Acoustic camouflage: moth wings as metasurface provide bimodal stealth against bat biosonar

Acoustic camouflage: moth wings as metasurface provide bimodal stealth against bat biosonar. Many moths have ultrasound-sensitive ears that detect echolocating bats. Moths without ears could gain protection by stealth acoustic camouflage, absorbing rather than reflecting sound. We found that body fur (Neil et al. subm) and wing scales (Neil et al. in prep) of moths but not butterflies absorbs bat biosonar. Thick body fur is a porous absorber outperforming technical solutions. Aerodynamic constraints render the scale layer on wings too thin (<1/10th of wavelength) for porous absorption requiring resonant functionality. We found individual moth scales are resonant absorbers (Shen et al, under review at PNAS) achieving broadband metasurface functionality (Shen et al, in prep) with implications for ultrathin and lightweight solutions for building acoustics. Stealth camouflage is only adaptive in flight though. Frequently, moths are resting on substrates from which bats glean them. High absorption lets resting moths stand out as a non-reflective 'black spot', while matching substrate reflectivity would provide camouflage. We believe that the resonant wing absorber offers both functionalities - being highly absorptive in flight and highly reflective on a substrate, by exploiting the viscoelastic boundary layer that covers every solid surface. Moths would only need to address their wings to their resting substrate, and many species (e.g. Geometridae) do exactly that.
In this interdisciplinary project into biological metasurfaces, we will confirm and quantify that moth wings have bimodal resonant functionality.