The photonic structures of butterfly wings are among the most anatomically diverse of all those in nature, giving rise to an unrivalled display of structural colours. These have recently become the focus of research by workers in a variety of disciplines, stimulated by their potential applications to technology (‘biomimetics’). This interest, together with the discovery of unpublished electron micrographs taken by the late Dr John Huxley (Natural History Museum, London), prompted this review of butterfly photonics in general. The current work provides a synopsis of the literature to date, covering the diversity and evolution of these optical structures and incorporating Huxley's work, which represents an important biomimetic and evolutionary database on its own. This review deals with butterfly photonic devices according to the parts of the butterfly scales on which they occur. In this way, the information is ripe for evolutionary study.

The structural origin of the weak iridescence on some of the dark feathers of the black-billed magpie, Pica pica (Corvidae), is found in the structure of the ribbon-shaped barbules. The cortex of these barbules contains cylindrical holes distributed as the nodes of an hexagonal lattice in the hard layer cross section. The cortex optical properties are described starting from a photonic-crystal film theory. The yellowish-green coloration of the bird's tail can be explained by the appearance of a reflection band related to the photonic-crystal lowest-lying gap. The bluish reflections from the wings are produced by a more complicated mechanism, involving the presence of a cortex second gap."

Iridescent butterfly wing colours result from the interaction of light with sub-micrometre structures in the scales. Typically, one scale contains one such photonic structure that produces a single iridescent signal. Here, however, we show how the dorsal wings of male Lamprolenis nitida emit two independent signals from two separate photonic structures in the same scale. Multiple independent signals from separate photonic structures within the same sub-micrometre device are currently unknown in animals. However, they would serve to increase the complexity and specificity of the optical signature, enhancing the information conveyed. This could be important during intrasexual encounters, in which iridescent male wing colours are employed as threat displays. Blazed diffraction gratings, like those found in L. nitida, are asymmetric photonic structures and drive most of the incident light into one diffraction order. Similar gratings are used in spectrometers, limiting the spectral range over which the spectrometer functions. By incorporating two interchangeable gratings onto a single structure, as they are in L. nitida, the functional range of spectrometers could be extended.Keywords: butterfly wing colour; iridescent signal; photonic structure; blazed diffraction grating; spectrometerLamprolenis nitida Godman & Salvin (1881) (Satyrinae: Nymphalidae), a New Guinean forest species (Parsons 1998), appears matt brown dorsally when illuminated and observed from above (figure 1a,b, background). However, when light is incident on the hindwing of a male in an anteroposterior direction, bright green to red iridescence is observed in backscatter (figure 1a, inset).If illumination and observation positions are then rotated approximately 1808 on the hindwing, further backscattered iridescence from blue to violet is visible, albeit more faintly, in the same location (figure 1b, inset). Iridescence in either direction was not observed from females. Since all of the colours observed from the hindwing do not occur in the order in which we would see them together in a single spectrum, despite originating from the same region on the wing, the two signals must originate from separate photonic structures on the same scale. Typically a single photonic structure provides a single optical signal (e.g. Bálint et al. 2005). Multiple signals from independent photonic structures within the same sub-micrometre device are currently unknown in animals, stimulating an investigation of L. nitida.Spectral measurements were taken with an Avantes Avaspec 2048/2 spectrometer. Hindwing samples were dissected from two males and adhered using double-sided tape to glass slides. The reflection was standardized using a white diffusive standard. The angle of incidence (q) was measured from the normal to the wing sample surface and chosen to be 758 for anteroposterior illumination and 608 for posteroanterior illumination. Measurements of the anteroposterior iridescence confirmed the observed backscattered shift from green to red with observation angle gettin...

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The structure and mechanism of pupal attachment are described for the nymphalid Greta oto using electron microscopy, and high-speed and time-lapse photography. The cremaster is composed of a 3-D array of hooked setae that engage with silk fibers spun into layers in a pad on the lower leaf surface. Each seta comprises a shaft terminating in a strongly curved hook, tipped with two lateral barbs. These hook into the silk pad, which is densely laid and built-up in the central portion, flattening out peripherally. Timelapse photography showed that silk pad construction by fifth instar larvae is completed in four distinct spinning movements, producing a random fiber arrangement. It is proposed that such a fiber arrangement provides isotropic strength, giving greater flexibility to the attachment. The cremaster is attached to the silk pad by a series of lateral movements of the pupa's posterior abdomen. This movement, together with the shape of the setal hooks, is thought to be integral to the attachment process. Tensile loading tests showed that attachment failure is due to the breakage of the silk pad, which undergoes gradual destruction before releasing the cremaster. The attachment was found to have high tensile strength and fracture toughness, both of which suggest that it has evolved for the dual purpose of preventing the pupa being pulled from the leaf by a predator and preventing the attachment being weakened by wind, which causes the pupa to swing.