Development of the synganglion and morphology of the adult nervous system in the mite Archegozetes longisetosus Aoki (Chelicerata, Actinotrichida, Oribatida).

Small arthropods show a highly condensed central nervous system, which is accompanied by the loss of the ancestral metameric organization. This results in the formation of one solid mass, a synganglion. Although numerous studies investigated the morphology of Archegozetes longisetosus, the organization of the nervous system is to date unknown. Using synchrotron X-ray microtomography, we investigated the organization of the nervous system in the adult stage and the development of the synganglion over all five free-living life stages (larva, proto-, deuto-, tritonymph and adult). The general morphology of the synganglion resembles that of other studied mites (in the classic sense) and ticks, being subdivided into a sub- and supraesophageal region, and consisting of cortex and neuropil. All nerves entering the walking legs except the first consist of two rami. This split is not based on a functional division into a motor and a sensory ramus, but both rami contain motor and sensory neurites. Within the synganglion, we found structures that resemble the ancestral metameric organization of the nervous system of arthropods. The development of the synganglion of A. longisetosus shows a more or less linear increase in volume, but cortex and neuropil grow at different rates over the five life stages. Between the second and third nymphal stage, the volume of the neuropil increases at a faster rate than the cortex.