Specialized hearing organs, known as tympanal organs, have evolved in at least seven different orders of insects. Tympanal organs are usually defined by the presence of a tympanal membrane (or eardrum). They are backed by an air-filled space or cavity and are innervated by a chordotonal sensory organ. In some insects, however, a recognizable tympanal membrane may not be easily identified by visual inspection, yet may possess tympanal hearing organs. In insects that possess them, tympanal hearing organs may mediate the detection of predators, prey, and potential mates and rivals. Unlike the ears of vertebrates, which are localized to cranial segments, the ears of insects may be found in a bewildering variety of locations on their bodies, depending on the species. The embryological and evolutionary origins of tympanal organs are related to ancestral states as proprioceptive chordotonal organs.