Wiring diagrams of the major pathways in the first visual neuropil of the pycnogonid Achelia langi . Pathways represented by >1 synaptic sites are shown. R-cells provide input into the system and are presynaptic to D- and A-cells. D-cells are predominantly postsynaptic to R-cells and, hence, are second-order neurons. A-cells are pre- and postsynaptic to both R- and D-cells and may play a feedback role.