Hearing, Organs of

specialized organs of animals and man that perceive and analyze sounds. In many invertebrates, extant fishes, and primitive chordates the organs of hearing perceive both sounds and low-frequency vibrations.

Among invertebrates, true organs of hearing have developed only in insects. In most cases they consist of tympanal organs and sometimes of other structures sensitive to sound, for example, chordotonal or Johnston’s organs. Tympanal organs exist in orthopterons, true bugs, and butterflies; they are situated on the legs, abdomen, or thorax. Consisting of a thin cuticular tympanic membrane connected to the tracheae, or air cavities, they are sometimes protected by a skeletal fold. A group of chordotonal sensilla is attached either to the membrane or to tracheae connected to the membrane. All types of such organs of hearing, including the Johnston’s organ on the second antennal segment of mosquitoes, perceive low-frequency vibrations. In some butterflies and orthopterons, the tympanal organs perceive the ultrasonic vibrations of certain animals, for example, the echolocation signals emitted by bats.

In vertebrates, including man, the organs of hearing are situated together with the organ of equilibrium in the inner ear. In most fishes, the receptor system of the organ of hearing is formed of modified epithelial cells (secondary sensory cells), which are the terminating points of the acoustic nerve’s fibers. This receptor system takes the form of a small projection on the round sacculus, which is situated in the inner ear. This projection continued to develop during the evolution of terrestrial vertebrates, and in mammals, including man, it became the highly complex cochlea and organ of Corti.

In terrestrial vertebrates, the middle ear and then the external ear developed after the inner ear. The middle ear is most perfected in mammals. A rudimentary external ear in the form of a skin fold protecting the tympanic membrane first appeared in crocodiles, which are highly evolved reptiles. This structure gradually developed in some birds. In mammals, it became a complex movable cartilaginous auricle. As some terrestrial mammals, for example cetaceans and pinnipeds, made a secondary transition to an aquatic mode of life, the external ear became reduced. In some aquatic mammals, for example dolphins, the middle ear became reduced as well. Secondary reduction of the middle ear is also characteristic of some amphibians, for example caecilians, and of some reptiles, for example, snakes.