catastrophic failure, which is not ideal at 30,000 feet.
So in the lab, the researchers joined a carbon-fiber-reinforced composite plate to an aluminum plate with a “Hi-Lok” pin, a fastener used in aerospace engineering to attach structures. Then they fabricated pieces of laminated carbon-fiber composite, shaped like the beetle's sturdy puzzle-edged medial suture, and also joined it to a milled aluminum plate that way. Next they tugged on the two ends of the carbon-fiber and aluminum structures and watched how well their newfangled suture held up compared to the materials attached by a traditional fastener. Things did not go well for the fastener. “When you look at the failure of the fastener, it failed catastrophically,” says Kisailus. “But when you look at the failure of the beetle mimic, it didn't fail catastrophically. You have the same delamination that we saw in the beetle, but it was happening in the carbon-fiber composite that we mimicked.”
Before the researchers can fully exploit the DIB’s powers, though, they have to unravel the mysteries of the biological materials at play. If the beetle isn’t incorporating additional minerals into its exoskeleton to strengthen it, is there something special about the proteins that make up the shell? “We don't know what those proteins are,” says Kisailus. “Are they hyperelastic?”
Finding the answer could one day help make aircraft and drones that resist catastrophic failure. For now, though, we’ll just have to put a pin in it.