There is no single evolutionary pathway by which structural colour evolves in beetles, and no single function for iridescence. Similar iridescence and colour mechanisms are likely to serve different functions in different taxa, not only crypsis but also sexual signalling, aposematism, mimicry, locomotory enhancement and thermoregulation; some may simply be an artefact of other structural constraints, such as the iridescence of the torridincollid plastron or the thin-film colour of an exposed flight wing.
Multilayer reflectors are the most widespread iridescence mechanism, and have evolved many times from similar precursors: stacked chitin layers (optically active or not) are present in all beetles, more so than other, less well-armoured insects. In this sense, the order Coleoptera can be considered preadapted for this particular form of iridescence.
It is clear that multilayer coloration is asymmetrically distributed across the beetle phylogeny: it occurs throughout the order but is most common in the large clades Phytophaga, Scarabaeidae and Buprestidae. In adults of all three groups, surface feeding and daytime activity on plant surfaces is common (Farrell 1998
; Bellamy & Nelson 2002
). Although there are some dazzling exceptions, the most common colour in these groups is green, with apparent disruptive coloration, aposematism and mimicry less widespread.
It is conceivable that, as various lineages of polyphagan beetles evolved to take full advantage of land plants (and in the process departing from their likely ancestral habits of concealed saprophagy (Farrell 1998
; Marvaldi et al. 2002
)) they were also subject to increased predation pressure from other invertebrates and vertebrates. If crypsis is the first (and arguably metabolically cheapest) line of defence against predation, and green pigments are rare or unavailable in animals, it can then be argued that multilayer reflectors and photonic crystals are inextricably linked to major evolutionary shifts towards plant feeding in polyphagan beetles. As reflectors that produce green coloration in any exposed condition (i.e. when sunlight is present), structural colour mechanisms grant their bearers an efficient and lasting camouflage among the green foliage of plants.
7.1 Recommendations for future research
When describing the coloration of an organism (in ecological, behavioural and taxonomic works alike), we encourage the biologist to determine the underlying colour mechanism(s); this is particularly important in iridescent taxa, where similar chromatic effects can be caused by very different and non-homologous structures. In this review, we have aimed to provide accurate descriptions, recognitory diagnoses and known distribution of the major mechanisms of iridescence and structural colours in Coleoptera. We also provide a glossary of optical terms of potential use to entomologists (appendix A) and a proposed terminology of iridescence mechanisms ().
Relationships between structural colours, optical mechanisms and visible reflectance in Coleoptera.
Because colour perception is subjective, varying between organisms, and even between human observers, and since we currently lack a fixed vocabulary for structural colours, it is particularly important to characterize chromatic features in the most objective fashion possible. The quickest and most objective method is to measure reflectance peaks with a spectroradiometer (e.g. Endler 1990
); this method is effective with colours produced by multilayer reflectors and three-dimensional photonic crystals. Measured reflectance spectra can then be used for precise characterization of multilayer colour effects with or without polarization (e.g. De Silva et al. 2005
; Deparis et al. 2008
). A less expensive alternative is that proposed by Aguiar (2005)
, who proposed sampling a colour from within a digital photograph of the specimen in question; graphics programs such as Adobe Photoshop and CorelDraw or Corel PhotoPaint automatically display the RGB coordinates of the sampled colour. This approach is more affordable than the former, but has two notable limitations: unlike spectroradiometry, it cannot detect UV and IR (nor quantify the amount of reflectance of a colour); it will also be affected by the light environment used to take the photograph.
When evaluating the proposed function of a coloration mechanism, it is also important to take into account not only the visual system of study organisms and their potential predators but also the organism's natural context, including optical properties of the substrate and the ambient light conditions (Endler 1990
). This approach has been applied broadly to the study colorations in vertebrates and butterflies, but rarely with beetles. Schultz (2001)
quantified the conspicuousness of iridescent anti-predator colorations of two tropical tiger beetles against natural backgrounds and under forest light. Using photoreceptor sensitivities obtained from the related species Onitis alexis
, Thery et al. (2008)
have determined that the iridescent colour of the crepuscular scarab C. lancifer
would be most effective as an intraspecific signal at dusk when the beetles were most active. The visual system of O. alexis
is dichromatic with receptor peaks in the UV and green (Warrant & McIntyre 1990
); the ancestral condition for beetles is trichromatic vision with peaks in UV, blue and green (Briscoe & Chittka 2001
, based on measurements for lampyrid beetles and owlflies). However, more research is needed on the spectral sensitivities of beetles for which iridescent colours may serve as signals.
There is a long entomological tradition of considering beetles and other insects within the context of the museum and laboratory, prizing large and seemingly conspicuous specimens for their jewel-like reflectance or unusual morphology. A similar habit persists in the fields of optics and biomechanics; individual structures are treated in isolation and evaluated on the basis of their material properties or as potential source material for the expanding technological field of biomimicry. Although a mechanistic, experimental approach is critical to elucidate how insect reflectors function, it is important to connect these structures to their role in the organism, which in turn necessitates a phylogenetic and ecological context.
The order Coleoptera is by any standard a prodigious showcase for the extraordinary creativity and flexibility of the evolutionary process. Many optical mechanisms in beetles remain to be fully explored; with more diligent integration of the experimental and evolutionary perspectives in research, we can achieve a fuller understanding of insect chroma.