The sensitivity of animals to low-frequency vibrations (Hadley and Williams, 1968; Dorward and Mclntyre, 1971; Hartline, 1971; Salmon and Horch, 1972) and their ability to produce vibrations in the substrate (Pearman, 1928; Emerson and Simpson, 1929; Haskell, 1955; Sismondo, 1980; Uetz and Stratton, 1982) have been recognized for some time. Yet, the analyses required to determine the biological usefulness of an ability to produce or detect vibration are still rare (Ewing, 1989). Merely being species-specific and stereotypical does not make an event a signal (Doherty and Gerhardt, 1984; Bradbury and Vehrencamp, 1998). Activities of animals set up airborne sound and substrate-borne vibration simultaneously (Gogala, 1985), but whether or not these are true signals depends on the environment and adaptations of animals to communication in that environment (Keuper et al., 1985). Logical arguments once suggested that physical limitations on communication via the substrate were severe (Schwartzkopff, 1974). In fact, when Brownell first described detection of Rayleigh waves by foraging sand scorpions in 1977, he expressed the conventional wisdom: "Natural solids are not considered important avenues of information transfer, since they are generally heterogeneous and inelastic, or the conduction velocity and wavelength of the signals they conduct are too large to convey biologically useful information other than to warn of a disturbing force nearby" (Brownell, 1977, p. 479). Yet, when he continued to question, he found that the nocturnal scorpion, Paruroctonus mesaensis, can interpret vibrations in sand to determine both direction and distance of prey species and that conduction velocities are actually much lower than had been assumed (Brownell, 1977, 1984). The scorpion does rather better in extracting information from vibrations in sand than preying-mantids and jumping spiders on land and water striders on the water surface (Brownell and Farley, 1979b). More recent investigations by Cocroft et al. (2000) have suggested that a vibration localization mechanism can function at even the smallest spatial scales in arthropods. Thus, as in most things, the more we search, the more we find. Animals, after all, in the world according to Barth, "... look at the world through windows that may differ drastically from our own" (1998, p. 228).
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