Mechanics of the transduction of sound in the tympanal organ of adults and larvae of locusts.

The mechanical transmission of sound in the tympanal organ of adults and 5th instar larvae of Locusta migratoria and Schistocerca gregaria has been investigated by means of stroboscopic measurements within a frequency range from 1-20 kHz. Frequency dependent spatial distributions of amplitudes and phases of oscillation on the tympanal membrane and the Müller's organ could be demonstrated. Cuticular structures on the membrane may act as a lever arm (e.g. elevated process) and cause a transformation of the (unidimensional) membrane motion into components of displacements in the Müller's organ perpendicular, as well as even parallel, to the membrane. Sites of maximum relative displacements at distinct frequencies are found to be correlated to the course of the dendrites of the acoustic receptor cells. Differences in morphology of the tympanal organ between the two species as well as between adults and larvae always correspond to differences in the mechanical properties (resonances etc.). Consequently, differences or changes in the neurophysiological response characteristics of the different receptor cells have been found. Based upon these findings a correlation between the anatomical and physiological classification of the receptor cell groups is presented.