Moth wings are acoustic metamaterials

Abstract

Intense predation pressure from echolocating bats has led to the evolution of a host of anti-bat defences in nocturnal moths. The wings of moths are covered in scales that exhibit intricate shapes and sculpted nano structures. Here, we reveal that this scale layer forms a metamaterial ultrasound absorber that is 111 times thinner than the longest absorbed wavelength. Both experimental and numerical analyses of individual scale vibrodynamics reveal that the first three resonances of moth scales lie within the typical echolocation frequency range of bats, whilst scale shape diversity on the wing results in a broad resonance distribution. Individual scales form the resonant unit-cells on the wing and collectively they generate hard to attain broadband deep-subwavelength absorption. Numerical modelling confirms that the acoustic properties of moth wings originate from the interaction of differently tuned single scale resonators, creating emergent acoustic performance beyond that of its elements, confirming broadband metamaterial absorber functionality. This sound absorber provides moth wings with acoustic camouflage against echolocating bats, it combines broadband absorption of all frequencies used by bats with light and ultrathin structures that meet aerodynamic constraints on wing weight and thickness.