ABSOLUTE STEEPNESS OF RAMPS AS AN ESSENTIAL CUE FOR AUDITORY PATTERN-RECOGNITION BY A GRASSHOPPER (ORTHOPTERA, ACRIDIDAE, CHORTHIPPUS-BIGUTTULUS L)

Male grasshoppers of the species Chorthippus biguttulus react to femal e songs with a characteristic turn towards the female. The probability of turning towards female song models was used to evaluate those para meters which are essential for a signal to be interpreted as female so ng. 1. The shape of sound pulses turned out to be most decisive; pulse s with ramps rising gradually over 3 and more ms were efficient (Figs. 2, 3), whereas rectangularly modulated pulses evoked only weak respon ses and only when pulse intervals were between 3 and 5 ms (Fig. 2). Th e decline of a pulse did not influence its efficiency (Fig. 3). In par ticular, pulses with sudden onsets and gradual declines were as weakly effective as rectangularly modulated ones and thus remarkably less ef fective than pulses with ramp-like onsets (Fig. 4). 2. Intensity tunin g curves suggest, that the absolute steepness of ramps (expressed as m ubar/ms) is detected by the grasshopper nervous system (Figs. 6, 7), p ossibly by processing the delay in excitation onset of at least two re ceptor types differing in threshold sensitivity. 3. The sawtooth shape of pulses in female signals is suggested to be adaptive with respect to directional hearing.