The physiology of the tettigoniid ear. I. The implications of the anatomy of the ear to its function in sound reception.

Ever since the anatomy of the Tettigoniid ear was described by Schwabe (1906) the previously inferred function of sound reception by the tympanal organ has undergone repeated anatomical (Sharp, 1922; Carpentier, 1924, 1927; Zeuner, 1936; Howse, 1968) and physiological (Pumphrey, 1940; Autrum, 1940, 1941; Haskell 1956; McKay, 1969,1970; Zhantiev, 1970; and others) investigations, but the function of the tympanal organ has usually been interpreted on the basis of the tympanal region alone, even though suggestions as to the importance of the prothoracic acoustic trachea are found in the work of von Siebold (1844), Graber (1876), Carpentier (1924, 1927), Zeuner (1936) and others. The Tettigoniids have cuticular folds protecting the external membranes and have developed a complex receptor, the crista acoustica, in addition to the subgenual organ and intermediate organ of the Gryllids (Schwabe, 1906; Howse, 1968; Freidman, 1972; Schumacher, 1973). The tracheal system which forms the tympanal air spaces behind the external tympanal membranes of Tettigoniids, takes origin from the large 'acoustic' portion of the double prothoracic spiracle which opens into a vesicle from which arises a horn-shaped trachea; this descends into the leg and splits in the region of the tympanal organ to form the anterior and posterior cavities. If the spiracular opening is large, if the trachea is horn-shaped, and if the trachea has no respiratory branches, then its importance in the functioning of the tympanal organ in sound reception may be greater than the resonance or damping function (Zeuner, 1936; Pumphrey, 1940) ascribed to it. This paper considers the importance of the tracheal system for sound reception in Tettigoniids and the implications of the findings for the physiology of the ear. Subsequent papers will consider the role of the cuticular folds, the external and central membranes, and some aspects of the biophysics of the whole system. Finally, the implications of the findings to the existing theories of acoustic orientation behaviour, and acoustic feedback will be considered.