Insects have ears in their legs that help them hear other insects and danger. Katydids, relatives of grasshoppers and crickets, were among the first insects to make sounds and they produce the broadest range of calls across the entire family range. Their ancestors were making and hearing sounds more than 200 million years ago, long before the family expanded dramatically during the Eocene era (56–34 million years ago). Present-day katydids hear sounds ranging from 600 Hz to 150 kHz, which is much broader than the hearing range of humans (0.02–20 kHz), and make sounds for mating and communication by rubbing parts of their wings against each other. But what did the most ancient katydid ancestor sound like and what frequency range was their hearing tuned to? To find out, Charlie Woodrow and Fernando Montealegre-Z from the University of Lincoln, UK, with Emine Celiker, from the University of Leicester, UK, decided to take a look at the wing and outer ear of an impeccably preserved fossil of the extinct katydid Eomortoniellus handlirschi embedded in amber, from the Natural History Museum, UK.

To understand the structure of the ear, the team used high-definition X-ray scans to examine the entire fossil, looking for structural details. The scans revealed a small opening in the outer ear that directed sound through a canal to the vibrating eardrum, called the tympanum. Both the ear canal and the tympanum, which likely vibrated in response to sound waves, were completely preserved. The team then focused on the external part of the ear, called the pinnae, which captures high-pitched sounds, to learn more about the frequency range of the ancient insect's hearing. They 3D printed a model of the pinnae, 30 times larger than the real ear, placed a microphone in the cavity at the centre and played sounds to the model, recording which frequencies, and how strongly, the model transmitted sound to the microphone. Rescaling their measurements down to the size of the ancient ear, the team discovered that E. handlirschi could have picked up sounds over 400 kHz, which is higher than the pitch of the echolocation calls used by bats for navigation.

Having figured out what the extinct katydids could hear, the team then measured the frequency range of the sounds they could have produced by measuring the teeth-like structure on the wing that the insects used to make sound. Based on the length of this structure, called the stridulatory file, and some mathematical calculations, the team estimated that the song was pitched around 32 kHz, in the ultrasound range. So, insects evolved the ability to communicate at this high pitch 56–34 million years ago.

The researchers also calculated how the ancient insect's ear canal vibrated in response to sound and how it amplified or dampened specific pitches before they reached the eardrums. The calculations indicated that the left and right ear canals naturally resonated at frequencies around 30.0 and 30.6 kHz respectively, which also matches the ultrasonic hearing range of modern katydids. In addition, the ears of E. handlirschi detected changes in sound pressure both internally and externally, allowing them to discern the direction of sound, similar to present-day katydids and crickets. The fact that sound moved more slowly through their ears (180 m s−1, compared with 343 m s−1 in the open air) suggested a dependency on delayed internal cues to determine the sound direction. Unlike their living relatives, which use the ears on their legs to hear their kin and predators, the extinct E. handlirschi utilized their legs primarily for directional hearing.

This study by Woodrow and colleagues expands our understanding of katydid communication strategies across time. And, although the specimen that they investigated is preserved in amber, it is fascinating to think that the vestiges of this ancient katydid's communication system are preserved in every katydid alive today.

An Eocene insect could hear conspecific ultrasounds and bat echolocation
Curr. Biol