Their exoskeleton contains a protein which fluoresces. Scorpion exoskeleton continues to fluoresce even after they dead, specimens preserved in alcohol and even fossilised scorpions continue to fluoresce. Only newly moulted scorpions don't fluoresce. We still don't know why they do this. Some have become popular pets and are endangered by over-collection for the pet trade.
Scorpions are vulnerable because they have the lowest rate of reproduction among arthropods.
They don't lay eggs and young must be raised by living parents, and it takes so long for females to produce babies. It also takes a long time for a scorpion to reach maturity, from 1-3 years. There are many scorpions only because they live and can stay alive for so long. Dr Stockwell also wrote the Tree of Life page on scorpions. Copyright Singapore Zoological Gardens Docents 2000.