Light wavelength biases of scorpions

Scorpions are negatively phototactic animals, and physiological data suggests that their photoreceptors are differentially sensitive to light wavelengths ranging from red to ultraviolet. However, behaviour modification resulting from exposure to different wavelengths has not been established. We monitored behavioural responses of animals in small circular arenas while they were presented with different wavelengths of light (red, green, UV, or no light) matched for intensity. In the first experiment using desert grassland scorpions, Paruroctonus utahensis, half of each arena received the light treatment, while the other half was shaded. The results indicated that the amount of time that scorpions spent on the light-exposed side varied depending on the treatment and that avoidance was greatest for UV light followed by green light. In subsequent experiments using both P. utahensis and striped bark scorpions, Centruroides vittatus, the entire arena was subjected to the particular light wavelength while animal locomotory activity was monitored. We found no significant difference in animal responses to randomized, sequential 30 min presentations of all four light treatments. Scorpion activity was greatest during the first 10 min of the 30 min trials; in the first 5 min period, the highest activity levels were in the UV light treatments, followed by the green light treatments. In behavioural tests to either green or IR light, animals moved sporadically and significantly faster under green light compared to IR light treatments. Taken together, we conclude that different wavelengths of light affect scorpion locomotory behaviour differently.