A rainforest katydid was the first invertebrate to have the 3-stage hearing system common among vertebrates.
Mammalian hearing starts with an airborne pressure wave thumping the eardrum. The drum jiggles tiny bones that translate large eardrum vibrations into smaller sloshes, which are sent to a liquid-filled chamber.
Katydid ears sit below the knees, with an eardrum on each side of a leg. They don’t use translating bones. Instead, plates attached to katydid eardrums do the job.
A pressure wave bends each drum inward. That motion pushes a small translator plate on each drum outward. The plate vibration sends smaller ripples into a liquid-filled chamber inside the leg.