Insect trapped in amber reveals clues on the evolutionary arms race in ancient Europe’s animal kingdom

• Extinct insect found preserved in amber for 44 million years reveals clues on the evolution of ultrasound communication
• Advanced scanning technology has allowed scientists to reconstruct the ancient mating call of the perfectly preserved specimen from the Natural History Museum
• The katydid evolved to best pick up the calls of its species but could also tune in to nearby echolocation calls of early bats

New research on an extinct katydid perfectly preserved in amber for 44 million years, reveals that the species, a katydid (also known as bush cricket), has used ultrasound frequencies to communicate for millions of years to keep predators from hearing them.

With the development of enhanced micro-scanning technology, researchers were able to gain an understanding of the mating call of the katydid. The preserved insect has called the Natural History Museum home since being officially named Eomortoniellus handlirschi 80 years ago.

The new scans have revealed it to be the earliest known species to communicate at ultrasound frequencies, far beyond the range of human hearing.

All the way back in the Eocene Epoch, the E. handlirschi became stuck in pine tree resin, which hardened encasing the bug. The preserved insect was buried in sediment before its coating transformed into amber over millions of years. It was uncovered in 1936 in an area then known as East Prussia, which is now split between Poland, Russia and Lithuania.

CT scans of the katydid revealed that the sap had got into the katydid’s ear canal, located on the inside of its legs. With the sap inside, the delicate structure of the insect’s ear was well-preserved.

Dr Charlie Woodrow, lead author of the new study conducted during his PhD at the University of Lincoln, now a researcher at Uppsala University, Sweden, says, ‘This discovery wouldn’t have been possible without such a well-preserved katydid, which highlights how important museum collections are in discovering specimens like these.’

‘This katydid was frozen in time at a crucial moment in the arms race between echolocating predators and insects. Shortly before this animal was fossilised, bats had developed the ability to echolocate, which may have driven the katydids to call at higher frequencies. At the same time, their ears were adapting to listen out for bats trying to hunt them down.’

Richie Howard, Fossil Arthropods Curator at the Natural History Museum, comments, ‘Charlie and his team have applied thoroughly modern and cutting-edge imaging techniques to an important specimen from our historic collection of amber from north-eastern Europe’s Baltic coast’.

‘This coming together of old material and modern methods is critical for palaeontological research as it unlocks new data to address questions we had no way of answering before – in this case regarding the soundscape of Eocene orthopterans.’

Evidence suggests that katydids developed the ability to send mating calls at increasingly higher frequencies to go undetected by their mammal predators, until they moved into ultrasonic sound. These sounds would have gone undetected until the first bats evolved to use laryngeal echolocation around 52 million years ago.

‘While katydids were likely already exploring high frequencies, both to avoid eavesdropping and to develop a greater diversity of signals, bats gave them a new impetus,’ explains Professor Fernando Montealegre-Zapata, University of Lincoln, who co-authored and supervised the new study.

‘It might seem strange that katydids kept singing at these high pitches once they could be overheard, but ultrasound dissipates quickly in the environment. This ensures that a distant bat won’t hear the singing katydid as the sound will break up before it can be heard. The morphological and physiological characters that these insects evolved in response to mammalian predators, mainly bats, are the current main traits that define the entire bush cricket family (the Tettigoniidae), which originated also in the Eocene.’

Using models of how sound travels and how living katydids produce sound, Charlie teamed up with Dr Emine Celiker, University of Leicester, to predict the frequency of E. handlirschi’s call to reconstruct its mating signal. They calculated that the katydid was probably best at hearing sounds of around 30kHz. This suggests that the insect’s hearing evolved to be best at listening in to the calls of its species, giving the best chance of drawing couples together for mating.

The team also found two other peaks in the katydid’s hearing at around 60 and 90kHz. This was likely to help the insect tune in to any nearby echolocation calls of early bats, which were around 40-65Hz.

The ability of the insects to listen to high frequencies would have been enhanced by the katydid’s pinnae, or earlobes in mammals. While only partly developed in E. handlirschi, evolution in the years after it was trapped in amber has allowed its relatives to listen to calls of over 100kHz.

‘It’s now important to identify more fossils to track these changes,’ Charlie says. ‘I think that more adults of this species, or its close relatives, will turn up as many are sold online to private amber collections as well as being held in public collections. It just takes the right people to notice it and study it.’

The findings of the study have been published in the journal Current Biology. 

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