Why are some scorpion species fluorescent under UV light?
As far as I can see from different articles, this seems to be a study in progress. Dr Kloock from the California state University states that scorpions might be more active when not exposed to UV light. If not fluorescing, they know that they are hidden and have a lesser chance of being preyed on. He argues that scorpions are less active on full moon nights rather than other days. (reference) Dr Kloock has also stated that the pigment could get used up on repeated exposure to UV light. He made an observation that scorpions whose pigments got used up were more active than those still fluorescing when exposed to UV light mimicking the moon and the stars. (reference) Dr Gaffin from the University of Oklahoma, in his studies have reported that scorpions might be converting UV light to turn it into the colour blue-green which is what they see best effectively turning their body into one big eye. His studies are consistent with Dr Kloock in that scorpions try to hide and avoid UV light (reference). Some scientists have also hypothesized that scorpions might glow to warn predators or that the pigments that make it glow are the accidental by products of certain chemical reactions in its body (reference). There is no definite proof on this subject though.