When exposed to ultraviolet rays, scorpions begin to glow.

The blue glow comes from the entire body of the animal, or rather from its exoskeleton. According to one version, this feature helps them to hunt better, and also protects them from attacks by other predators. 

Other scientists believe that the ability to glow is one of the components of mating games. But the most popular theory suggests that the whole body of a scorpion performs a kind of "eye" function and receives UV rays.