A tiny visual system - retinula axons and visual neuropils of Neobisium carcinoides (Hermann, 1804) (Chelicerata, Arachnida, Pseudoscorpiones)

Only a few studies have examined the visual system of Pseudoscorpiones until now. To fill this knowledge gap we analysed the axonal trajectories and neuropil architecture of the visual system of the pseudoscorpion Neobisium carcinoides (Hermann, 1804) with different neuroanatomical techniques. The R-cell axon terminals were identified with Cobalt fills and the morphology of the visual neuropils and the protocerebrum generally is described by means of the osmium-ethyl gallate procedure and TEM. N. carcinoides has two lateral eyes on each side of the prosoma. The R-cells of the eyes per hemisphere are linked to a first and a second visual neuropil, located in the dorsolateral protocerebrum. The first visual neuropil is subdivided into two hemineuropils, one for each lateral eye, while the second neuropil is not. Furthermore, the two-layered arcuate body is found − isolated from the visual neuropils − in the midline of the protocerebrum at a dorsoposterior position. These findings allow a detailed comparison of the pseudoscorpion visual system with that of other previously studied taxa and shed new light on lateral eye evolution in Chelicerata.