A

FIGURE 3 Schematic illustrations of structures associated with hearing in crickets (Ensifera). (A) The locations of the tibial ears and the H-shaped tracheal system connecting the contralateral tympanal membranes and spiracles are shown. (Illustration by M. Nelson.) (B and C) Light micrographs of the larger posterior and smaller anterior tympanal membranes located on the tibia of the forelegs in GGryllus bimaculatus. (Courtesy of A. C. Mason.)

FIGURE 3 Schematic illustrations of structures associated with hearing in crickets (Ensifera). (A) The locations of the tibial ears and the H-shaped tracheal system connecting the contralateral tympanal membranes and spiracles are shown. (Illustration by M. Nelson.) (B and C) Light micrographs of the larger posterior and smaller anterior tympanal membranes located on the tibia of the forelegs in GGryllus bimaculatus. (Courtesy of A. C. Mason.)

membrane vibrates in response to sound waves arriving only to the external surface of the tympanal membrane. In insects whose ears are sensitive to high-frequency sounds (like many bat-detecting moth ears), the sound shadow of the body is sufficient to cause a difference in sound amplitudes arriving to each ear, which provides useful directional cues to the animal. Many insects using lower frequency sounds are faced with the difficulty of being able to localize sounds that arrive almost simultaneously to both ears. They overcome this problem using pressure-difference receivers in which the sound impinges on both the outer and the inner surfaces of the membrane, and the membrane vibrates in response to the difference in pressure between the two sides (e.g., Fig. 3).

A third and very unusual way to localize sounds has been "invented" by certain parasitic tachinid and sarcophagid flies. These flies localize their hosts (field crickets, katydids, and cicadas) by hearing and then homing in on the mating calls. The ears possessed by these flies are conventional chordotonal organs, containing 70 or more scolopidia. However, the fly's eardrums are mechanically connected, which confers an acute sense of directionality such that they can detect and localize their calling hosts/prey from a distance of tens of meters, even from high in the air. Such ears appear to be unique to these parasitoid flies.

The frequencies detectable by insect tympanal ears range from a few kilohertz (e.g., the water boatman Corixa) to over 100 kHz (e.g., some tettigoniid species). This broad bandwidth of frequency sensitivity is used by some insects for mate calling and in others it enables them to detect their predators (e.g., echolocating bats). Some insects have the ability to discriminate between different sound frequencies, which may be achieved at the level of the tympanal membrane or the chordotonal organ. To date, most insect ears studied appear to be tone deaf (i.e., different tones are indistinguishable).

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