Hearing or “sound detection” requires the detection of vibrations, through gas, liquid or solid. In addition to hearing with your ears, you can detect sound when airwaves vibrate your body. During the vibrations, mechanoreceptors are stimulated creating a distinct sensation. Humans commonly experience these vibrations (sounds) when attending rock concerts, fireworks displays or stuck in traffic behind some guy with the bass cranked.
The “ears” of insects are present in a variety of locations, shapes and sizes. From this, we conclude that hearing in insects has independently evolved several times. Most insect “ears” consist of a very thin area of cuticle connected to receptors that respond to vibrations. Sound receptors can evolve by parts of the cuticle becoming thinner in the vicinity of the vibration sensor. The thinner cuticle is more sensitive to vibrations. The size and shape of the sound organ “tune” the organ to sounds of specific frequencies. Larger organs typically detect lower pitch frequencies.The “ears” of katydids and crickets are on the front legs, as discussed in a previous post. The sound receptors are typically in a trough surrounded by thicker cuticle. Sound receptors can be preserved in the formation of fossils.
Scientists Roy Plotnick and Dena Smith studied collections of insect fossils that date to 50 million years ago from the Eocene, Green River formation in Colorado. In a paper in Journal of Paleontology, Plotnick and Smith describe sound organs in a number of fossil crickets (Gryllidae) and a katydid (Tettigoniidae). The sound organs are in a similar location and similar in structure to sound organs on extant species. It is likely that these ancient insects used their ears in a manner similar to the way extant species use them, for communication with mates and for detection of predators, primarily bats.