Moth Wing Scales Form a Natural Acoustic Metamaterial

The pursuit between bats and moths takes place in an ultrasonic world beyond the range of human hearing. Nocturnal bats emit high-frequency sounds and use the returning echoes to determine the location, size, and flight direction of their prey. In response, many moths have evolved ultrasound-sensitive hearing organs that allow them to turn sharply, dive, or stop flying when a bat approaches. Some species also produce ultrasonic clicks that interfere with a bat's ability to track or identify them. Moths that lack ultrasound-sensitive hearing must rely on other forms of defence. One possible strategy is to reduce the strength of the echoes reflected from their bodies, making them more difficult for a bat's biosonar to detect.

Moths are lepidopteran insects, and their bodies are covered by a thick, fur-like layer of scales that can attenuate bat ultrasound in much the same way as a porous sound absorber. The scales covering the wings, however, form a much thinner layer, generally less than 0.3 mm thick. In addition to contributing to thermoregulation, anti-stick properties, and visual camouflage, this intricate structure may perform another function in some moths: absorbing the ultrasound that bats use to locate flying prey.

Researchers compared two moth species that lack ultrasound-sensitive hearing, Antheraea pernyi and Dactyloceras lucina, with two diurnal butterfly species, Graphium agamemnon and Danaus chrysippus. Circular wing samples 6 mm in diameter were taken from the leading and trailing regions of the forewings. The strength of the echoes returned by each sample was measured both before and after the scales had been removed. Sound was directed perpendicularly at the wing surface, the angle at which a wing produces its strongest echo, allowing the researchers to compare changes in reflected sound energy.

Antheraea pernyi（Image source：Kugamazog， CC BY-SA 2.5 ）

Antheraea pernyi（Image source：Charles J. Sharp， CC BY-SA 4.0 ）

Graphium agamemnon（Image source：ManaskaMukhopadhyay， CC BY-SA 4.0 ）

Danaus chrysippus（Image source：Charles J. Sharp， CC BY-SA 4.0 ）

Within the 20- to 60-kHz frequency range used by many bats to detect flying insects, the scales of both moth species substantially reduced echo strength. Depending on the species and wing region, mean target strength decreased by approximately 3 to 5 dB. The strongest individual result was measured in a wing sample from D. lucina, where target strength fell by 5.5 dB, corresponding to the absorption of about 72% of the incident sound energy. The researchers also measured how much sound passed through the wing or was scattered away in other directions. Both effects were negligible, indicating that the weaker echoes were primarily caused by genuine sound absorption. In comparison, the scales of the two butterfly species produced no similar reduction and, in some cases, slightly increased echo strength.

Scanning electron micrographs of the scale structures of moths (left) and butterflies (right)（Courtesy of Neil TR et al. (2020) ）

One of the most remarkable features of this system is that the moth scale layer is far thinner than the wavelengths it absorbs. At 20 kHz, for example, the wavelength of sound in air is approximately 17 mm, vastly greater than a wing scale layer measuring less than 0.3 mm. The ability of such an ultrathin and lightweight structure to absorb bat ultrasound is an impressive example of deep-subwavelength sound absorption.

Moth wing scales can be divided into two main types: shorter base scales that form an underlying layer and longer cover scales that overlap them. These scales differ in length, width, shape, stalk stiffness, and the angle at which they are inserted into the wing membrane. When sound of an appropriate frequency reaches a scale, the scale can resonate through bending, side-to-side waving, or twisting motions. This resonance transfers acoustic energy from the surrounding air into the scale, where the energy is gradually dissipated through the intrinsic damping properties of the material.

A single resonator generally absorbs sound over only a narrow frequency band. A moth wing, however, contains large numbers of scales with different morphologies and resonant frequencies. Measurements made with a scanning laser Doppler vibrometer showed that the collective resonances of the two moth species were distributed relatively evenly across the full tested range of 20 to 160 kHz. Both base scales and cover scales could vibrate in several different modes, many of which fell within the ultrasonic frequencies used by bat biosonar. The resonances of different scales therefore complemented one another, allowing the wing to absorb sound over a much broader frequency range. Butterfly scales also resonated, but their more uniform shapes caused their resonances to cluster within only a few restricted frequency bands.

To test whether differently tuned scales coupled through a shared wing membrane could produce broader and stronger absorption than scales acting independently, the researchers constructed a numerical model containing an array of 16 scales. When all 16 scales had identical properties, the array generated a single absorption peak centred around one frequency. When the scales were tuned to have resonances ranging from 30 to 45 kHz in 1-kHz intervals, the absorption band became substantially wider. The mixed array also produced a higher absorption peak and greater total absorption than any of the uniform arrays.

Because all of the scales were attached to the same flexible wing membrane, neighbouring scales influenced one another's vibrational responses. The acoustic performance of the complete array therefore exceeded the simple sum of the contributions from the individual scales. Its properties emerged from the interaction among differently tuned resonators coupled through a common substrate.

This collective behaviour lies at the heart of an acoustic metamaterial. The unusual physical properties of a metamaterial arise from the arrangement and interaction of structural elements that are smaller than the wavelengths on which they act. Each scale on a moth wing functions as a microscopic resonant unit cell, while the shared wing membrane couples thousands of these elements into an integrated array. Together, the scales form a biological metamaterial absorber that provides broadband acoustic camouflage against echolocating bats.

The discovery also suggests new directions for engineering. Artificial acoustic metamaterials often rely on complex geometries, repeated resonators, and additional damping layers to attenuate sound within a limited thickness. Moth wings show that morphologically diverse ultrathin resonators, a flexible substrate, intrinsic damping, and a nonuniform arrangement can generate broadband absorption within an exceptionally thin structure. A deeper understanding of how moth scales are coupled and organized may eventually lead to thinner and lighter sound-absorbing panels for noise-control devices, vehicles, and other applications in which space and weight are tightly constrained.

Author: Shui-Ye You

Reference:

Neil TR et al. (2020). Moth wings are acoustic metamaterials. PNAS.