STIMULUS TRANSMISSION IN THE AUDITORY RECEPTOR ORGANS OF THE FORELEG OF BUSH-CRICKETS (TETTIGONIIDAE) I - THE ROLE OF THE TYMPANA

The auditory organs of the tettigoniid are located just below the femo ral tibial joint in the forelegs. Structurally each auditory organ con sists of a tonotopically organized crista acustica and intermediate or gan and associated sound conducting structures; an acoustic trachea an d two lateral tympanic membranes located at the level of the receptor complex. The receptor cells and associated satellite structures are lo cated in a channel Filled with hemolymph fluid. The vibratory response characteristics of the tympanic membranes generated by sound stimulat ion over the frequency range 2-40 kHz have been studied using laser vi brometry. The acoustic trachea was found to be the principal structure through which sound energy reached the tympana. The velocity of propa gation down the trachea was observed to be:independent of the frequenc y and appreciably lower than the velocity of sound in free space. Stru cturally the tympana are found to be partially in contact with the air in the trachea and with the hemolymph in the channel containing the r eceptor cells. The two tympana were found to oscillate in phase, with a broad band frequency response, have linear coherent response charact eristics and small time constant. Higher modes of vibration were not o bserved. Measurements of the pattern of vibration of the tympana showe d that these structures vibrate as hinged flaps rather than vibrating stretched membranes. These findings: together with the morphology of t he organ and physiological data from the receptor cells, suggest the p ossibility of an impedance matching function for the tympana in the tr ansmission of acoustic energy to the receptor cells in the tettigoniid ear. (C) 1998 Elsevier Science B.V.