Many insects have body hair that is able to vibrate in response to sound waves of certain frequencies, allowing to detect various types of sound. Others have more elaborate sound pickup organs - the tympanic organs - usually located in the paws.
Tympanic organs consist of a membrane (tympanic membrane) that vibrates in response to certain sound frequencies.
Vibration stimulates mechanoreceptors located beneath the tympanic membrane, generating nerve impulses that are transmitted to the brain ganglia, where they will be interpreted as sounds.
Thanks to their sensory hair and tympanic organs, certain moths can detect the high-frequency sounds emitted by their predatory bats. This way they locate the enemy and can make a strategic escape.
Hearing of reptiles and birds
In reptiles and birds, and also in mammals, besides the inner and middle ears, there is the outer ear. The tympanic membrane is not exposed, but located in a tubular depression of the head, which constitutes the outer ear.
The middle ear of birds and reptiles resembles that of amphibians. They consist of an air-filled tube with an ossicle inside. This ossicle has one end attached to the tympanic membrane and the other to a sacral projection called cochlea.
Sound waves that propagate through the outer ear canal reach the tympanic membrane, causing it to vibrate. These vibrations are transmitted through the middle ear ossicle to the cochlea. The vibration of the cochlea wall moves the liquid inside stimulating the sensory cells present there.
Hearing in mammals
In mammals, as well as reptiles and birds, the structures responsible for hearing are the outer ear, O middle ear and the cochlea. The semicircular canals, saccule, and utricle, as we have seen, are responsible for balance.
The outer ear of mammals is a channel that opens to the outside in the ear. The ear is a projection of the skin, supported by cartilaginous tissue, that acts as a sound-collecting shell. The epithelium lining the outer ear canal is rich in wax-secreting cells whose function is to trap dust particles and microorganisms, thus protecting the inner parts of the ear.
The middle ear, separated from the outer ear by the eardrum (tympanic membrane), is a narrow, air-filled canal located within the temporal bone. Within the middle ear of mammals there are three small bones, aligned in sequence from the eardrum to the inner ear. These ossicles are called hammer, anvil and stirrup.
The middle ear communicates with the throat through a flexible channel, the Eustachian tube.
The function of the Eustachian tube is to balance the pressures of the ear and the external environment.. When we quickly climb or descend a saw, we have a sensation of pressure in our ears, which results from the imbalance between atmospheric pressure and air pressure in our middle ear. As we rise, the atmospheric pressure decreases relative to that of the ear, so that the eardrum is pressed from the inside out.
When we go down, the opposite occurs: the atmospheric pressure increases relative to that of the ear, and the eardrum is pressed inward. When the Eustachian tubes open, the pressures inside and outside the ear equal. The opening of the tubes is facilitated by swallowing, so that eating, chewing gum, or even swallowing saliva, facilitates the setting of the ears to variations in external pressure.