Somatotopic mapping of chordotonal organ neurons in a primitive ensiferan, the New Zealand tree weta Hemideina femorata: I. femoral chordotonal organ

Abstract

The femoral chordotonal organ (FCO) in orthopteran insects comprises several hundred sensory neurons, making it one of the most complex insect proprioceptors. The sensory neurons are suspended from the proximal femur, connecting distally to ligaments and to a needle-like apodeme extending from the proximal tibia. They monitor the position and movement of the tibia. To address how this complexity depends on evolutionary status and function, the morphology of the FCO neurons in the primitive orthopteran Hemideina femorata was investigated by staining small populations of identified afferents. As in crickets, the FCOs in all legs of the weta comprise partly fused ventral and dorsal scoloparia, with the former containing two groups of somata, the ventral group (VG) and the dorsal group (DG). However, the dendrites of the DG insert into thin connective tissue attached to the ventral side of the dorsal ligament, forming a "third scoloparium." The VG afferents terminate mainly in the motor association neuropils, whereas afferents from the dorsal scoloparium neurons terminate exclusively in the vibratory neuropil as do the afferents from the subgenual organ, a substrate vibration detector. Several afferents originating in the DG have extensive terminations in the motor association-, vibratory-, and auditory-processing neuropils, indicating lesser functional specialization than in the other groups. The evolutionary development of the FCO is discussed from a comparative viewpoint.