Have you ever tried hunting for scorpions at night using an ultraviolet light? They fluoresce! Here is a scorpion near Portal, at night under the “normal” illumination of a camera flash:
And here is the same beast under a UV light:
Quite impressive. That led me to the question of why they glow an eerie green under UV light, and google came through again. An unidentified substance in the scorpion’s cuticle causes the fluorescence. When they have just molted, they don’t glow. The fluorescence increases as the cuticle hardens.
One notion is that the fluorescent layer functions as a UV sensor. Indeed, the scorpions that I found reacted to the UV flashlight by trying to hide from it.
Amazingly, this very tough fluorescent layer is preserved in fossils––and the fossils of scorpions fluoresce as well!