Immunolocalization of histamine in the optic neuropils of Scutigera coleoptrata (Myriapoda: Chilopoda) reveals the basal organization of visual systems in Mandibulata.

Myriapods play a crucial role in considerations of evolutionary transformations of arthropod nervous systems. The existing descriptions of the identity and connectivity of myriapod optic neuropils are contradictory. This study asks if the first and second optic neuropil of the scutigeromorph centipede Scutigera coleoptrata correspond to the optic neuropils of Hexapoda and malacostracan Crustacea, the lamina and medulla which are linked by neurites that are arranged in a characteristic optic chiasm. To identify photoreceptor axons, we used immunohistochemistry against histamine which is the universal transmitter of arthropod photoreceptors. Our results provide evidence that the two optic neuropils of S. coleoptrata correspond to the lamina and medulla of Hexapoda and Malacostraca and strongly argue against a correspondence of the optic neuropils in branchiopod crustaceans and scutigeromorphs, as was previously suggested. We conclude that these two retinotopic optic neuropils and an outer optic chiasm are part of the ground pattern of Mandibulata and that the visual systems of branchiopod crustaceans were simplified from this ground pattern.