March 1, 2024

The mystery of moths’ warning sound production explained in new study

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Yponomeuta tymbal – image of a timbal showing a row of microtimbals. Credit: Hernaldo Mendoza Nava

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Yponomeuta tymbal – image of a timbal showing a row of microtimbals. Credit: Hernaldo Mendoza Nava

The workings of ultrasonic warning sounds produced by the wings of a species of moth have been revealed by researchers at the University of Bristol.

Scientists recently discovered that moths of the genus Yponomeuta (the so-called ermine moths) have developed a very special acoustic defense mechanism against their echolocating predators – bats.

Ermine moths produce ultrasonic clicking sounds twice per wingbeat cycle, using a tiny rippled membrane on their hindwings. Surprisingly, these moths do not have hearing organs and are therefore not aware of their unique defense mechanism, nor do they have the ability to control it through muscular action.

In the study, published in Annals of the National Academy of Sciences, an interdisciplinary team of engineers and biologists from Bristol shows how the individual ridges of a wavy patch on the hindwings of ermine moths break due to folding of the wings during flight. The sudden passage of these resources causes an adjacent membrane to vibrate, significantly amplifying the strength and direction of the sound produced. Due to its passive action during flight, this sound-producing organ is known as an “aeroelastic timbal”.

Marc Holderied, professor of Sensory Biology in the School of Biological Sciences, explained: “Our goal in this research was to understand how the ripples in these tymbals can bend and break in a choreographed way to produce a broadband click chain. With this study, we unfold the biomechanics that trigger the buckling sequence and shed light on how clicking sounds are emitted through tymbal resonance.”

Tethered flight — Slow-motion video of sound production during flight by Yponomeuta malinellus. Credit: Hernaldo Mendoza Nava

The study’s first author, Hernaldo Mendoza Nava, who investigated the mechanics of the aeroelastic tymbal when he was a Ph.D. student at the EPSRC Center for Doctoral Training in Advanced Composites for Innovation and Science at the Bristol Composites Institute (BCI), said: “The Sound production and radiation are linked to mechanical vibration, for example in the head of a drum or a speaker.

“In ermine moths, instantaneous buckling events act like beating the edge of a drum, stimulating a much larger part of the wing to vibrate and radiate sound. As a result, these millimeter-sized tymbals can produce ultrasounds at the equivalent level of a human conversation lively.”

To discover the mechanics of the aeroelastic tymbal, Hernando combined cutting-edge techniques from biology and mechanical engineering. Biological characterization of wing morphology and material properties led to detailed computational simulations of the snap-through response and sound production that match the moth’s recorded signals in frequency, structure, amplitude and direction.

Rainer Groh, Senior Lecturer in Digital Structural Engineering at BCI, added: “The integration of multiple methods in the sciences with a consistent flow of information across discipline boundaries in the spirit of ‘team science’ is what made this study unique and a success. Furthermore, without incredible modern imaging, data analysis and computing capabilities, it would not have been possible to discover the mechanics of this complex biological phenomenon.”

The discovery will help researchers understand many other insect species with similar sound-making mechanisms, filling an anti-bat acoustic defenses page in the book on the ancient arms race between echolocating bats and their prey.

Yponomeuta wing and tymbal – complete wing with insertion of a close-up Yponomeuta tymbal. Credit: Hernaldo Mendoza Nava

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Yponomeuta wing and tymbal – complete wing with insertion of a close-up Yponomeuta tymbal. Credit: Hernaldo Mendoza Nava

Structural buckling and sound production are rarely studied together, despite being reciprocal phenomena. Furthermore, buckling occurs as a sudden large deformation that can be attractive as a shape-changing mechanism in the field of morphing structures, such as in the aerospace industry, where engineers seek to optimize the aerodynamic performance of wings.

Alberto Pirrera, Professor of Nonlinear Structural Mechanics at BCI, concludes: “In the domain of engineering design, nonlinear elastic responses, such as buckling and fitting instabilities, have traditionally been perceived as failure modes to be avoided. In our research , we have advocated a paradigm shift and demonstrated that buckling events can be strategically harnessed to imbue structures with intelligent functionality or greater mass efficiency. Yponomeuta’s aeroelastic timbal embodies the concept of beneficial nonlinearity.”

“The natural world once again serves as a source of inspiration.”

The research team predicts that, through bioinspiration, aeroelastic tymbals will encourage new developments in the context of transformed structures, acoustic structural monitoring and soft robotics.

More information:
Sound production induced by buckling in the aeroelastic tymbals of Yponomeuta, Proceedings of the National Academy of Sciences (2024). DOI: 10.1073/pnas.2313549121.

Diary information:
Proceedings of the National Academy of Sciences

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