![]() Step three: the waves travel down the crista acustica, whose cells respond to the various frequencies and send that information to the insect’s nervous system. Step two: the tympanal plate sends these vibrations into the fluid-filled auditory vesicle, amplifying them in the process. Step one: the two eardrums captures sound, just as ours do. The effect is the same: large faint vibrations are converted into small strong ones. And just as our eardrums have 17 times more surface area than our stirrups, the katydid’s eardrum has 13 times more surface area than its tympanal plate. This plate is the katydid’s version of our three ear bones – a stiff lever that converts the air-filled outer ear with the fluid-filled auditory vesicle. It looks like just another part of the insect’s hard outer cuticle, but using lasers, Montealegre-Z found that it vibrates in time with the eardrum. The AV is connected to each eardrum by a completely new structure called the tympanal plate (TP). Their role is the same – to analyse frequencies. It contains a line of sensitive cells called the crista acustica, like the hair cells that line our cochlea. For a start, it’s only found in the first pair of legs where the katydid’s ears are. Other scientists had assumed that the AV was a simple blood vessel, but Montealegre-Z knew that couldn’t be right. The first– the acoustic vesicle (AV)-is like an uncoiled version of our cochlea, a tapering hollow fluid-filled tube. But Montealagre-Z also discovered two new organs. The scans revealed a pair of eardrums (or tympanal membranes) on each knee. He analysed the legs of living katydids using a CT scanner designed for tiny objects. Fresh from recreating the sound of a Jurassic cricket, he turned to studying a katydid called Copiphora gorgonensis, a green insect with an orange face, topped with a unicorn-like spike and comical beady eyes. Even though countless numbers of these insects had been dissected, no one had really understood the structures of these ears.įernando Montealegre-Z from the University of Bristol has filled in the rest of the gaps. Grasshoppers, crickets and locusts all have knee-ears that, at just a fraction of a millimetre long, are among the tiniest ears in the animal kingdom. And that’s why, in the rainforests of South America, a katydid-a relative of crickets-hears using the same three-step method that we use, but with ears that are found on its knees. Different branches often evolve similar solutions to life’s problems. But good adaptation are rarely wasted on just one part of the tree of life. ![]() And voila – we hear something.Īll mammal ears work in the same way: capture sound convert and amplify and analyse frequencies. The signals from these cells are passed to the auditory nerve and decoded in the brain. They’re like a reverse piano keyboard that senses rather than plays. Each cell responds to different frequencies, and are neatly aligned so that the low-frequency ones are at one end of the tube and the high-frequency ones at another. These perform the third step: frequency analysis. Ignore the whisk for now – the shell is the cochlea, a rolled-up tube that’s filled with fluid and lined with sensitive hair cells. These vibrations enter the inner ear, which looks like a French whisk poking out of a snail shell. They transmit all the pressure from the relatively wide eardrum into the much tinier tip of the stirrup, transforming large but faint air-borne vibrations into small but strong fluid-borne ones. The bones perform the second-step: convert and amplify. On the other side, the eardrum connects to three tiny well-named bones-the hammer, anvil and stirrup-which link the air-filled outer ear with the fluid-filled inner ear. This is the tympanum, or more evocatively, the eardrum. The sound waves pass through the bits of your ear you can actually see, and vibrate a membrane, stretched taut across your ear canal. Your ears can not only detect these oscillations, but decode them to reveal a Bach sonata, a laughing friend, or a honking car. Sounds are just waves of pressure, cascading through sparse molecules of air. Every time you put on some music or listen to a speaker’s words, you are party to a miracle of biology – the ability to hear.
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