Tiny organs below insect’s knees have a structure similar to those in humans

By Susan Milius

Web edition: November 15, 2012
Print edition: December 15, 2012; Vol.182 #12 (p. 11)

A rainforest katydid doesn’t talk like a mammal, or walk like a mammal, but it does hear with the first mammal-like, three-stage sound-sensing system known outside vertebrates.

“The beauty about the katydid ear is that it does the same job in a way that is much simpler,” says sensory biologist Daniel Robert of the University of Bristol in England. And of interest to researchers designing miniature hearing devices, the Copiphora gorgonensis katydid ear is smaller than a rice grain.

When mammals hear a sound, airborne pressure waves thump against the eardrum and send ripples through a liquid-filled chamber where tuned cells pick out the various frequencies. The sophisticated transition comes from a trio of tiny bones that the eardrum jiggles in just the right way to translate large thumps over its broad area into intelligible sloshes in the narrow chamber.

Katydids don’t have ear bones. Instead, their eardrums do the translating themselves. Katydid ears sit below the knees with a drum on each side of the leg. An airborne pressure wave bends a large zone on each eardrum inward, and that motion forces a small plate on each drum to rise outward. The plate vibration sends appropriate sloshes through a liquid-filled chamber inside the leg, where detector cells sense various frequencies, Robert and colleagues report in the Nov. 16 Science.