Functional role of the N-cells as anterior thoracic stretch receptors in aterrestrial isopod (Armadillidium vulgare)

Although the pill bug, a terrestrial isopod, is articulated in all its body segments, anterior thoracic stretch receptors are not of the muscle recept or organ (MRO) types, but homologues of the N-cells of decapod crustaceans. Since the thorax of decapod crustaceans shows immobility due to the carapa ce, the N-cells of the thorax have been regarded as retrograde stretch rece ptors derived from abdominal MROs. The pill bug digs a burrow by coordinati on of movements of the head, tergites, and thoracopods. Based on this behav ior, we tried to specify the function of the N-cells morphologically and ph ysiologically. We could identify innervation of putative accessory neurons to the N-cells, which have never been shown in the N-cells of decapod crust aceans. This identification was made by backfill of Lucifer Yellow to the t hird nerve roots of the first to third thoracic ganglia. Furthermore, the i nhibitory intersegmental reflex, which is normally found among abdominal se gments of crayfish, proved to be between a stretch-activated N-cell and an ordinary thoracic stretch receptor activated by a stretch stimulus. Flexion and extension of anterior segments in digging behavior suggest a functiona l role for the N-cells as positional detectors rather than as retrograde st retch receptors.