Tiny insect’s ears, located on its hind legs, work just like ours

Despite the fact that a katydid’s ears are located on its hind legs, the rain forest insect’s hearing works in a strikingly similar fashion to human hearing, according to a new study published Thursday in the journal Science.

Mammalian hearing is enabled by a three-part system. First, a sound arrives at the eardrum, causing it to vibrate. Then, those vibrations in the air are converted to vibrations in liquid by the middle ear, which is made up of those three bones everyone loved to memorize in middle school biology: the hammer, anvil and stirrup. Together, those three bones act to transform airborne vibrations on one side and liquid-borne waves on the other, in the inner ear.

Then, the hair cells in our inner ear convert those waves into signals that get sent to the auditory nerve, where the brain receives the auditory information. Each hair cell is activated by a different sound frequency, allowing the brain to receive specific information about what sound is being heard.

If that all sounds too complicated to you for a bug’s hearing system, well, you’re wrong. Using advanced techniques for imaging the katydid’s tiny ear and measuring how it responds to sound, the authors of the new study, led by Fernando Montealegre-Zapata of the University of Lincoln in England, discovered that the katydid’s ear is also made up of three distinct parts that play the same roles as the three parts of the human ear.

The key to this discovery was finding the equivalent of the hammer, anvil and stirrup. In the katydid, an outer membrane—much like the eardrum—is connected to a stiff lever-like plate called the “tympanal plate.” Together, the membrane and plate effectively convert sounds from vibrations in the air to waves in liquid, where cells much like those in our inner ears sense them.

In an article accompanying the study, neurobiologist Ronald Hoy of Cornell University says the discovery is a remarkable example of convergent evolution, in which two distantly related species evolve the same solution to a problem—in this case, how to hear. In fact, Hoy writes, the finding should lead researchers to re-evaluate their knowledge for how many invertebrates hear, including the cricket, which is closely related to the katydid.

If he’s right, it may just turn out that our hammer, anvil and stirrup aren’t so special after all.

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