Short answer
Light reflected from the top and bottom of a transparent layer a few hundred nanometres thick interferes, reinforcing some wavelengths and cancelling others; stacking many such layers makes the reinforced band brighter and more saturated. Organisms build these stacks from whatever they have — chitin, melanin in keratin, guanine, cellulose, lipid, even photosynthetic membrane — and the colour that results depends on layer spacing, refractive-index contrast and, above all, the angle of view.
A single transparent film reflects a little light at each surface. If its optical thickness is comparable to a wavelength, the two reflections interfere and the film takes on a colour that drifts with thickness and angle, as in a soap bubble. Single films in organisms are weak colourists. The cortex of a kingfisher's feather barb, a few micrometres thick, adds ripples to the reflectance spectrum that are measurable from a small spot and average away over a larger area. The buttercup petal's smooth epidermis behaves as a mirror-like film above a starch layer: it adds a directional gloss to a yellow that is itself made by pigment. In both cases the film modifies a colour made another way.
Stack the layers with alternating high and low refractive index and each interface adds a reflection in step with the others for one band of wavelengths. Reflectance climbs steeply with the number of layers. The Japanese jewel beetle's wing case shows the pattern cleanly: about 16 layers in the green areas and 12 in the purple stripes, with refractive indices between roughly 1.6 and 1.7, give reflectance bands 100–150 nm wide peaking near 530 nm and 700 nm. In Anna's hummingbird the high-index layers are sheets of melanosomes separated by keratin. In the neon tetra and many other fish they are plate-like guanine crystals in cytoplasm, a material whose very high refractive index makes a strong reflector from few layers.
Multilayers have arisen in lineages that share no relevant tissue. Plants make them from cellulose: in the fruit of Pollia condensata, microfibrils are laid down in a helix through the cell wall, and the repeat distance of the helix sets the reflected colour. Viburnum tinus uses globules of lipid arranged in rough layers. Shade begonias stack their own thylakoid membranes at regular spacing inside modified chloroplasts, so the light-harvesting tissue is itself the reflector. Squid use plates of protein. That convergence is good evidence that layered reflectors are easy to evolve; it says nothing by itself about what each one is for.
For any flat multilayer the reflected band moves towards shorter wavelengths as the light becomes more oblique, because the path difference between successive reflections shrinks. The jewel beetle's green slides towards blue and its reflection becomes strongly linearly polarised at glancing angles. A helical stack does something a flat one cannot: it reflects circularly polarised light of one handedness. The scarab Chrysina gloriosa reflects left-circular light, and Pollia fruit contain cells of both handednesses in the same tissue. People cannot see either kind of polarisation without a filter, which is one reason human descriptions of these surfaces are incomplete.
Several organisms combine a multilayer with a larger-scale shape that broadens or stabilises its colour. Morpho scales carry their layers on rows of tall ridges, so that blue is returned across more than 100° in one plane while remaining tightly confined in the other; some scale microstructures reflect up to three quarters of the incident blue light. The emerald swallowtail's scale multilayer is modulated in shape so that it returns both yellow and blue iridescence, the blue by a retro-reflection process, and the wing looks bright green. The Pollia fruit's cell-by-cell variation in layer spacing gives a speckle of colours rather than one. A multilayer is a component, and the scale it sits on decides what the animal looks like.
| Organism | Structure | Mechanism | Made from | Change with angle |
|---|---|---|---|---|
| Morpho butterflies (Morpho spp.) | Dorsal wing ground scales | Multilayer reflector | Chitin ridges on each scale carrying stacked lamellae with air between them; the stack interferes and the row of ridges diffracts. | Moderate. Blue is returned over a wide spread of angles in one plane and a narrow one in the other, so the wing stays blue as it tilts and then abruptly goes dark. |
| Japanese jewel beetle (Chrysochroa fulgidissima) | Wing cases (elytra), epicuticle | Multilayer reflector | Epicuticle stacks of about 16 layers (green areas) and 12 layers (purple areas) with refractive indices between roughly 1.6 and 1.7. | Strong. Both bands move to shorter wavelengths as the light becomes more oblique, and the reflection becomes strongly polarised. |
| Asian jewel beetle (Sternocera aequisignata) | Wing cases (elytra) | Multilayer reflector | Multilayer cuticle reflector. | Strong. Hue changes continuously with viewing angle. |
| Glorious scarab (Chrysina gloriosa) | Exoskeleton cells about 10 micrometres across | Multilayer reflector | Chitin fibrils laid down in a helical (cholesteric-like) stack, arranged in cells that resemble the focal conic domains of a liquid crystal. | Strong. Iridescent; the reflected light is left circularly polarised. |
| Anna's hummingbird, male (Calypte anna) | Throat and crown feather barbules | Multilayer reflector | Stacked layers of melanosomes separated by keratin within each barbule. | Strong. Near-specular: the colour is seen only where light, feather and viewer line up, and is dark otherwise. |
