"All scorpions will glow like this when exposed to black light, with the only exception being if they just molted. The cause of this "glowing" phenomenon is the chemical makeup of a scorpion's exoskeloton(sic) fluoresces when exposed to certain wavelengths of ultraviolet light." Bug Guide
Some spiders also fluoresce.
Some animals see UV, so if you are huntin said animals you should use detergent wiothout optical brighteners.
(Wave a UV light over your shirt.. oopsies, you glow.)