Fig. 3. Cycle-to-cycle frequencies (f_C) of single sound pulses for representative S. borellii and S. vicinus at 25° C. The f_C is not constant and decreases by 5–10% late in the call (especially so in this S. vicinus individual). The instantaneous jumps in frequency late in the pulses (asterisks) are believed to be associated with the disengagement of file and plectrum. This is followed by a period of undriven vibration, during which the sound amplitude drops exponentially. Although the patterns given in this figure are typical, a number of individuals of both species showed different degrees of f_C stability and sometimes lacked a noticeable change in frequency at the start of the sound amplitude's exponential decay.