Tympanal mechanics in the parasitoid fly Ormia ochracea : intertympanal coupling during mechanical vibration

Abstract

The acoustic parasitoid fly Ormia ochracea locates its host, a singing field cricket, by means of a pair of small tympanal organs which are less than 2 mm in width. Nevertheless, laser vibrometric evidence shows that this tympanal system is directionally sensitive to sound through the action of a flexible intertympanal bridge that mechanically couples the tympana. Biomechanical data, a mechanical analogue and an analytical model lead to a testable prediction about the vibratory behavior of this tympanal system: if intertympanal coupling occurs, a force applied only unilaterally in non-acoustical conditions should be transmitted, at least to some degree, to the contralateral ear. This paper presents new experiments of direct mechanical stimulation that test the prediction of mechanical coupling. Stimulation on only one side of the intertympanal bridge elicited a contralateral mechanical response. Thus, coupling of the tympanal membranes through a flexible intertympanal bridge is demonstrated by mechanical as well as acoustical stimulation. These experiments also test for the possible presence of a pressure-difference system in O. ochracea. Intertympanal coupling is shown not to depend on the integrity of the air space backing the tympanal system, thus eliminating this possibility.