To hide objects from view, our technologies have a long tradition of learning from nature. The ‘dazzle’ camouflage used for ships during the First World War took inspiration from the work of naturalists such as Abbott Thayer and John Graham Kerr, who had pointed out how disruptive patterning in animal markings confuses the eye of predators. British military camouflage strategies in the Second World War were guided by the advice of zoologist Hugh Cott, a disciple of Kerr.
With the advent of sonar and radar detection methods, however, the challenge was of another order: to reduce not optical visibility but the reflection of acoustic or microwave/radio electromagnetic signals. Stealth camouflage on today’s fighter aircraft typically uses coatings and composites that absorb radio waves, potentially reducing the radar scattering cross-section of a jet to the equivalent of a golf ball. Metamaterials — structures made from arrays of artificial building blocks of perhaps macroscopic size with tailored properties such as response to incident waves1 — have meanwhile been explored for making cloaking devices against microwaves2 or acoustic waves3. Nature might seem a less likely place to find inspiration for such highly engineered systems.
But we should never underestimate nature’s ingenuity. Optical metamaterials that refract and scatter light in adaptive ways are already familiar in the living world, for example in the photonic crystals found on strongly coloured, microstructured insect cuticles or butterfly wings4,5. Now it appears that acoustic stealth technology too was discovered first by natural selection. Neil et al. report evidence that the intricate array of scales on some moth wings acts as an acoustic metamaterial to reduce echoes from ultrasound6. This, they say, is probably an adaptive property that reduces the visibility of moths to the sonar searches of their predators, bats.
The researchers measured the acoustic reflections from wing sections of two moth species (native to Africa and China) in the ultrasonic range of 20–160 kHz typically used by bats for locating prey. They made measurements both with and without the tiny scales that normally cover the wings, which are known already to provide thermoregulation and optical camouflage. The presence of scales significantly reduces the reflected signal from the wings by absorption. In contrast, for two species of butterfly that are not subject to bat predation, the scales actually increase the acoustic reflection.
Acoustic absorption doesn’t itself demand metamaterial properties, of course, and the moths achieve it on their thorax with a thick layer of hair-like scales7. But the layer of scales on the wings is much thinner than the acoustic wavelengths of bat sonar (around 17 mm at 20 kHz) — that’s an inevitable aerodynamic constraint — and so can’t obviously attain strong absorption from a simple mechanical response. Moth scales do show a resonant response to acoustic signals in the relevant frequency range, but resonant absorbers tend to have a narrow bandwidth.
Neil et al. find, however, that moth scales have variations in size and shape that both experimental measurements and finite-element modelling reveal to possess a wide range of resonances, turning the scale layer into a metamaterial acoustic array with a broad absorption band spanning from 20 to 160 kHz. Again, this contrasts with butterfly scales, which tend to be rather uniform in shape and size. What’s more, the modelling studies suggest that the positioning of the scales on a flexible membrane base creates coupling and blending of the acoustic modes to produce an emergent broadband response — an essential attribute, perhaps, for species preyed on by many different varieties of bat that use different frequencies for their sonar.
Aviation buffs will know very well that some of the earliest commercial biplanes of the 1920s and 30s were the de Havilland Moths, so-named by Geoffrey de Havilland because the light wings could be folded back against the plane’s body for ease of transport by towing. Evidently, there is much more benefit to be gleaned from the natural engineering of the moth’s wing than that.
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Ball, P. New lessons for stealth technology. Nat. Mater. 20, 4 (2021). https://doi.org/10.1038/s41563-020-00887-z