Why do scorpions glow under ultraviolet light?

Today's question:

I often read that scorpions glow under a black light. I have never seen an explanation for why this is. Do you know?

Two questions here: How do scorpions glow under ultraviolet light, and why do they do it? The first one is pretty easy. Scorpions have certain proteins in their exoskeletons that react to ultraviolet rays and give them that creepy blue glow. The real question is why? What's the purpose?

Over the years scientists who consider such matters — arachnologists — have come up with a number of ideas.

One idea is that the glow might help scorpions find other scorpions, given the way they tend to blend in with their desert surroundings.

Some think they use it to dazzle their prey because many animals can detect ultraviolet light. And some say that glow is a kind of sunscreen, which doesn't make much sense because scorpions are nocturnal. And there is a school of thought that it doesn't serve any purpose at all. It's just a fluke of evolution..

However, the latest thinking, based on research at California State University, suggests another idea. Scorpions do not like heat and bright light. They lose some of what little water they have to evaporation. So they use their sensitivity to UV rays to determine if it's dim enough for them to crawl out and get to business.

It's true that really hungry scorpions will go out on the hunt regardless of light levels, but they do tend to be less active on moonlit nights, especially under a full moon, even though there is a lot less UV light from the moon than the sun.