Topographic mapping of the axons of the femoral chordotonal organ neurons in the cricket Gryllus bimaculatus

Central projections of the femoral chordotonal organ (FCO) neurons in the cricket Gryllus bimaculatus were investigated by selectively staining small numbers of axons. The FCOs in all legs consist of partly fused ventral and dorsal scoloparia in the proximal femur. The ventral scoloparium neurons can be reliably divided into two groups: the ventral group neurons (VG), which are arranged in a sequentially smaller manner distally, and dorsal group neurons (DG), which simply aggregate in the proximal region near the dorsal scoloparium. All axons of the FCO projected to the ipsilateral half of the respective thoracic ganglion. The VG axons possessed dorso-lateral branches in the motor association neuropile and antero-ventral branches dorso-lateral to the anterior ventral association centre. However, the more proximally the somata were situated, the more medially the main neurites terminated. The DG axons showed some variations: some axons of the distally located neurons possessed dorso-lateral branches and terminated on the boundary region of the mVAC, while the other axons terminated exclusively in the medical ventral association centre (mVAC), including the ventral part, which receives auditory sensory neuron projections. All axons of the dorsal scoloparium neurons projected exclusively into the dorsal part of the mVAC; however, the ventrally located neurons projected more ventrally than did the dorsally located neurons. The above characteristics were nearly identical in the pro- and metathoracic FCOs. These results suggest that the cricket FCO axons are roughly organized in a somatotopic map and are broadly differentiated in their function.