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  Figure 6. Mechanically coupled ears of the fly Ormia ochracea. Top: cyan arrow points to location of the eardrums, which are hidden by the head; red arrow shows approximate angle of view in the image below, after removal of the head. Middle: left and right eardrums (prosternal tympanal membrane [PTM]) are linked by a cuticular bridge, high- lighted in pink. Black arrow points to tympanal pits (TP), the attach- ment sites of the inner ear neuronal components. Scale bar: 200 μm. Modified from Robert et al. (1994). Right: sound pressures at the two eardrums are indistinguishable (A), but vibration of the ipsilateral ear- drum is earlier and larger (B). From Robert et al. (1996).
It is said that necessity is the mother of invention. This ad- age applies not only to human creativity but also when the “mother” in question is Mother Nature, whose method of invention is natural selection. Examples of remarkable bio- logical adaptations that meet particular needs are abundant throughout the natural world but perhaps no more so than in the feats of “engineering” that allow insects to overcome the many constraints imposed by their diminutive size and exploit acoustics in their daily lives.
I thank Natasha Mhatre and Arthur Popper for helpful com- ments; John Cooley, Norman Lee, and Vicki Powys for gen- erously contributing photographs; and Fernando Monteal- gre-Z for kindly supplying a video.
Gerald Pollack is professor emeritus of biology at McGill University, Montreal, QC, Canada. He has studied the neu- roethology of insect sensory systems throughout his career, beginning with his graduate work on blowfly chemore- ception and feeding behavior with Vin- cent Dethier. His interest in insect hear-
ing and communication, which began with his postdoctoral work with Ron Hoy, persists in his “retirement” where, as a volunteer “postdoc” with Andrew Mason, University of Toronto Scarborough, Toronto, ON, Canada, he continues to work on the auditory neuroethology of tree crickets and parasitoid flies.
Albert, J. T., and Göpfert, M. C. (2015). Hearing in Drosophila. Current Opinion in Neurobiology 34, 79-85. doi:10.1016/j.conb.2015.02.001.
Balakrishnan, R. (2016). Behavioral ecology of insect communication. In Pollack, G. S., Mason, A. C., Popper, A. N., and Fay, R. R. (Eds.). Insect Hearing. Springer International Publishing, Cham, Switzerland, pp. 49-80. doi:10:1007/978-3-319-28890-1.
Bennet-Clark, H. C. (1970). The mechanism and efficiency of sound pro- duction in mole crickets. Journal of Experimental Biology 52, 619-652.
Bennet-Clark, H. C. (1987). The tuned singing burrow of mole crickets. Journal of Experimental Biology 128, 383-409.
 Mechanically Coupled Fly Ears
Not only are the eardrums of the parasitoid fly Ormia ochracea close together, they are nearly coplanar on the ventral surface of the fly’s chest, so that there is essentially no “fly” between them. As a result, the IID is immeasurably small. The maximum ITD for a source located perpendicular to the intereardrum axis is only about 1.4 μs. The ability of this system to generate direc- tion-dependent differences in eardrum vibration results from the linking of the two eardrums by a thin bridge of cuticle, the relatively stiff material that forms an insect’s “shell” (Figure 6).
The left and right halves of the bridge are linked by a flex- ible joint that permits them to flex in one of two ways. Equal sound pressures on the two sides generate a tendency to os- cillate inward and outward in synchrony, like the flapping wings of the butterfly. However the direction-dependent, al- beit small, ITD gives the side nearest the sound source a slight head start, driving a rocking or seesaw-like motion. The over- all mechanical response of the system is the sum of these two tendencies, resulting in a seesaw in which the two sides both pivot and flex about the midpoint. Remarkably, this complex motion pattern results in a difference in vibration amplitude for a cricket-like sound frequency of up to 12 dB, and the time shift between motion on the two sides is amplified to up to 50 μs (Miles et al., 1995). Moreover, because the response laten- cies of nerve cells is inversely related to stimulus strength (and thus to vibration amplitude), the ITD at the neuron level is even greater, up to 250 μs (Mason et al., 2001).
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