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Determining the azimuth (direction in the horizontal plane) of a source is based on differences in the timing and/or intensity of sounds at the left and right ears, known respectively as interaural time difference (ITD) and interaural intensity difference (IID). Any sound that arises from off the midline will travel further to the offside ear than to the nearer one and thus will arrive later. The difference in path length depends on the direction of the sound source and the distance between the ears; the more lateral the sound and the larger the interaural dis- tance, the greater the ITD.
For insects, maximum possible ITDs are small. For example, an interaural dis- tance of 1 cm, which is possible only in relatively large insects such as crickets (Figure 1A) and cicadas (Figure 1B), would generate an ITD of only about 34 μs for a sound source perpendicular to the midline. The sophisticated nervous systems of birds and mammals can process such miniscule time differences, but so far as is known, the simpler nervous systems of insects cannot.
IID depends both on source azimuth and on how effectively sound is blocked by whatever separates the two ears, which in turn depends on its size relative to the wavelength of the sound and on its sound-absorptive properties. Sound diffracts readily around objects that are small relative to its wavelength, as is the
Figure 1. A brief introduction to the insect groups discussed in this article. Scale bars indicate approximately 1 cm. Readers can hear sounds from each of these insects at http://acousticstoday.org/gpollack. A: Field crickets (order Orthoptera, family Gryllidae, subfamily Gryllinae). As the name suggests, most species live in relatively open areas, often in simple burrows or under rocks, logs, leaf litter, and the like. The figure shows a fall field cricket, Gryllus pennsylvanicus, singing outside its burrow. B: Cicadas (order Hemiptera, family Cicadidae). Cicadas spend most of their lives underground where, as larvae, they feed on tree roots. When nearly adult they emerge at the surface, climb a tree, and molt to adulthood. The periodic cicadas emerge in enormous numbers every 13 or 17 years when their loud songs result in many sleepless nights. The photograph shows a 17-year cicada, Magicicada septendecim. C: Parasitoid flies (order Diptera, families Tachinidae and Sar- cophagidae). Larvae (maggots) of parasitoid flies burrow into their hosts that they devour from the inside. Mature larvae emerge from the host, killing it in the process, before forming pupae, the stage during which they transform from larva to adult. The photograph shows Ormia ochracea (right) alongside its host in Florida, the southeastern field cricket (Gryllus rubens). D: Katydids (Orthoptera, Tettigoniidae). Katydids live in vegetation. They vary considerably in size, shape, and color according to species. The photograph shows the short- winged meadow katydid, Conocephalus allardi. E: Mole crickets (Orthoptera, Gryllidae, Gryllotalpidae). With forelimbs specialized for digging, these large insects excavate elaborate burrows from which they sing. The photograph shows the prairie mole cricket, Gryllotalpa major. F: Tree crickets (Orthoptera, Gryllidae, Oecanthinae). These slender insects live in bushes and trees. Their songs are rather low in frequency (2-4 kHz), making them haunt- ingly attractive to at least this human listener. The photograph shows the black-horned tree cricket, Oecanthus nigracornis. Photos in A and F courtesy of J. E. Lloyd, used with permis- sion from http://entnemdept.ufl.edu/Walker/buzz/; photo in B courtesy of John Cooley, used with permission from http://magicicada.org/magicicada/; Photo in C courtesy of N. Lee, used with permission; photos in D and E courtesy of T. J. Walker, used with permission from http://entnemdept.ufl.edu/Walker/buzz/.
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