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

How ultrasonic warning sounds produced by the wings of a moth species work work has been revealed by researchers at the University of Bristol.

Scientists have recently discovered that moths of the genus Yponomeuta (called stoat butterflies) have developed a very special acoustic defense mechanism against their echolocating predators: bats.

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

In the study, published in Proceedings of the National Academy of Sciences, An interdisciplinary team of engineers and biologists from Bristol shows how the individual ridges of a wavy area in the hindwings of stoat butterflies break due to wing folding in flight. The sudden appearance of these features causes an adjacent membrane to vibrate, greatly amplifying the strength and direction of the sound produced. Because of its passive actuation in flight, this sound-generating organ is known as the “aeroelastic tymbal.”

Marc Holderied, professor of sensory biology in the School of Biological Sciences, explained: “Our goal in this research was to understand how the undulations of these timpani can distort and break in a choreographed way to produce a chain of wide-ranging clicks. band. study, we revealed the biomechanics that trigger the buckling sequence and shed light on how click sounds are emitted through tymbal resonance.

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

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

To uncover the mechanics of the aeroelastic tymbal, Hernando combined cutting-edge techniques from biology and engineering mechanics. Biological characterization of wing morphology and material properties ultimately led to detailed computer simulations of instantaneous response and sound production that matched recorded butterfly signals in terms of frequency, structure, amplitude and direction.

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

This discovery will help researchers understand many other insect species with similar sound-producing mechanisms, filling a page of anti-bat acoustic defenses in the book on the centuries-old arms race between echolocating bats and their insect prey.

Structural buckling and sound production are rarely studied together, although they are reciprocal phenomena. Additionally, buckling occurs as a sudden, large deformation that may be of interest 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 the wings.

Alberto Pirrera, professor of nonlinear structural mechanics at BCI, concludes: “In the field of engineering design, nonlinear elastic responses, such as buckling and snapping instabilities, have traditionally been viewed as failure modes to avoid. we advocate a paradigm shift and have demonstrated that buckling events can be strategically exploited to imbue structures with intelligent functionality or improved mass efficiency. Yponomeuta’s aeroelastic tymbal embodies the concept of beneficial nonlinearity.

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

The research team anticipates that through bioinspiration, aeroelastic timpani will encourage new developments in the context of structure morphing, acoustic structure monitoring and soft robotics.