Scorpion fluorescence comes from the hyaline layer of the exoskeleton, a thin outer band of the cuticle. Two identified fluorophores are beta-carboline and 7-hydroxy-4-methylcoumarin. Under long-wave ultraviolet at roughly 365 nanometres, they re-emit at blue-green wavelengths, and the whole animal lights up against dark ground.
The effect is remarkably durable. It persists in preserved museum specimens and has been reported in scorpions recovered from amber and rock, though intensity fades. A newly moulted scorpion glows only weakly until its fresh cuticle hardens, which is a useful field cue that the animal has recently shed.
Why scorpions fluoresce is unresolved. Proposals include converting ultraviolet into wavelengths the scorpion can detect so the whole body acts as a light sensor, protection against ultraviolet damage, or fluorescence being an incidental by-product of cuticle chemistry with no function at all. The glow is real and reliable; its purpose is not established.