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. We used male and female P. utahensis collected during March 20 from a sandy region about 30 km southeast of Monahans, Texas. In the laboratory, we kept the animals in individual glass jars with about 2–3 cm of sand covering the bottom. We fed the scorpions one waxworm ( Achroia grisella) every 3–4 weeks and watered them twice per week. We filmed the scorpions from beneath a Wavelength Response Trials The room temperature remained within a range of 21–24 ☌, and the room lighting followed a 16:8 h light:dark cycle. Most trials (70%) met the criteria for validity (12 or more ICIs of less than 45 s). Animal movements were noticeably sporadic under certain wavelengths of light compared to no-light trials. The distribution of the first 20 crossings for the 10 min trials for scorpions under 395 nm and no light showed that stimulated animals (e.g. Twenty of the 40 animals had valid trials for all four Discussion 395 nm) had a disproportionate number of crossings early in their excursions compared to no-light controls (Fig. 2). Scorpions moved in sporadic bursts under cyan-green and UV wavelengths of light compared to yellow light and no light. Furthermore, their response to UV was greater than what would be expected based on previously published physiological studies of retinal sensitivity. Also, scorpions significantly altered their behaviour under green light when their eyes were blocked by foil compared to scorpions without their eyes blocked. The behaviour of eyes-blocked scorpions under UV was less affected. We thank Dr Douglas Mock, Dr Ingo Schlupp, Dr Mariëlle Hoefnagels, Dr Eric Abraham, Elise Knowlton, Andrea Jordan and Tristan Barker for valuable advice, technical support and consultation services. Thanks also to Jay Vinnedge for assistance with the eye-block behavioural trials. Special thanks go to Greg Blass for making the initial forays into scorpion UV-stimulated behaviour.
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