FUNCTIONAL ROLE OF THE N-CELLS AS ANTERIOR THORACIC STRETCH RECEPTORS IN A TERRESTRIAL ISOPOD (ARMADILLIDIUM VULGARE)

ABSTRACT

Although the pill bug, a terrestrial isopod, is articulated in all its body segments, anterior thoracic stretch receptors are not of the muscle receptor organ (MRO) types, but homologues of the N-cells of decapod crustaceans. Since the thorax of decapod crustaceans shows immobility due to the carapace, the N-cells of the thorax have been regarded as retrograde stretch receptors derived from abdominal MROs. The pill bug digs a burrow by coordination of movements of the head, tergites, and thoracopods. Based on this behavior, we tried to specify the function of the N-cells morphologically and physiologically. We could identify innervation of putative accessory neurons to the N-cells, which have never been shown in the N-cells of decapod crustaceans. This identification was made by backfill of Lucifer Yellow to the third nerve roots of the first to third thoracic ganglia. Furthermore, the inhibitory intersegmental reflex, which is normally found among abdominal segments 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 functional role for the N-cells as positional detectors rather than as retrograde stretch receptors.