Physical and physiological properties of the tettigoniid (?grasshopper?) ear

Abstract

1. The hearing threshold for test sound frequencies above 2 kHz and the directional properties of the intact tettigoniid ear (8 kHz test sound) remain unchanged if sound is prevented from acting on theouter surface of the tympana (Figs. 6, 14). This cannot readily be understood from present theories on tettigoniid hearing. 2. No changes of the hearing threshold above 0.5 kHz or the directionality at 8 kHz can be observed if onlyone tympanum, anterior or posterior, is damped. The hearing threshold shows an increase only afterboth tympana have been damped (Figs. 7, 17). 3. The ear becomes very insensitive to sound and looses the directional properties (8 kHz test sound), characteristic of the intact ear, if the tympanal trachea is blocked at the spiracle on the thorax (Figs. 5, 16). This demonstrates the importance of the sound pathway via the tympanal trachea. 4. The directional sensitivity, i.e. the hearing threshold for a given sound frequency, depends on the relative position between the direction of sound incidence and the tympanal spiracle on the thorax (Figs. 12, 13). The symmetry axis of the directivity pattern (8 kHz test sound) is fixed in relation to the longitudinal axis of the body and does not change if the foreleg is moved into different “walking positions” (Fig. 15). 5. The optimum frequency of hearing of the tettigoniid ear is changed by altering the length of the tympanal trachea (Figs. 9, 19). 6. Two theories, namely the horn and the resonator theory, relating to the acoustical function of the tympanal trachea are discussed.