What arachnid is nocturnal and glows under ultraviolet light?

The arachnid that is nocturnal and glows under ultraviolet light is the scorpion. Many scorpions fluoresce blue-green under a black light because chemical compounds in their hardened outer cuticle absorb ultraviolet radiation and re-emit it as visible light.

Scorpions spend much of the day sheltering beneath rocks, bark, soil, or cracks, then emerge at night to hunt insects, spiders, and other small animals. Their UV fluorescence makes them surprisingly easy to find after dark, so researchers, park rangers, and pest-control workers commonly use handheld ultraviolet lamps during nighttime surveys.

The glow is not the same as bioluminescence: scorpions do not produce their own visible light in darkness. Fluorescence depends on an external UV source. It can also change as the animal molts, because the compounds are associated with the mature cuticle. Spiders and ticks are also nocturnal arachnids, but the vivid UV glow is a classic scorpion trait.