Imagine lying on the forest floor at night, staring up into a canopy twinkling with jewels. Some are bright, hard gold. Many are emerald green, or shine like a polished granny smith apple under a spotlight. A very few are ruby red. You reach up to pluck one of them from the dark trees. But as you extend your hand, the gem whirs away to a point out of reach.
Chrysina limbata, left and Chrysina resplendens, right. (Images from museum.unl.edu.)
This is a sight you might find in the American southwest. There, pine, juniper, and oak trees drip with gems like Audrey Hepburn's neck outside Tiffany's. This isn't because someone actually managed to break into that jewelry store and transport the loot to a forest in Arizona. It's because the trees are decorated with bugs called jewel scarabs.
These curious beetles are not your normal black, bulbous, boring scarabs (which aren't actually boring at all: just ask the Ancient Egyptians if you want a monologue about how godly they are). The jewel scarabs are outrageously bright, unnaturally metallic, and daringly iridescent: impaled on a pin inside a box labeled "Oh My!" in the back room of the Smithsonian's insect collection, the golden beetle in front of me actually looks like a drop of gold.
A box of bugs in the Oh Wow! Smithsonian entomological collection. Several of the small, very shiny gold and granny smith apple green ones are jewel scarabs. (Image by Lauren DiVito.)
Imagine these jewel bugs winging around a forest clearing. With their dazzling good looks, they'd attract the attention of every creature in the forest. If I had a back that looked like a million bucks I'd probably show it off too, but when you're only a bug, you have to be careful. Predators are everywhere. You could get eaten by a blackbird or squished by a lizard. Standing out from your surroundings like a squid in a tree is not a plus when you're a one-inch-long beetle and don't happen to possess the powers of an Egyptian sun god.
As the theory of natural selection has taught us, organisms want, above all else, to have features that make them more likely to last in the big bad world. We know that wearing brightly colored garb does not make a bug faster, stronger, bigger, healthier, brainier, or in any way better at surviving than its less flashy friends. So according to Charles Darwin (and all of modern science), natural selection should have caused the brightest, most visible bugs to be killed by beady-eyed predators a long time ago. It should have left the dullest, most camouflaged ones to survive and create the next generation. But it didn't.
One possible advantage these bugs have begins with the way light hits that lustrous, shiny surface. Most light in the world is a jumble of squiggly waves, springing helter skelter from the sun or a light bulb. Some light in the world is organized, its waves put in line like disciplined school children crossing the road. Sunglasses are one kind of light organizer: like a strict teacher, they rap unruly light waves on the knuckles and send them back home, and let only the dignified and orderly waves pass through.
When it comes to organizing light, sunglasses are crude compared to jewel scarabs. These bugs are professional light manipulators, veritable titans of disguise. They can wrangle the light hitting them as random rays into precise alignment, creating a spiral shape. And by exerting control over rowdy waves, they make themselves appear daringly flashy to certain eyes and completely camouflaged to others.
Chrysina Gloriosa, a beetle that lives in the American southwest and has a special way of detecting and creating a rare kind of light, called circular polarized light. (Image by K. Robacker.)
How can they possibly manage to be simultaneously ostentatious and hidden? A study published in 2010 by Parrish Brady and Molly Cummings at the University of Texas suggests that members of the species Chrysina gloriosa use their ability to bully light into order to signal other beetles. While members of their own species are sensitive to this rare form of light, other animals' eyes cannot detect it. The bugs' predators, therefore, see these jewels as mere dull blobs that blend maddeningly well into their surroundings, while their potential mates can't miss them.
While it should be noted that the bugs' unusual way of ordering light is distinct from their bright, reflective colors, the authors of the paper suggest that both factors play into the bug's ecology and way of living in the wild. The complexity of the games this little bug plays with light, both in polarizing it to communicate and absorbing it to let off shocking hues, is mindboggling.
To find out more about jewel scarabs, visit the National Museum of Natural History's Insect Collection, which is one of the three largest in the world.
This post was inspired by a lecture by Gary Hevel for Biodiversity Week, one of the Smithsonian Science Educational Academies for Teachers (SSEATS).