Moths fight against echolocating bats with sounds of their own

Under the darkness of night, bats use soundwaves to find moths to eat. However, these insects are not completely defenseless against bats. Some moths use their wings to produce an ultrasonic warning sound against winged mammals. The findings are described in a study published February 5 in the journal Proceedings of the National Academy of Sciences (PNAS).

[Related: Why artificial light—and evolution—trap moths.]

A genus of moths called Yponomeuta (or ermine moths) click twice per wingbeat cycle using a small ridged membrane located in their hindwing. These moths do not have hearing organs and thus do not appear to be aware that they are even making these sounds and they can’t even control the sound using muscular action.

Slow-motion video of in-flight sound production by Yponomeuta malinellus. CREDIT: Hernaldo Mendoza Nava

Defenses like these can help the moths thwart the bats by annoying them and save them from becoming a meal, even if only temporarily. Decoding the mechanics of how this works in moths could help researchers better understand more intricate aspects of the way insects produce sounds for self-defense.

In the study, a team of engineers and biologists from the University of Bristol in England looked at how the individual ridges that make up a corrugated patch in the ermine moth’s hindwings snap. This sudden snap-through buckling vibrates the membrane that is next to the hindwing. The strength and direction of the sound is then amplified like how the skin of a drum or a speaker makes a sound louder. This sound-producing organ in moths is called an aeroelastic tymbal.

“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,” study co-author and mechanical engineer Hernaldo Mendoza Nava said in a statement. “As a result, these millimeter-sized tymbals can produce ultrasounds at the equivalent level of a lively human conversation.”

To study the mechanics behind the ermine moth’s aeroelastic tymbal, the team merged the biological concept of how the wing is formed with material principles from engineering. This fusion of biology and engineering helped the researchers build a detailed computer simulation of the wing’s snap-through response and sound production that matched actual recordings of the moth signals in frequency, structure, amplitude, and direction.

Structural buckling and sound production, as when the wings generate noise, are not always studied together, even though they both affect one another. Looking at how these two actions work in tandem has applications in aerospace, where engineers are constantly trying to make wings more aerodynamic. Buckling and snap-through instabilities are called nonlinear elastic responses that generally don’t follow the rules of aerodynamics and cause strain. They generally considered something to avoid in engineering, but this new research shows that buckling and snap-throughs could be used in wing design.

[Related: How do sound waves work?]

“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,” study co-author and mechanical engineer Alberto Pirrera said in a statement. “Yponomeuta’s aeroelastic tymbal embodies the concept of beneficial nonlinearity. The natural world, once again, serves as a source of inspiration.”

The team hopes that studying the month’s aeroelastic tymbals will encourage new developments in morphing structures, acoustic structural monitoring, and soft robotics.