High Frequency Muscles Used in Sound Production by a Katydid. I. Organization of the Motor System

During stridulation the forewings of Neoconocephalus robustus are rubbed against one another at a frequency of 145-212 per second. Despite the high frequency the forewing muscles are synchronous muscles; each contraction is preceded by a muscle action potential. 2. The direct flight muscles of the mesothorax are wing openers during singing; the indirect flight muscles are wing closers. The sound pulse is produced on the closing stroke of the wings. 3. Singing is preceded by warm-up during which all forewing muscles are activated synchronously. In early warm-up the muscles are activated in short bursts, often at a regular frequency. Later warm-up activity is continuous. Muscle activity stops briefly at the transition from warm-up to singing. 4. Muscle activity patterns during singing indicate that the motor output results from an endogenous pacemaker which fires at the singing frequency. There are probably at least three neuronal elements in series between the pacemaker and the forewing muscles. The phasing between opener and closer muscles results from a fixed delay of approximately 3 msec between opener and closer portions of the command chain.