MORPHOLOGICAL AND PHYSIOLOGICAL DIFFERENCES OF THE AUDITORY-SYSTEM IN3 RELATED BUSH-CRICKETS (ORTHOPTERA, PHANEROPTERIDAE, POECILIMON)

The auditory system of three closely related bushcrickets was investig ated with respect to morphological and physiological differences. The size of the acoustic vesicle in the prothorax cavity and the size of t he acoustic spiracle were compared to differences in auditory tuning o f the tympanic nerve and differences in the directionality. The result s indicate that a small auditory vesicle and auditory spiracle provide reduced sensitivity in the high frequency range (above 10-15 kHz), bu t increase sensitivity at low frequencies (below 10 kHz). The directio nality of the hearing system deteriorates at frequencies between 10 an d 25 kHz in species with a small spiracle and trachea. The evolutionar y implications of these differences of the auditory systems are discus sed. They are considered to be influenced more by ecological factors t han bioacoustical ones.