Investigating Vibroacoustic Properties in Moth Wings for Acoustic Camouflage
Certain moths have special scaled wings that exhibit acoustic camouflaging properties, hiding them from bats' echolocation. Researchers at the University of Bristol sought to model this effect to better understand the vibroacoustic phenomena at play — seeing potential for broadband acoustic camouflage in other areas.
Have you ever looked at the ground, or a tree branch, or a leaf on a bush — and all of a sudden, it moves? Plenty of insects and arachnids camouflage themselves from predators by blending in with their surroundings. For instance, the orchid mantis has wings that look just like the delicate buds of an orchid flower; the Phasmatodea, also known as the “stick bug”, has arms and legs that bear an uncanny resemblance to little brown twigs; and the Luna moth has fluorescent green wings that perfectly match bright leaves on a tree.
However, this type of visual camouflage is a moot point when trying to avoid one of the main predators of insects: Bats do not see with their eyes, but instead navigate and search for food using echolocation. So what is a bug to do? As it turns out, certain types of moths, like the Bunaea alcinoe, or the cabbage tree emperor moth, have scaled wings that provide acoustic camouflage, protecting them from bats' advanced sonar detection.
Researchers from the University of Bristol used numerical modeling to study this wing scale phenomenon and see how we can potentially apply these acoustic camouflaging capabilities to other areas.
Echolocation Meets Its Match
For over 65 million years, bats have sought out moths as a source of food. Some moths can detect the signals of approaching bats, while others defend themselves with poison, or clicking sounds that can startle the bats enough to fly away. The cabbage tree emperor moth is both deaf and nontoxic, but it is not helpless. It simply relies on a more passive defense strategy: acoustic camouflage (also called acoustic cloaking).
How do moths use acoustic camouflage to fend off bat attacks? To find out, we can take a closer look at their wings. Moth wings are solid, thin membranes made up of chitin, a long-chain polymer derived from glucose. Stiff wing veins hold these membranes in place. Looking even closer, the upper and lower surfaces of the moth wing are covered in arrays of overlapping scales, like the tiles on a roof. Each scale is porous and of a complex structure. “The highly sculptured scale structure implies sophisticated evolutionary adaptations, analogous to the highly organized nanoscale photonic structures for visual signaling," said Zhiyuan Shen, researcher at the University of Bristol.
These wing scales are less than 0.25 mm long, making them smaller than 1/10th of the wavelength that bats use for echolocation, using signals of frequencies from 11 kHz to 212 kHz (Ref. 1). The University of Bristol researchers hypothesized that moth wings can be categorized as ultrathin absorbers with subwavelength thickness, acting as resonant absorbers, in their paper "Biomechanics of a Moth Scale at Ultrasonic Frequencies". To investigate their hypothesis, the group sought to capture the governing physical phenomena of the wing scales and show that moth scales can achieve high absorption coefficients at resonance. To do so, they turned to numerical modeling...
Advanced Imaging Techniques Meet Numerical Simulation
The project started with a few moth pupae, which were cultivated in the lab until they reached maturity. Researchers collected samples of the moth wings, which then underwent two types of advanced imaging techniques: scanning electron microscopy (SEM) and confocal microscopy. The SEM technique involved mounting sections of the moth wing to adhesive carbon tabs, which were then coated with a thin, 5-nm layer of gold. The scales were imaged under high-vacuum and variable-pressure modes and magnified to get a large, clear image. For the confocal microscopy process, the team immersed a single moth scale in glycerol and sealed it between two microscopy slides. Then they used autofluorescence to get ultraclear images.
Once the clear, high-quality images of the moth wings had been produced, the team was able to extract 3D data from the images into a 3D isosurface model, which they saved in the STL format in the MATLAB® software and imported into the COMSOL Multiphysics® simulation software using LiveLink™ for MATLAB®. Using the COMSOL Multiphysics® model, the team identified the ideal unit cell of the moth wing scale and parameterized it to study the effective material properties.
Next up, the team was ready to perform their vibroacoustic analysis of the scale. They used the Periodic boundary condition in COMSOL Multiphysics® to model the single unit cell instead of an entire scale array, which saved computational effort and memory. “We can simplify our model to a few scales and use the Periodic boundary condition to expand the structure into an array. If we made an actual array model, it would be too big for the computer to handle,” said Shen. The team then modeled the scale vibrations at ultrasonic frequencies using a macroscale FEM model in COMSOL Multiphysics®, allowing them to calculate the vibration of the scale. “COMSOL® is really good at coupled problems. We needed both acoustics and solid mechanics to understand how the ultrasonic wave is coupled with the scale structure," said Shen.
The team also built two models to analyze the damping effect of the moth scale and the ultrasonic properties of an entire moth wing made up of such scales. The former consisted of a single scale with one end fully clamped, while the latter added Rayleigh damping to the material and was used to calculate the absorption coefficient of the scaled array.
Comparing Calculations to Measurements
In order to see how the calculated vibrations of the moth scale compare to the real-world behavior of moth scales, the team then turned their attention to a laser Doppler vibrometer (LDV), which they used to characterize the vibrational behavior of the single scale. The LDV results showed good agreement with the calculated resonances for the first and third modes, which differed by just 2.9% and 1.0%, respectively. The calculated resonances were 28.4, 65.2, and 153.1 kHz, compared to LDV results of 27.6, 90.8, and 152.3 kHz. The 28% deviation for the second mode can be explained by the simplified curvature of the moth scale, the fact that the perforation rate of the scale was modeled as constant when it actually varies, and inconsistencies in the incident sound wave during the LDV measurements.
It is interesting to note that the calculated modes of the moth scale overlap and span the biosonar range used by bats for echolocation (typically 20–180 kHz). To see if this was just a coincidence, the researchers repeated the analysis for a similar unit cell that mimics the structure of a butterfly wing scale. This time, the modes fell outside of the bats' biosonar range at 88.4, 150.9, and 406.0 kHz. It makes sense, evolution-wise: Moths are nocturnal and are often within bats' crosshairs, whereas butterflies are active during the day and do not need to protect themselves from the fanged creatures. This comparison supports the theory that moths may have evolved to acoustically camouflage themselves from bats.
Inspiring New Uses for Acoustic Camouflage
This research project marks the first effort to characterize the biomechanics and vibrational behavior of moth scales, both numerically and experimentally. The results demonstrate that multiphysics modeling software can be used to accurately capture moth scale behavior, paving the way for further simulation-driven analysis in this area. In the future, the University of Bristol team aims to expand the current periodic model into a full 3D model of an array of moth scales.
The research itself also has far-reaching implications outside of the animal kingdom. By being able to understand the vibroacoustic behavior of moth scales, researchers can start to develop macroscopic structures with the same acoustic camouflage capabilities. “If we can make materials mimicking moth scales, applications can include high-efficiency ultrasonic acoustic absorbers. If we can find a material with a thickness of only 1/100th of the wavelength it’s working with, that would be a big improvement for acoustic design,” said Shen.
In the future, we can expect to see enhanced noise mitigation materials used in building design and defense technology with acoustics camouflaging capabilities — proving that when you take inspiration from nature, you will be pleasantly surprised by what you can achieve.
- G. Jones and M. Holderied, "Bat echolocation calls: adaptation and convergent evolution", Proc Biol Sci., vol. 274(1612), pp. 905–912, 2007.
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