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How ermine moths effortlessly produce ultrasonic warning sounds 

A new study from the University of Bristol sheds light on the complex defensive mechanisms employed by nature. The researchers have unraveled the mystery behind the ultrasonic warning sounds produced by ermine moths, a species known as Yponomeuta. 

This discovery not only provides fascinating insights into the natural world but also holds potential implications for the fields of engineering and technology.

Ermine moths 

Ermine moths, which lack hearing organs and are unaware of the sounds they produce, have developed an ingenious acoustic defense mechanism to deter their predators, primarily echolocating bats. 

The researchers found that these moths generate ultrasonic clicking sounds at an astonishing rate of twice per wingbeat cycle. This sound production is facilitated by a minute corrugated membrane located in their hindwings, known as an “aeroelastic tymbal.”

Natural defense strategy 

The study reveals that the unique sound production mechanism is a result of the snap-through buckling of individual ridges on the corrugated patch in the moth’s hindwings during flight. The buckling causes a sudden vibration of an adjacent membrane, significantly amplifying the sound’s strength and direction. 

This process occurs passively, without any muscular action from the moth, highlighting the efficiency and elegance of this natural defense strategy. 

“Our goal in this research was to understand how the corrugations in these tymbals can buckle and snap through in a choreographed way to produce a chain of broadband clicks,” explained Professor Marc Holderied. “With this study, we unfolded the biomechanics that trigger the buckling sequence and shed light on how the clicking sounds are emitted through tymbal resonance.”

Ultrasonic sound production 

Study first author Hernaldo Mendoza Nava investigated the mechanics of the aeroelastic tymbal as a PhD student at the EPSRC Centre for Doctoral Training in Advanced Composites for Innovation and Science of the Bristol Composites Institute (BCI).

“Sound production and radiation is linked to mechanical vibration, for example in the skin of a drum or a loudspeaker,” said Mendoza. 

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

Interdisciplinary research 

The team used advanced techniques from both biology and engineering mechanics to achieve their findings. Detailed computer simulations, along with biological characterization of the wing’s morphology and material properties, enabled the researchers to accurately model the snap-through response and sound production, closely matching recorded moth signals.

“The integration of various methods across the sciences with a consistent information flow across discipline boundaries in the spirit of ‘team science’ is what made this study unique and a success,” said co-author Rainer Groh. “In addition, without the amazing modern capabilities in imaging, data analysis and computation, uncovering the mechanics of this complex biological phenomenon would not have been possible.”

Broader implications 

The study not only contributes to our understanding of insect defense mechanisms against predators but also has broader implications for engineering and technology. 

The phenomenon of structural buckling and sound production, though rarely studied together, presents exciting opportunities for developing morphing structures with enhanced functionality or efficiency, particularly in fields such as aerospace.

“In the realm of engineering design, nonlinear elastic responses, such as buckling and snap-through instabilities, have traditionally been perceived as failure modes to be avoided,” said Professor Alberto Pirrera. 

“In our research, we have been advocating a paradigm shift and have demonstrated that buckling events can be strategically leveraged to imbue structures with smart functionality or enhanced mass-efficiency. Yponomeuta’s aeroelastic tymbal embodies the concept of beneficial nonlinearity. The natural world, once again, serves as a source of inspiration.”

The study is published in the journal Proceedings of the National Academy of Sciences

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