What makes scorpions glow under UV light? What substance (or class of substances) is responsible for the neon blue-colored fluorescence we observe when we shine UV light on scorpions?



Do note, I want to know what substance makes them glow and not why they glow... the latter being more suited for the Bio.SE.
As mentioned in this comment, which links to this paper, and this comment, which links to this Wired article, there are at least two molecules:

$\beta$-carboline:



4-methyl-7-hydroxycoumarin:



$\tiny\text{Yes I poached those comments, they could have been answers.}$