Home Search Collections Journals About Contact us My IOPscience 515 million years of structural colour This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2000 J. Opt. A: Pure Appl. Opt. 2 R15 (http://iopscience.iop.org/1464-4258/2/6/201) The Table of Contents and more related content is available Download details: IP Address: 172.16.0.32 The article was downloaded on 29/03/2010 at 09:45 Please note that terms and conditions apply. J. Opt. A: Pure Appl. Opt. 2 (2000) R15–R28. Printed in the UK PII: S1464-4258(00)14590-8 REVIEW ARTICLE 515 million years of structural colour Andrew Richard Parker Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK E-mail: [email protected] Received 7 June 2000, in final form 10 August 2000 Abstract. Structures that cause colour or provide antireflection have been found in both living and extinct animals in a diversity of forms, including mirror-reflective and diffractive devices. An overview of this diversity is presented here, and behavioural and evolutionary implications are introduced. Keywords: Animal structural colours, fossil colours, reflectors, antireflectors, eyes 1. Introduction particularly the case when the dimensions of the structures are of the order of a few wavelengths of light. Since the above Animal pigments have long received scientific attention. categories are academic, I place individual cases of structural Bioluminescence, or ‘cold light’, resulting from a chemical colours in their most appropriate, not unequivocal, category. reaction, is also well understood. However, another major The array of structural colours found in animals today category of colour/light display in animals has more recently results from millions of years of evolution. Structures attracted the attention of biologists: structural colouration. that produce metallic colours have also been identified in Structural colouration involves the selective reflectance of extinct animals (e.g. Towe and Harper 1966, Parker 1998a). incident light by the physical nature of a structure. Although Confirmation of this fact, from ultrastructural examination of the colour effects often appear considerably brighter than exceptionally well-preserved fossils such as those from the those of pigments, structural colours often result from Burgess Shale (Middle Cambrian, British Columbia), 515 completely transparent materials. million years old, permits the study of the role of light in Hooke (1665) and Newton (1730) correctly explained ecosystems throughout geological time, and consequently its the structural colours of silverfish (Insecta) and peacock role in evolution. feathers, respectively, and Goureau (1842) discovered that This review introduces the types of structural colour the colours produced from the shells of certain molluscs found in invertebrates, and hence animals in general, without and the thin, membranous wings of many insects resulted delving deeply into the theory behind them. Examples of from physical structures. Nevertheless, until the end of their functional and evolutionary implications are introduced. the nineteenth century pigments were generally regarded However, this field is very much still in its infancy. as the cause of animal colours. Accurate, detailed studies of the mechanisms of structural colours commenced with 2. Mechanisms causing structural colour in Anderson and Richards (1942) following the introduction of animals the electron microscope. Invertebrates possess the greatest range of structural 2.1. Multilayer reflectors (interference) colours known in animals and will therefore be used to provide examples in this review. Within invertebrates, Light may be strongly reflected by constructive interference structural colours generally may be formed by one of three between reflections from the different interfaces of a stack of thin films (of actual thickness d) of alternately high mechanisms: thin-film reflectors, diffraction gratings or n structures causing scattering of light waves. Some structures, and low refractive index ( ). For this to occur, reflections from successive interfaces must emerge with the same phase however, rather fall between the above categories, such as and this is achieved when the so-called ‘Bragg condition’ photonic crystals (figure 1). In some cases this has led is fulfilled. The optical path difference between the light to confusion in the identification of the type of reflector. reflected from successive interfaces is an integral number of For example, the reflectors in some scarab beetles have wavelengths and is expressed by the equation: been categorized by different authors as multilayer reflectors, three-dimensional diffraction gratings and liquid crystal 2nd cos = (m + 1 )λ displays. Perhaps all are correct! It is not always easy to 2 predict how variations in the ‘optical’ dimensions or design from which it can be seen that the effect varies with angle of of a structure will alter its effect on light waves. This is incidence (, measured to the surface normal), wavelength 1464-4258/00/060015+14$30.00 © 2000 IOP Publishing Ltd R15 Review article reflected waves incident waves (in phase) (in phase) phase 2 n change a n f actual no phase thickness d f change Figure 1. Scanning electron micrograph of a cross section of the wall of a cylindrical spine of the sea mouse Aphrodita sp. n (Polychaeta, a bristle worm). The wall is composed of small a cylinders with varying internal diameters (increasing with depth in transmitted waves the stack), arranged in a hexagonal array, that form a photonic crystal. Scale bar represents 8 µm. Figure 2. Schematic diagram of thin-film reflection. The (λ) and the optical thickness of the layers (nd). There is a direction of wave (straight line) and profile of electric (or phase change of half a wavelength in waves reflected from magnetic) component are illustrated. Incident waves are indicated every low to high refractive index interface only. The optimal by a solid line, reflected waves by broken lines. Refraction occurs at each media interface. The refractive index of the film (nf )is narrow-band reflection condition is therefore achieved where n nd greater than the refractive index of the surrounding medium ( a). the optical thickness ( ) of every layer in the stack is a Constructive interference of the reflected waves is occurring. As quarter of a wavelength. In a multilayer consisting of a large the angle of incidence changes, different wavelengths number of layers with a small variation of refractive index the constructively interfere. At normal incidence constructive process is more selective than one with a smaller number of interference occurs where nf × df = λ/4. layers with a large difference of index. The former therefore gives rise to more saturated colours, corresponding to a reflectance has a narrower bandwidth. A narrow bandwidth, narrow spectral bandwidth, and these colours therefore vary less conspicuous reflection is sometimes selected for in more with a change of angle of incidence. Both conditions animals, as will be discussed later in this paper. Multilayer can be found in animals—different coloured effects are reflectors polarize light incident at Brewster angles. This is appropriate for different functions under different conditions. about 54◦ for a quarter-wave stack of guanine and cytoplasm. For an oblique angle of incidence, the wavelength of light that At very oblique angles, all wavelengths are strongly reflected. interferes constructively will be shorter than that for light at Multilayer reflectors are common in animals. They normal incidence. Therefore, as the angle of the incident are usually extra-cellular, produced by periodic secretion light changes, the observed colour also changes. and deposition, but sometimes occur within cells. Guanine Single-layer reflectors are found in Nature, where light is (n = 1.83) is a common component in invertebrate reflectors reflected, and interferes, from the upper and lower boundaries because it is one of the very few biological materials with (figure 2). A difference in the thickness of the layer a high refractive index and is readily available to most provides a change in the colour observed from unidirectional invertebrates as a nitrogenous metabolite (Herring 1994). polychromatic light. The wings of some houseflies act as However, arthropods, including insects, crustaceans and a single thin film and appear to have different colours as spiders, have largely ignored guanine in favour of pteridines a result of this phenomenon (Fox and Vevers 1960). A (Herring 1994). Also surprising is the fact that the reflector single quarter-wavelength film of guanine in cytoplasm, for material of closely related species, e.g. the molluscs Pecten example, reflects about 8% of the incident light (Land 1978). (scallop) and Cardium (cockle), may differ (Herring 1994). However, in a multilayer reflector with 10 or more high index Multilayers produce effects in beetle cuticle from highly layers, reflection efficiencies can reach 100% (Land 1972). metallic colours (‘ideal’ system) to rather dull greens (‘non- Thus, animals possessing such reflectors may appear highly ideal’ system in combination with scattering; figure 3) metallic. (Parker et al 1998c), and colours from the wings of many The reflectance of the multilayer system increases very butterflies. Often in butterflies, layers of chitin (n = about rapidly with increasing number of layers (Land 1972). If 1.56) are supported by vertical vanes of the scales. Air the dimensions of the system