All scorpions fluoresce under ultraviolet light, such as an electric black light or natural moonlight.

The blue-green glow comes from a substance found in the hyaline layer, a very thin but super tough coating in a part of the scorpion’s exoskeleton called the cuticle.

Scientists have noticed that, right after a scorpion molts, or sheds its shell, it doesn’t glow until the new cuticle hardens.

According to scorpion expert Dr. Scott A. Stockwell, this could mean that the substance that causes fluorescence is a byproduct of the hardening process itself, or it might be secreted not long after the creature molts.

Whatever its source, the glowing property is surprisingly long-lasting. When scorpions are preserved in alcohol, the liquid itself sometimes glows under UV light.

And the hyaline layer is amazingly durable: It can survive millions of years, Stockwell says; it’s often found in scorpion fossils even when all other parts of the cuticle have vanished. What’s more, even fossilized hyaline fluoresces!

Still, scientists don’t know what purpose the fluorescence serves. Some theories:

• It protects scorpions from sunlight.

• It helps them find each other.

• It might confuse their prey.

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