The femoral chordotonal organ: A bifunctional orthopteran ( Locusta migratoria) sense organ?

1. 1. The femoral chordotonal organ (feCO) in a middle leg of a locust consists of a proximal and distal scoloparium both being attached to the femoral-tibial joint by a common receptor apodeme. 2. 2. Both scoloparia show marked differences in the number and size of their sensory cells and axons. 3. 3. Specific central cobalt backfills of the two scoloparia revealed marked differences in the terminal areas within the mesothoracic ganglion which do not overlap. 4. 4. Recordings from the sensory nerve of the proximal scoloparium revealed phasic and phasic-tonic units as well as many tonic units. A strong response to 4μm vibration of the receptor apodeme at 50–300 Hz was observed. 5. 5. The behavioural roles of the two scoloparia were studied by simultaneous intracellular recordings from extensor and flexor tibiae motor neurons while the receptor apodeme was passively moved. 6. 6. Intact animals showed the well-documented resistance reflexes. 7. 7. Distal scoloparium ablation abolished the resistance reflex, but revealed weaker arousal or startle effects. 8. 8. Proximal scoloparium ablation did not change the occurrence of the resistance reflex. 9. 9. We conclude that the resistance reflex is mediated only by the distal scoloparium. The role of the proximal scoloparium remains obscure although the weak arousal effects and potential vibration sensitivity may provide hints of its function.