Data matrix with pycnogonid eye features (this study) compared to median eyes of other arthropods (citations for exemplary taxa used are given above).

A, eye nerves paired and arranged in bilateral symmetry (0) or unpaired (1); B, visual neuropils paired and arranged in bilateral symmetry (0) or unpaired ocellar centre (1); C, number of visual neuropils innervated by R-cell axons greater than one (0) or equal to one (1); D, bifurcation of subsets of visual fibres targeting two different neuropils present (0) or absent (1); E, second visual neuropil with visual fibre terminals in close vicinity to arcuate body present (0) or absent (1). Due to absence of median eyes, Myriapoda are omitted; “-“ indicates that the feature has not been studied.*characters of fused rudimentary median eye.