Diffraction of Life - biosonar camouflage, cloaking and concealment

点击翻译按钮获取中文摘要

Invisibility cloaks are fantastic devices in popular culture from Harry Potter to Star Wars. The science behind cloaking has been advanced to a level that brings a future real-life invisibility cloak within our reach. In fact, several cloaks with partial functionality have already been realised with so-called metamaterials - assemblies of multiple elements engineered to have properties not yet found in nature. An even more promising field for the development of a functional cloaking device is not for light, but for sound - acoustic cloaks. Because the wavelengths of sound are longer than those of light waves, it is easier to design and build acoustic metamaterials and hence effective cloaks. Indeed, the most advanced acoustic cloak can now completely hide an object on a surface - a so-called carpet cloak. As a metamaterial it consists of partly overlapping perforated plates, arranged much like roof tiles.While we know of no metamaterials for light in nature, is this also true for acoustic metamaterials? Which organism would need such a device to hide itself acoustically? We propose the answer lies in the 65MY old arms race between echolocating bats and their moth prey. A 'biosonar cloak' against bats would reduce predation pressure on the moths and therefore offer substantial evolutionary benefits. Interestingly, the layers of scales on a moth's body surfaces bear remarkable structural resemblance to an acoustic carpet cloak.We hypothesise that moth wings are an acoustic metamaterial engineered by nature. We will investigate whether the scales on the moths have acoustic properties that hide the moth from an echolocating bat. In our pilot study, we have developed a 'biosonar visualizer' that creates acoustic images revealing the reflective nature of body parts. This technique is closely related to medical ultrasound imaging (tomography). We also use a laser scanner to measure how the layer of scales vibrates in response to ultrasound. From these preliminary data we find a surprising range of interesting adaptations: First, scales on a (dead and dried) moth wing change wing reflectivity by a factor of four. In another very exciting discovery, we find that the long tails of Luna moths reflect strong echoes such that they attract the bat's attention and attack away from the moth's body. We also find that the eye spots, used in a visual display to startle an approaching predator, also stand out acoustically. Finally, we have evidence that moths choose the places to rest and adjust their wing position to reduce contrast and blend into the substrate acoustically. This pilot data make clear that there is a wide and promising unstudied field of echoacoustic adaptations. In analogy to visual camouflage, we introduce an entirely novel field of research - biosonar camouflage. We identify several possible strategies for camouflage. One strategy is for the moth to reflect very little ultrasound (cloaking) when in flight, thus reducing the distance over which a bat can detect it. Another strategy involves mimicking the echoes of a resting surface. In this scenario, the moth resting on the bark of a tree is acoustically blending with its environment.Our research will establish what acoustic properties and sound processing mechanisms have evolved in moth scales in response to bat biosonar. Since the industrial revolution, the world has become a noisy place where man-made sounds are pervasive throughout our living habitats. Acoustic pollution is a source of discomfort and stress for humans and animals. We will use our understanding of moth wings to 3D print scaled prototypes with acoustic metamaterial properties at audible frequencies. Thereby we contribute new bio-inspired solutions to current noise control challenges at the low frequencies so important to human speech and comfort.