1. The structural and physiological properties of a chordotonal organ found in the metathoracic femoral segments of the locust and grasshopper are described.
2. Recordings from the afferent fibres of this mechanoreceptor reveal two patterns of activity: a static or tonic discharge and a dynamic or phasic, rapidly adapting discharge. The frequency of the tonic discharge varies with the femur-tibia angle, while the phasic discharge occurs in response to angular movements of the tibia of > 3'. The frequency of the phasic response is related to the velocity of tibial displacement. The responses recorded during flexion of the tibia are different from those recorded during extension.
3. Removal of either of the metathoracic chordotonal organs produces significant changes in walking and postural behaviour.
4. Leg reflexes involving the femoral chordotonal organ and femoral muscles appear to play a very significant role during postural and walking activity.