On each front tibia of a bush-cricket there is a pair of slit-like openings backed by tympanal membranes. Sound reaches these membranes from two directions: externally, and internally along an air-filled acoustic trachea that opens at a large spiracle on the side of the thorax and narrows as it runs down the leg.
That tube behaves like an exponential horn, which both amplifies the incoming sound and slows it, so the two paths arrive out of step. The resulting pressure difference gives the animal directional hearing across a very small body. Behind the membrane lies the crista acustica, a row of sensory units that vary systematically in size along its length, so different positions respond best to different frequencies. The arrangement produces a tonotopic map, the same organising principle as the mammalian inner ear, arrived at independently.
Bush-cricket hearing extends well into the ultrasonic range, which matters because bats hunt them. What is less clear is how much of that high-frequency sensitivity serves bat detection and how much serves communication, since many katydid songs themselves contain strong ultrasonic components.