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Acoustic metamaterial helps moths avoid being eaten by bats

09 Dec 2020
Moth scales
Ultrasound absorbers: false-colour 3D representation of a 0.21×0.28 mm wing section of the moth Lasiocampa quercus showing the structure, diversity, and arrangement of base scales (orange) and cover scales (blue and yellow). (Courtesy: Simon Reichel/Thomas Neil/Zhiyuan Shen/Marc Holderied)

Natural acoustic metamaterials found on the wings of some moths could help the insects avoid being eaten by bats – according to Marc Holderied and colleagues at the UK’s University of Bristol. By doing a combination of simulations and experiments, the team found that coupled vibrations of wing scales enable the moths to absorb ultrasound over a broad range of frequencies. The discovery could lead to the development of bio-inspired sound proofing materials with the potential to perform far better than current designs.

As a key source of prey for echolocating bats, moths are under evolutionary pressure to evade capture. While many moth species are very good at hearing the ultrasound used by bats, many other species lack this ability. Instead, these insects use acoustic camouflage to avoid detection. This involves sound waves being scattered or absorbed by sub-wavelength size structures on the moth’s wings.

Scientists have studied similar metamaterial structures on butterfly wings, which can have special optical properties that produce vibrant iridescent colours. However, these scales are not suitable for dampening sound, leading Holderied’s team to ask what is different about moths’ wings? In their study, the researchers investigated sound dampening mechanisms in two earless species of moth –  Antheraea pernyi and Dactyloceras lucina.

Microscope and tomography

They first used a combination of scanning electron microscopy and micro-computed tomography to study the shapes and arrangements of the wing scales of both species. The images revealed overlapping tile arrangements of paddle-shaped scales, with stalks attached to stiff, lightweight membranes. These layers were typically less than 0.3 mm thick, which is far shorter than the ultrasound wavelengths used by bats.

In subsequent experiments, Holderied and colleagues showed that these layers could significantly dampen sound waves within a broad range of ultrasound frequencies. Sound at frequencies as low as 20 kHz is absorbed and the layers display a maximum absorption of 72% at 78 kHz.

Using computer models, the researchers worked out the mechanisms responsible for these advanced absorption properties. The scales vary significantly in terms of their sizes and shapes – depending on their location on the wing. The team found that this leads to variations in the natural resonant frequencies of the scales. Furthermore, their models revealed a strong coupling between the vibrations of neighbouring scales on shared membranes. This means that collectively, the scales act to dampen acoustic waves over a broad range of frequencies. This is wholly unlike the acoustic behaviour of butterfly wings, whose uniform scales can only resonate in small, localized clusters.

The wings of the two moth species are the first confirmed examples of natural acoustic metamaterials. Their design could be mimicked to create lightweight, ultrathin soundproofing panels, with the potential to out-perform the thick, porous absorbers currently used for sound insulation.

The research is described in Proceedings of the National Academy of Science.

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