When life began on our planet, profound silence prevailed. The only sounds to be heard were those of thunderbolts and the roar of breakers against the sombre desolate dills by the primordial seas, but those sounds were of no interest to most animals. It was only after the animals themselves had achieved a higher stage of evolution and learned to wander about and devour one another that faint sounds began to be distinguished on the Earth. They were sounds of a biological origin produced by animals themselves. This prompted the development of the acoustic analyser and the sound signalling.
Extremely multifarious receptor devices, from wide-range to those only detecting a very narrow band of sounds, had to be evolved to cope with the gamut of sound sources.
Certain bats can best hear very high-pitched sounds of up to 300 kilocycles, but they can also hear very low sounds. Their auditory organ has a range of fifteen octaves. The nocturnal moths on which these bats feed have no use for such an enormous sound range. The tympanic organ of their wings is only able to pick up the ultrasonic signals of the bats. The organ for this limited function is similarly very simple in design. It consists of a membrane, air-sacs and two sensory nerve cells. Their sole function is to perceive the sound produced by the bat and to give the command so that the moth quickly changes direction of flight.