| Common kingfisher (Alcedo atthis) | Back and tail feather barbs | Coherent scattering, quasi-ordered | Spongy keratin-and-air nanostructure of slightly different dimensions in cyan and blue barbs, inside a cortex a few micrometres thick; the orange barbs hold pigment granules instead. | Weak. Scatterometry shows some angle-dependence, increasing towards shorter wavelengths; far less than a multilayer. |
| Neon tetra (Paracheirodon innesi) | Lateral stripe iridophores | Multilayer reflector | Stacks of thin reflecting platelets (guanine crystals) in cytoplasm, tilted together like a Venetian blind to change their spacing. | Strong. Colour and direction of reflection change together as the platelets tilt. |
| Squid (Loliginid squids) | Skin iridophores beneath the chromatophores | Multilayer reflector | Stacks of protein platelets separated by cytoplasm; in some squid the stack is tuned by a cholinergic, non-synaptic nervous signal. | Strong. Iridescent and partly polarised. |
| Marble berry (Pollia condensata) | Fruit skin cell walls | Multilayer reflector | Cellulose microfibrils stacked helically in the cell wall, with layer thickness varying from cell to cell; no blue pigment. | Strong. Iridescent; each cell reflects its own colour, giving a pixelated look. |
| Laurustinus (Viburnum tinus) | Fruit skin cell walls | Multilayer reflector | Globular lipid inclusions in a disordered multilayer within the cell wall, over a dark anthocyanin layer. | Moderate. A metallic sheen; the disorder broadens the reflection. |
| Shade-dwelling begonias (Begonia spp.) | Epidermal chloroplasts (iridoplasts) | Multilayer reflector | Thylakoid membranes stacked at regular intervals so that the light-absorbing tissue is itself a photonic multilayer. | Strong. Iridescent blue reflection. |
| Peacock spikemoss (Selaginella willdenowii) | Upper leaf cuticle | Multilayer reflector | A layered lamellar structure in the upper cuticle, absent from green leaves of the same plant. | Strong. Iridescent blue. |
| Creeping buttercup (Ranunculus repens) | Petal epidermis over a starch layer | Thin-film interference | A smooth, partly transparent, pigment-bearing epidermal layer that reflects like a mirror, backed by a starch layer that scatters yellow diffusely. | Strong. The gloss is highly directional; the yellow beneath is diffuse. |
| Soap film | A water film between two surfactant layers | Thin-film interference | A single transparent film whose thickness is comparable to the wavelength of light. | Strong. Colour depends on film thickness and viewing angle. |
| Organism | Mechanism | Reported spectral behaviour | Limit |
|---|---|---|---|
| Morpho butterflies (Morpho spp.) | Multilayer reflector | Some scale microstructures reflect up to 75% of incident blue light over more than 100° in one plane and about 15° in the other. | Measured on detached single scales from two species; the genus varies. |
| Japanese jewel beetle (Chrysochroa fulgidissima) | Multilayer reflector | Reflectance bands 100–150 nm wide peaking near 530 nm (green) and 700 nm (purple) at normal incidence. | Measured at normal incidence on a nearly flat surface; curvature changes what is seen. |
| Common kingfisher (Alcedo atthis) | Coherent scattering, quasi-ordered | The thin-film cortex adds oscillations to spectra from small areas that average out over larger areas; its contribution is small but not negligible. | One species; three feather types. |
| Marble berry (Pollia condensata) | Multilayer reflector | Described by its authors as a more intense reflection than any previously reported biological material. | One species; how dispersers respond to the fruit was not measured. |
| Shade-dwelling begonias (Begonia spp.) | Multilayer reflector | Reflects blue while increasing capture of the green light that dominates forest shade; quantum yield rose 5–10% under low light. | Here the reflected blue is a side-effect of a structure whose measured benefit is light capture, not display. |
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
The wing cases of the jewel beetle Chrysochroa fulgidissima carry epicuticle stacks of about 16 layers in green areas and 12 in purple areas, giving reflectance bands 100–150 nm wide that peak near 530 nm and 700 nm at normal incidence and shift to shorter wavelengths at oblique incidence.
FactStrong evidence
Some microstructures in single Morpho wing scales reflect up to 75% of incident blue light over an angular range of more than 100° in one plane and about 15° in the other.
Caveat: Measured on detached scales of two species under laboratory illumination.
Source: Quantified interference and diffraction in single Morpho butterfly scales
FactStrong evidence
The blue of Pollia condensata fruit is produced by helicoidally stacked cellulose microfibrils in the cell wall; layer thickness varies from cell to cell, and individual cells reflect circularly polarised light of either left or right handedness.
FactStrong evidence
The iridescent chloroplasts of shade-dwelling Begonia are photonic multilayers of thylakoid membrane, and they raised photosynthetic quantum yield by 5–10% under low light in direct measurement.
Caveat: One genus; the measured benefit is light capture, and the visible blue is its by-product.
Source: Photonic multilayer structure of Begonia chloroplasts enhances photosynthetic efficiency
Colourwise interpretationModerate evidence
The repeated, independent appearance of multilayer reflectors in chitin, keratin, guanine, cellulose, lipid and protein shows that the structure is readily evolved, but does not indicate a shared function.
Based on: Drawn from the range of materials in the dataset's multilayer records, whose proposed functions run from courtship display to camouflage to photosynthesis and whose evidence grades differ.
Source: Photonic structures in biology; Pointillist structural color in Pollia fruit
Reviewed 6 October 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.