lattices have recently been discovered in the iridescent cuticle of certain beetle species (A. R. Parker, V. L. Welch & D. Driver, personalcommunication), but the effect is far better understood in the inanimate structures associated with gem opals. In these, strong colouredregions are created by aggregates of 150–350 nm diameter amorphous silica spheres arranged hexagonally in layers that are usuallystacked randomly. Some opal specimens exhibit parallel domains ofordered packing that are commonly face-centered cubic but whichoccasionally are hexagonally close-packed25. This ordered structureis understood to be the result of precipitative settling of silica gel solution: the opal’s colouring is purely artefactual.Anti-reflection structuresAlthough the character of most natural partial PBG systems is associated with bright colour or broad angle reflectivity, a specific form ofnanostructure minimizes reflectivity at a surface over broad angles orfrequency ranges. The structure gradually matches the opticalimpedance of one medium with that of its neighbour across the interface. This is invariably achieved by incorporation of arrays of taperedelements, described as nipple arrays, across the boundary. Considerable research into such nipple arrays, which are commonly found onarthropodal ommatidial surfaces (Fig. 7a), suggests a distinct visualselection advantage associated with their use in compound eyes26: thereduced reflectivity from the ommatidial surface enhances the photoncollection efficiency of the visual system. The evolutionary development of these anti-reflective nanostructures must have occurred veryearly—they have been found on the ommatidial surfaces of ambertrapped diptera from the Eocene period27. Further evidence thatthese photonic structures enhance transmission through interfacescomes from their presence on the dorsal and ventral surfaces of transparent diurnal moth wings28 (Fig. 7b). Additionally, they are seen toremove the Brewster-mediated disparity in reflection of differentpolarizations from the wing (P.V. and J.R.S., unpublished data), a relevant feature within a forum where visual polarization sensitivity isubiquitous15,29.As well as enhancing transparency in certain insects, nanostructures may be associated with the colour black on certainlepidopteran wing scales. Although the dark melanin-based pigmentation in such scales is the absorbing medium, optical scattering