Flash photography of the wings of a decticine katydid, Metrioptera sphagnorum (F. Walker), during stridulation, indicates that major trains of sound pulses of both song modes coincide with wing closures. The song mode characterized by intense ultrasonics is generated by tooth–scraper interactions over an interval of approximately 60 teeth at the lateral end of the file. A more medial file region is employed for the audio-dominated mode. Tooth removal confirms the role of different file regions in mode generation. Each pulse of the ultrasonic mode closure is a wave train of 10–15 waves of uniform amplitude, approximating a pure tone frequency of 33 kHz.