Pretty patterns on your skin are great for camouflage, but can they tell more than tales of concealment? The loliginid family of squid congregate in schools that can be found at depths as great as 400 m. Most of these squid are effectively transparent, making them almost invisible at these depths, yet they have all of the specialised camouflage mechanisms found on their opaque cousins. One of these structures, the iridophore, consists of stacks of chitin plates that are separated by cytomplasmic spaces. When visible light enters these structures it is reflected. If the optical thickness of the plates and spaces separating them is one quarter of the wavelength of some of the wavelengths of incident light, only that wavelength will be reflected from the interface. The rest of the light will be transmitted into the squid’s body. Consequently only certain colours will be reflected by the iridophore. This will...

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