I’ve got ears just like yours (Image: Daniel Robert and Fernando Montealegre-Z)

All the better to hear you with, my dear. A chance discovery has revealed that some insects have evolved mammal-like ears, with an analogous three-part structure that includes a fluid-filled vessel similar to the mammalian cochlea.

Fernando Montealegre-Z at the University of Lincoln, UK, and colleagues were studying the vibration of the tympanal membrane – a taut membrane that works like an eardrum – in the foreleg of Copiphora gorgonensis, a species of katydid from the South American rainforest, when they noticed tiny vibrations in the rigid cuticle behind the membrane.

When they dissected the leg behind that membrane, they unexpectedly burst a vessel filled with high-pressure fluid.


The team analysed the fluid to confirm that it was not part of the insect’s circulatory system and concluded instead that it played a cochlea-like role in sound detection. In most insects, sound vibrations transmit directly to neuronal sensors which sit behind the tympanal membrane.

Mammals have evolved tiny bones called ossicles that transfer vibrations from the eardrum to the fluid-filled cochlea. The analogous structure in the katydid is a vibrating plate, exposed to the air on one side and fluid on the other.

Smallest ear

In mammals, the cochlea analyses a sound’s frequency – how high or low it is – and the new structure found by the team appears to do the same job. Spanning only 600 micrometres, it is the smallest known ear of its kind in nature.

The team have since found similar structures in four related katydid species. One is thought to be able to detect frequencies up to 190 kilohertz. By comparison, humans can hear up to about 20 kHz and dogs up to about 60 kHz. “It’s the most ultrasonic animal on the planet,” says Montealegre-Z.

“This is an amazing example of convergent evolution in hearing structures between very distantly related animals,” says Hannah ter Hofstede at the University of Cambridge, who was not involved in the study. “It suggests selection for a common solution to a specific problem.”

Most insects have a calling song in the frequency range that humans find audible, she says, and many appear to rely on changes in the timing of the calls for species recognition, which makes fine-scale resolution of higher frequencies unnecessary. But katydids can have calls with a large variety of frequencies, many of them ultrasonic. “Within a tropical community, there can be more than 100 different species of katydids singing at night,” she says.

It may also help with detecting the echolocation frequencies of predatory bats. “The pressure, evolutionarily speaking, is that a frequency discrimination system will be lifesaving,” says team member Daniel Robert at the University of Bristol, UK.

“You need to tell your mates apart from the bats.”

Journal reference: Science, doi.org/jrt