Researchers closely studied the wings of two moth species from China and Africa, and found that their tiny scales form a "metamaterial" that absorbs bat sonar — thus becoming an acoustic cloaking device.
Pretty darn cool. The paper is paywalled here, but Nature has a good lay-persons' writeup …
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. [snip]
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.
BTW, that photo of a moth above — by Andrew Synder Photography, and CC-2.0-licensed on his Flickr stream — is just some random moth, I don't think it's one of the species they studied.
