Comparative morphology of central neuropils in the brain of arthropods and its evolutionary and functional implications.

Most insects and decapod crustaceans possess an assemblage of midline neuropils, the central complex. Recent phylogenetic studies show a sister-group relationship between hexapods and decapods, suggesting that central complexes in both groups are homologous structures derived from a basal ancestral neuropil. This ancestral archetype of the central complex (lacking the protocerebral bridge) might be represented in the chilopods. Until recently, diplopods were regarded as closely related to chilopods and united within the taxon "Myriapoda". The entire lack of a midline neuropil in diplopods, however, renders the monophyletic origin of the class Myriapoda unlikely. In this study we used a palette of immunocytochemical and neuroanatomical methods to investigate mid-line neuropils in hitherto poorly examined arthropod groups. Of special interest for resolving arthropod phylogeny are onychophorans, who are believed to be an evolutionary ancient group that resembles the ancestors of modern arthropods. Striking similarities in central brain neuroarchitecture of the onychophoran Euperipatoides rowellii and of a chelicerate species, however, suggest a close phylogenetic relationship between these two groups. Our findings imply that onychophorans either represent the oldest form of the chelicerates or that extant onychophorans have developed from chelicerate-like ancestors by neoteny.