Ultrastructural comparison of the eyespot and ocelli of scorpions, and implications for the systematics of Chaerilidae Pocock, 1893

An eyespot, a smooth cuticular structure situated posteroventral or ventral to the lateral ocelli, has been documented in at least 60 species in 46 genera and six families of scorpions. The eyespot was confirmed to be absent in 89 genera, 115 species and seventeen families. In the present contribution, the ultrastructure of the eyespot, median and lateral ocelli of the Vietnamese chaerilid scorpion, Chaerilus julietteae Lourenço, 2011, is investigated using transmission electron microscopy. As in the buthid scorpion, Parabuthus transvaalicus Purcell, 1899, the eyespot of C. julietteae differs from the median and lateral ocelli in lacking a dioptric apparatus and pigment granules in the retinula and pigment cells, implying that its function is nonvisual. The presence of an eyespot is documented across Chaerilidae Pocock, 1893, confirming that it is diagnostic and probably synapomorphic for the family. Based on ultrastructure and position, the eyespot of scorpions appears to be a highly modified lateral ocellus that lost its lens and visual function during disintegration of the compound lateral eye.