Activity and directional sensitivity of leg campaniform sensilla in a stick insect

1. The mechanoreceptive campaniform sensilla, which are arranged in groups on insect legs, were studied in the stick insect Cuniculina impigra. In middle and rear legs, the most posterior trochanteral campaniform group (group 1) is oriented so as to be stimulated by compressional cuticular forces acting on the posterior articulation of the trochanter with the coxa. 2. Recordings from nerve Tr2, which innervates this campaniform group, confirmed that the sensilla were directionally sensitive to cuticular stress generated by imposed or active movements of the femur. Different individual sensilla had different thresholds of response, but all responded qualitatively in the same way to a given stimulus. 3. Posteriorward horizontal deflection of the femur-trochanter relative to the coxa (at right angles to the normal plane of movement) produced a strong excitation of the group 1 sensilla. Anteriorward horizontal deflection caused a sharp reduction of on-going background activity in the group. Imposed vertical movements of the femur usually had little effect on the activity of the campaniform sensilla. 4. Active depression of the femur-trochanter by the animal reduced on-going background activity in proportion to the strength of the depression movement. Elevation that simulated muscle-generated lifting of the leg strongly increased activity. 5. It is concluded that campaniform group 1 functions to signal passive horizontal and active vertical movements around the trochantero-coxal joint of the leg. This dual pattern of response means that the animal cannot interpret signals from this campaniform group without also having information about muscle activity.