Learn How Insects Hear

Sound is created by vibrations carried through the air. By definition, an animals capacity to hear means that it has one or more organs that perceived and interpreted those air vibrations. Most insects have one or more sensory organs that are sensitive to vibrations transmitting through the air. Not only do insects hear, but they may actually be more sensitive than other animals to sound vibrations. Insect sense and interpret sounds in order to communicate with other insects and to navigate their environments. Some insects even listen for the sounds of predators in order to avoid being eaten by them.   There are four different types of auditory organs that insects may possess.   Tympanal Organs Many hearing insects have a pair of tympanal organs that vibrate when they catch sound waves in the air. As the name hints, these organs catch the sound and vibrate in much the way that a tympani, the large drum used in the percussion section of an orchestra, does it when its drum head is struck by a percussion mallet.   Like the tympani, the tympanal organ consists of a membrane tightly stretched on a frame over an air-filled cavity. When the percussionist hammers on the membrane of the tympani, it vibrates and produces a sound; an insects tympanal organ vibrates in much the same way as it catches sound waves in the air. This mechanism is exactly the same as found in the eardrum organ of humans and other animal species. Many insects have the ability to hear in a manner quite similar to the way we do it.   An insect also has a special receptor called the chordotonal organ, which senses the  vibration of the tympanal organ and translates the sound into a nerve impulse. Insects that use tympanal organs to hear include grasshoppers and crickets, cicadas, and some butterflies and moths. Johnstons Organ For some  insects, a group of sensory cells on the antennae form a receptor  called the Johnstons organ, which  collects auditory information. These group of sensory cells is found on the pedicel, which is the second segment from the base of the antennae, and it detects vibration of the segment(s) above. Mosquitoes and fruit flies are examples of insects that hear by using the Johnstons organ. In fruit flies, the organ is used to sense the wing-beat frequencies of mates, and in hawk moths, it is thought to assist with stable flight. In honeybees, Johnstons organ assists in the location of food sources.   Johnstons organ is a type of receptor found only no  invertebrates other than insects. It is named for the physician Christopher Johnston (1822-1891), a professor of surgery at the University of Maryland who discovered the organ. Setae The larvae of Lepidoptera  (butterflies and moths)  and  Orthoptera  (grasshoppers, crickets, etc.) use small stiff hairs, called setae, to sense  sound  vibrations. Caterpillars often respond to vibrations in the setae by exhibiting defensive behaviors. Some will stop moving completely, while others may contract their muscles and rear up in a fighting posture. Setae hairs are found on many species, but not all of them use the organs to sense sound vibrations.   Labral Pilifer A structure in the mouths of certain hawkmoths enables them to hear ultrasonic sounds, such as those produced by echolocating bats. The labral pilifer, a tiny hair-like organ, is believed to sense vibrations at specific frequencies. Scientists have noted a distinctive movement of the insects tongue when they subject  captive hawkmoths to sounds at these particular frequencies. In flight, the hawkmoths can avoid a pursuing bat by using the labral pilifer to detect their echolocation signals.