Why Scorpions Glow Under Black Light

A bucket full of scorpions under black light. Flickr (CC By 2.0)

Some animals are just over the top, you know? Take scorpions: Is it not enough that these stinging arachnids can survive some of the harshest climates in the world, live 25 times longer than your average cockroach, give birth to live young (and some species don't even need a male to reproduce), live up to 48 hours without oxygen, and eat almost anything they can subdue — even other scorpions? No, it's not enough, because scorpions also glow electric scarab green under ultraviolet light.

Why they do this is a bit of a mystery, but it makes them pretty easy to study. All a scorpion researcher has to do to find scorpions is go out into the desert at night with a black light, and watch those suckers light up like Christmas trees. Chemically speaking, nobody's exactly sure what causes scorpions to glow, but we know it's powerful stuff — when a scorpion is preserved in alcohol, the alcohol itself fluoresces! Unbelievably, scorpion fossils have even been induced to glow under black light after hundreds of millions of years. What we do know is the chemicals that make a scorpion so rave-ready are in the outer layer, or cuticle, of its exoskeleton — scientists call it the hyaline layer. Because scorpions molt their exoskeleton every so often in order to grow, researchers have observed that, until the mushy outer shell has entirely hardened, the hyaline layer doesn't fluoresce under UV light.

This is all very strange. Why would an animal evolve to glow under an ultraviolet light? Researchers have posited a bunch of different ideas: Scorpion fluorescence might help them find each other in the dark, protect them from sunlight, or even confuse their prey. But as one 2011 study suggested, scorpions use their exoskeletons to detect UV light — mostly because they want to avoid it (they're night hunters after all, and a scorpion will always find the darkest place to hang out during the day or even in the moonlight). Further study found that they appear to be using their entire bodies as giant UV-seeking eyeballs — if they sense they're glowing at all, it's time to scurry off somewhere darker.