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

声学伪装：飞蛾的翅膀作为变形表面提供了对抗蝙蝠生物声纳的双峰隐身。许多蛾子都有超声波敏感的耳朵，可以探测到回声定位蝙蝠。没有耳朵的飞蛾可以通过隐形声学伪装获得保护，吸收而不是反射声音。我们发现了身体的毛发(Neil et al.Subm)和翼鳞(Neil et al.飞蛾而不是蝴蝶吸收蝙蝠的生物声纳。厚厚的体毛是一种性能优于技术解决方案的多孔吸波材料。空气动力学的限制使得机翼上的鳞片层太薄(波长的1/10)，不适合需要共振功能的多孔吸收。我们发现个别蛾的鳞片是共振式吸音器(沈等人，在PNAS进行审查)，实现了宽带变形表面功能(沈等人，在PREP中)，这意味着超薄和轻质的建筑声学解决方案。不过，隐形伪装只适用于飞行。经常，飞蛾栖息在蝙蝠收集它们的基质上。高吸光度使休眠蛾成为一个不反射的黑点，而匹配的底物反射率将提供伪装。我们相信，通过利用覆盖每个固体表面的粘弹性边界层，谐振式机翼吸振器提供了两种功能--在飞行中具有高吸收率，并且在衬底上具有高反射率。飞蛾只需要将它们的翅膀放在它们休息的底物上，而许多物种(例如尺蛾科)正是这样做的。在这个涉及生物超表面的跨学科项目中，我们将确认并量化飞蛾翅膀具有双峰共振功能。

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.