The nymphal eyes of Parabuthus transvaalicus Purcell, 1899 (Buthidae):: An accessory lateral eye in a scorpion

In P. transvaalicus nymphs, 5 pairs of lateral ocelli each composed of a corneal lens, R-cell units forming a latticed rhabdom, arhabdomeric cells and pigment cells are present. In addition, we found a pair of unpigmented accessory sense organs situated ventroposteriorly to the lateral ocelli in prenyrnphs as well as in first nymphs. They are composed of primary, rhabdomeric sensory cells, and we infer that they represent a second type of lateral eye. They also comprise sensory units, but lenses and screening pigment are lacking. Their position and cellular architecture corresponds well with that of the rudimentary lateral eye of the xiphosuran, Limulus. The occurrence of a bipartite lateral visual system in Chelicerata and Arthropoda is discussed.