Scorpion fluorescence and reaction to light

Scorpions are largely solitary, nocturnal arachnids that glow a bright cyan-green under UV light. The function of this fluorescence is a mystery. Previous studies of four species from three families have shown that scorpion lateral and medial eyes are maximally sensitive to green light (around 500 nm) and secondarily to UV (350–400 nm). Scorpions are negatively phototactic, and we used this behaviour to assay the responses of desert grassland scorpions, Paruroctonus utahensis, to 395 nm UV light, 505 nm cyan-green light, 565 nm green light and no light within small, circular arenas. Based on the eye sensitivity data, we predicted maximal response to 505 nm, followed by lower responses to 395 and 565 nm. In our experiments, however, scorpions responded most intensely (abrupt bouts of locomotory activity) to 395 nm and 505 nm. Next, we ran trials under 395 and 505 nm on scorpions with their eyes blocked. Scorpions with blocked eyes were much less likely to move under 505 nm than under 395 nm and were much less likely to move under 505 nm than were control animals (those without their eyes blocked). These results suggest an active role for fluorescence in scorpion light detection. Other studies indicate that photosensitive elements in scorpion tails are sensitive to green light. We therefore propose that the cuticle may function as a whole-body photon collector, transducing UV light to cyan-green before relaying this information to the central nervous system. Scorpions may use this information to detect shelter, as blocking any part of the cuticle could diminish the signal.

► Scorpions are negatively phototactic and glow a cyan-green under UV light.
► We assayed responses to UV and green light when scorpions’ eyes were blocked and unblocked.
► Scorpions’ responses to UV were more intense than that predicted from retinal sensitivity.
► Scorpions with their eyes blocked altered their behaviour more under green light than under UV light.
► We propose that scorpion fluorescence influences scorpions’ perception of light.