Tag Archives: Daniel Robert

Moths with sound absorption stealth technology

The cabbage tree emperor moth (Thomas Neil) [downloaded from https://www.cbc.ca/radio/quirks/nov-17-2018-greenland-asteroid-impact-short-people-in-the-rain-forest-reef-islands-and-sea-level-and-more-1.4906857/how-moths-evolved-a-kind-of-stealth-jet-technology-to-sneak-past-bats-1.4906866]

I don’t think I’ve ever seen a more gorgeous moth and it seems a perfect way to enter 2019, from a November 16, 2018 news item on CBC (Canadian Broadcasting Corporation),

A species of silk moth has evolved special sound absorbing scales on its wings to absorb the sonar pulses from hunting bats. This is analogous to the special coatings on stealth aircraft that allow them to be nearly invisible to radar.

“It’s a battle out there every night, insects flying for their lives trying to avoid becoming a bat’s next dinner,” said Dr. Marc Holderied, the senior author on the paper and an associate professor in the School of Biological Sciences at the University of Bristol.

“If you manage to absorb some of these sound energies, it would make you look smaller and let you be detectable over a shorter distance because echoe isn’t strong enough outside the detection bubble.”

Many moths have ears that warn them when a bat is nearby. But not the big and juicy cabbage tree emperor moths which would ordinarily make the perfect meal for bats.

The researchers prepared a brief animated feature illustrating the research,

Prior to publication of the study, the scientists made a presentation at the Acoustical Society of America’s 176th Meeting, held in conjunction with the Canadian Acoustical Association’s 2018 Acoustics Week, Nov. 5-9 at the Victoria Conference Centre in Victoria, Canada according to a November 7, 2018 University of Bristol press release (also on EurekAlert but submitted by the Acoustical Society of America on November 6, 2018),

Moths are a mainstay food source for bats, which use echolocation (biological sonar) to hunt their prey. Scientists such as Thomas Neil, from the University of Bristol in the U.K., are studying how moths have evolved passive defenses over millions of years to resist their primary predators.

While some moths have evolved ears that detect the ultrasonic calls of bats, many types of moths remain deaf. In those moths, Neil has found that the insects developed types of “stealth coating” that serve as acoustic camouflage to evade hungry bats.

Neil will describe his work during the Acoustical Society of America’s 176th Meeting, held in conjunction with the Canadian Acoustical Association’s 2018 Acoustics Week, Nov. 5-9 at the Victoria Conference Centre in Victoria, Canada.

In his presentation, Neil will focus on how fur on a moth’s thorax and wing joints provide acoustic stealth by reducing the echoes of these body parts from bat calls.

“Thoracic fur provides substantial acoustic stealth at all ecologically relevant ultrasonic frequencies,” said Neil, a researcher at Bristol University. “The thorax fur of moths acts as a lightweight porous sound absorber, facilitating acoustic camouflage and offering a significant survival advantage against bats.” Removing the fur from the moth’s thorax increased its detection risk by as much as 38 percent.

Neil used acoustic tomography to quantify echo strength in the spatial and frequency domains of two deaf moth species that are subject to bat predation and two butterfly species that are not.

In comparing the effects of removing thorax fur from insects that serve as food for bats to those that don’t, Neil’s research team found that thoracic fur determines acoustic camouflage of moths but not butterflies.

“We found that the fur on moths was both thicker and denser than that of the butterflies, and these parameters seem to be linked with the absorptive performance of their respective furs,” Neil said. “The thorax fur of the moths was able to absorb up to 85 percent of the impinging sound energy. The maximum absorption we found in butterflies was just 20 percent.”

Neil’s research could contribute to the development of biomimetic materials for ultrathin sound absorbers and other noise-control devices.

“Moth fur is thin and lightweight,” said Neil, “and acts as a broadband and multidirectional ultrasound absorber that is on par with the performance of current porous sound-absorbing foams.”

Moth fur? This has changed my view of moths although I reserve the right to get cranky when local moths chew through my wool sweaters. Here’s a link to and a citation for the paper,

Biomechanics of a moth scale at ultrasonic frequencies by Zhiyuan Shen, Thomas R. Neil, Daniel Robert, Bruce W. Drinkwater, and Marc W. Holderied. PNAS [Proccedings of the National Academy of Sciences of the United States of America] November 27, 2018 115 (48) 12200-12205; published ahead of print November 12, 2018 https://doi.org/10.1073/pnas.1810025115

This paper is behind a paywall.

Unusually I’m going to include the paper’s abstract here,

The wings of moths and butterflies are densely covered in scales that exhibit intricate shapes and sculptured nanostructures. While certain butterfly scales create nanoscale photonic effects [emphasis mine], moth scales show different nanostructures suggesting different functionality. Here we investigate moth-scale vibrodynamics to understand their role in creating acoustic camouflage against bat echolocation, where scales on wings provide ultrasound absorber functionality. For this, individual scales can be considered as building blocks with adapted biomechanical properties at ultrasonic frequencies. The 3D nanostructure of a full Bunaea alcinoe moth forewing scale was characterized using confocal microscopy. Structurally, this scale is double layered and endowed with different perforation rates on the upper and lower laminae, which are interconnected by trabeculae pillars. From these observations a parameterized model of the scale’s nanostructure was formed and its effective elastic stiffness matrix extracted. Macroscale numerical modeling of scale vibrodynamics showed close qualitative and quantitative agreement with scanning laser Doppler vibrometry measurement of this scale’s oscillations, suggesting that the governing biomechanics have been captured accurately. Importantly, this scale of B. alcinoe exhibits its first three resonances in the typical echolocation frequency range of bats, suggesting it has evolved as a resonant absorber. Damping coefficients of the moth-scale resonator and ultrasonic absorption of a scaled wing were estimated using numerical modeling. The calculated absorption coefficient of 0.50 agrees with the published maximum acoustic effect of wing scaling. Understanding scale vibroacoustic behavior helps create macroscopic structures with the capacity for broadband acoustic camouflage.

