The ability of organism to sense sound.
"An insect will be said to hear when it is demonstrably responsive to sound." 
"...the only distinction made between hearing and the tactile senses is based on the intensity factor and is quite arbitrary. Atthepresenttimenoconfusionarisesinpracticebecausethesensitivity of the auditory organs so far investigated is of a different order from the sensitivity of end-organs usually regarded as tactile. But the distinction is not fundamental, and it can be regarded as certain that further work will demonstrate the existence of end-organs intermediate in sensitivity between “hearing” and “tactile” end-organs. There will then be no justification for attempting a sharp separation." 
"Moulton discusses definitions of hearing, primarily in the vertabrate context. Of Pumphrey's definitions, there mentioned, that of 1950 '(an animal hears when it behaves as if it has located a moving object (a sound source) not in contact with it') is rejected as making hearing a definitively directional phenomenon.
His earlier definition ('demonstrable responsiveness to sound) is on the other hand too broad for some workers, and Dijkgraaf incorporates it as only the first of two mandatory criteria, returning to von Buddenbrock for his second, though removing the undue restriveness of von Buddenbrock's resonant membranes: (1) demonstravle sensitivity to air- or waterborne sound; together with (2) detection of these stimulii with special detectors primarily used for this purpose. He terms (1) by itself, without (2), merely sound reception; and responsiveness to sound or vibration reaching the animal through the solid substratum, vibration perception.
Though these distinctions between vibration perception, sound perception, and "echter Gehörsinn" remain as artifical as they were in 1937, they are related more realistically to everyday experience than the attempt to treat all that shudders as sound and every reaction to it as a hearing response - and all that glisters as gold. Zoology, indeed, science itself, is founded on the abritary subdivision of intergrading series into finite classes, simply as working units for study; in the present context we can recognize the infinite series without having to reject the useful working distinctions between its parts." 
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