Three neural groups in the femoral chordotonal organ of the cricket Gryllus bimaculatus: central projections and soma arrangement and displacement during joint flexion.

Abstract

The arrangement of neuronal somata and their displacement during joint flexion together with the central projection of the pro- and metathoracic femoral chordotonal organs (FCOs) in the cricket were investigated. The FCO consists of the partially fused ventral and dorsal scoloparia in the proximal femur. The ventrally located neurones (the ventral group) form chain-like rows in which somata became sequentially smaller distally and project their axons ipsilaterally to the dorso-lateral regions, giving off abundant branches and terminating in the region between the dorsal intermediate tract and the ventral intermediate tract in the thoracic hemiganglion. The dorsal scoloparium, composed of small, simply aggregated neurones, projects exclusively to the medioventral association centre (mVAC), which is known to be an auditory neuropile. In addition, another neural cluster (the dorsal group) was found in the proximo-dorsal region of the ventral scoloparium. This was composed of simply aggregated neurones with axons giving off sparse branches dorso-laterally and terminating in the peripheral region inside the mVAC. The somata of these three groups were displaced distally by flexion of the femoro-tibial joint: the ventral group showed the greatest displacement, with the degree of movement depending upon soma location, while the dorsal group and dorsal scoloparium neurones were hardly displaced, possibly because of their strong connection with the cuticle. These properties were similar in both the prothoracic FCO and the metathoracic FCO. Taken together, the above points suggest that there is greater functional differentiation of the FCO than was previously thought.