Those nanoscale photonic effects caused by butterfly scales are something I’d usually describe as optical effects due to the nanoscale structures on some butterfly wings, notably those of the Blue Morpho butterfly. In fact there’s a whole field of study on what’s known as structural colo(u)r. Strictly speaking I’m not sure you could describe the nanostructures on Glasswing butterflies as an example of structure colour since those structures make that butterfly’s wings transparent but they are definitely an optical effect. For the curious, you can use ‘blue morpho butterfly’, ‘glasswing butterfly’ or ‘structural colo(u)r’ to search for more on this blog or pursue bigger fish with an internet search.

‘Tsunamis’ at the nanoscale and the hearing abilities of locusts

It’s turning into a rather sound-oriented day (Nov. 6, 2013) given my earlier posting (Pop and rock music lead to better solar cells). This time I’m featuring locusts and their hearing abilities as per a Nov. 6, 2013 University of Bristol press release (also on EurekAlert),

The remarkable mechanism by which the tiny ears of locusts can hear and distinguish between different tones has been discovered by researchers from the University of Bristol. Understanding how the nanoscale features of the insect eardrum mechanically process sound could open up practical possibilities for the fabrication of embedded signal processing in extremely small microphones.

Here’s an image illustrating how the locust perceives a sound wave,

The shape of the nanoscale wave on a locust's eardrum Courtesy University of Bristol

The shape of the nanoscale wave on a locust’s eardrum Courtesy University of Bristol

There’s also this image of a locust and a section of a locust eardrum,

A locust with ion beam milling showing a section of eardrum  Courtesy of University of Bristol

A locust with ion beam milling showing a section of eardrum Courtesy of University of Bristol

The press release offers more details about locusts and their eardrums and about the research (Note: Links have been removed),

Unlike a microphone membrane, the eardrum of the locust is a complicated structure which is used to process the information contained in an incoming sound.  In order to survive, the locust needs to be able to distinguish between the friendly sounds of fellow locusts in its swarm and the sounds of a hunting bat approaching.  These sounds differ in their tonal composition: locust sounds are raspy and noisy while bat echolocation calls have distinctly higher frequencies.

Using a set of laser beams shining on the locust, Dr Rob Malkin of Bristol’s School of Biological Sciences and colleagues were able to observe the effects of incoming sound waves on the eardrum.  They found that the locust eardrum behaved in a most unusual way, quite unlike a microphone membrane or the eardrums of other animals.

The researchers first confirmed a result the Bristol team observed a few years ago, namely that the eardrum generates concentric waves of vibrations that shoal in a tsunami-like fashion as they travel from one side of the membrane to the other. [emphasis mine]  The new, detailed analysis shows that eardrum waves caused by low frequency sounds travel completely across the membrane, where low-frequency-sensitive nerve cells attach to the membrane.  Remarkably, high frequency waves travel only half that far, and stop at the attachment point of high frequency neurons.

Using data and computer modelling, Dr Malkin, an aerospace engineer working in bio-inspired sensor research, quantified this mechanical behaviour.  He said: “It rapidly became evident that the distribution of the vibrational energy was odd…quite unlike what normal materials do when waves travel through them.”

The researchers then discovered a surprising effect: the energy density contained in the travelling wave was amplified as the wave travelled across the eardrum.  The team measured that, as the high frequency waves converge onto one point, the amplification can be as high as 56,000 times.  This energy localisation is remarkable because it is purely mechanical; at this stage only cleverly arranged material within the eardrum membrane does the job.

To understand how this effect is possible in such a small structure, the team used a combination of mathematical modelling with nanoscale measurements and structural visualisation.  They employed a focussed ion beam at Bristol’s Interface Analysis Centre to gain knowledge of the structural features of the locust’s eardrum then fed this information into analytical models in order to unveil the contributions of different eardrum attributes.  Thus, they established that a particular combination of attributes generates the phenomenon; geometry, tension, stiffness and mass distribution all turn the locust eardrum into a little mechanical processing device.

Professor Daniel Robert, who led the research team and is funded by the Royal Society, said: “Other animals, including mammals such as ourselves, analyse tonal differences using very refined mechanisms in the cochlea.  Hearing in these animals is a three-step process, from capturing sound with an eardrum to amplifying vibrations through middle ear bones and then transmitting them to the cochlear frequency analyser.  Locusts do not enjoy the luxury of such a complicated, large and biologically expensive to build apparatus.  Instead their ears evolved to be much simpler with sound capture, local amplification and frequency analysis all taking place within one structure.”

Dr Malkin added: “This is a feat of miniaturisation and simplification; we now need to make a similar sensor and test it.”

As is often the case, the story proceeds slowly. First, the researchers notice a locust eardrum’s ability to generate concentric waves of vibrations and compare them to tsunamis. They then spend years trying to understand the mechanism responsible for  the locust’s eardrum ‘tsunami’ and, now that’s been accomplished, the researchers will try in the coming years to create a sensor based on the principles they’ve observed. I look forward to hearing more about the story as it develops in the next decade or so. Meanwhile, here’s a link to and a citation for the researchers’ latest paper,

Energy localization and frequency analysis in the locust ear by Robert Malkin, Thomas R. McDonagh, Natasha Mhatre, Thomas S. Scott, and Daniel Robert. Published 6 November 2013 doi: 10.1098/​rsif.2013.0857 J. R. Soc. Interface 6 January 2014 vol. 11 no. 90 20130857

This article is open access.