Common design in a unique midline neuropil in the brains of arthropods

Most insects possess an assemblage of midline neuropils in their protocerebrum called the central complex. Recent studies have identified comparable assemblages in the malacostracan protocerebrum. Studies of Drosophila melanogaster locomotory mutants suggest that in insects one role for the central complex might be to orchestrate limb actions. This is anecdotally supported by comparisons amongst insects suggesting that elaboration of central complex architecture correlates with complexity of limb motor repertoires. The present account describes immunocytochemical and neuroanatomical observations that reveal common design principles amongst midline neuropils in four arthropod clades, the hexapods, crustaceans, chilopods, and chelicerates and the absence of midline neuropils in diplopods. The chilopod midline neuropil, which is columnar and stratified and lacks chiasmal axons to the dorsal protocerebrum or connections to discrete satellite regions, may represent the plesiomorphous condition. The complete absence of a midline neuropil in diplopods supports previous neuroanatomical studies suggesting that the 'Myriapoda' are an artificial paraphyletic group. The columnar and layered arcuate midline neuropils of chelicerates are compared with columnar and layered midline neuropils of chilopods. No midline neuropil has been identified in a lophotrochozoan outgroup, the Polychaeta.