Short answer
A diffraction grating is a surface of evenly spaced ridges that spreads white light into its spectrum by angle; some beetles, the rainbow peacock spiders and the petals of some flowers have them. Whether the resulting rainbow is seen and used is a separate question, and for flowers the evidence is openly disputed: bees can learn the cue in the laboratory, but optical measurements suggest it is too faint under natural light to count.
Both effects are interference, but they sort light differently. A multilayer reflects one band strongly in the mirror direction and changes that band as the angle changes. A grating sends each wavelength into its own direction, so a single ridged surface shows the whole spectrum at once, laid out side by side, with further fainter copies at wider angles. The spacing of the ridges must be comparable to the wavelength of light, typically somewhat under or around a micrometre. The signature is unmistakable under a point source — an ordered run of spectral colours, like the surface of a compact disc — and that dependence on a point source is the mechanism's weakness outdoors.
A review of beetle iridescence lists diffraction gratings as one of three mechanism groups in the order, alongside multilayers and three-dimensional photonic crystals; they occur in several families as fine parallel ridges on otherwise dark cuticle. The most elaborate gratings known are on the abdominal scales of two Australian peacock spiders, Maratus robinsoni and M. chrysomelas. Each scale, about 40 by 10 micrometres, carries a two-dimensional nanograting wrapped over a curved surface, and that curvature separates wavelengths at least twice as finely as a flat grating of the same period. In Morpho butterflies the regular row of ridges on each scale adds a diffractive component to what is mainly a multilayer colour.
In 2009 a study in Science reported that the petals of Hibiscus trionum and some tulips carry cuticular striations that act as diffraction gratings, and that bumblebees trained on artificial targets learned to use the iridescence as a cue and could still identify the target as its appearance changed. Five years later a different group examined petals of many species with an imaging scatterometer. They confirmed that striated petals diffract when a single cell is lit by a narrow beam, but found the iridescent signal vanished under illumination resembling natural light, and concluded that pigment, not structure, determines how flowers look. Both findings stand. They answer different questions: what a bee can learn, and what a flower in a field presents.
This is a tested function that remains contested — which is why the dataset records the grade and the dispute separately.
A grating's colours are separated by direction. Under the sun alone, a viewer at one position receives one narrow band from each patch of grating and sees saturated colour. Under a whole sky, light arrives from every direction, each direction contributes a different band to the same viewing position, and the bands add back towards white. Irregular ridges make this worse by smearing each band. A petal is curved, its cells are imperfectly aligned, and it is usually lit by sky as well as sun, so its diffracted light is a small addition to a strong pigment colour. A peacock spider's display partly escapes the problem by performing in direct sunlight on a tiny, precisely built scale that the male orients himself.
| 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. |
| Beetles with grating-bearing cuticle (Several families of Coleoptera) | Outer cuticle with fine parallel ridges | Diffraction grating | Regular parallel ridges on the cuticle surface, usually over melanised cuticle. | Strong. The whole spectrum is spread out by angle, so colours sweep across the body as it turns. |
| Rainbow peacock spiders (Maratus robinsoni, M. chrysomelas) | Abdominal scales about 40 by 10 micrometres | Diffraction grating | Two-dimensional nanogratings on curved, three-dimensional scale surfaces. | Strong. Small movements sweep the reflected colour through the spectrum. |
| Flower-of-an-hour (Hibiscus trionum) | Petal cuticle with parallel striations | Diffraction grating | Cuticular ridges forming a diffraction grating, over anthocyanin-pigmented cells. | Strong. Diffractive, where it can be seen at all. |
| Organism | Colour | Proposed function | Evidence grade | What the evidence is |
|---|---|---|---|---|
| Morpho butterflies (Morpho spp.) | Brilliant metallic blue | Long-range visibility to other Morpho, suggested from the breadth of reflection. | Proposed, untested in cited work | The cited work is optical; it does not test who sees the flash or what follows. |
| Beetles with grating-bearing cuticle (Several families of Coleoptera) | A faint rainbow sheen over a dark body | Putative only; reviews list several untested suggestions. | Not established | The review treats functions of beetle iridescence as putative. |
| Rainbow peacock spiders (Maratus robinsoni, M. chrysomelas) | Patches that flash through the whole spectrum | Display to females during courtship. | Comparative or correlational | The scales are presented in the male's courtship display; the cited work does not manipulate them to measure female response. |
| Flower-of-an-hour (Hibiscus trionum) | A faint blue-to-ultraviolet sheen over dark pigment at the petal base | A cue for pollinating bees. | Experimentally tested; contested | Bumblebees learned to use iridescence as a cue on artificial targets in the laboratory; a later optical study found the signal too weak to matter under natural light. |
Why: Any angle-dependent colour gets the same label, though a grating and a multilayer behave differently and imply different structures.
Fix: Light it with one small source. A full ordered spectrum visible at once means a grating; one hue that slides as you tilt means a multilayer.
Why: The photograph used a flash or a single lamp close to the petal — a point source — which is the condition under which a grating performs best.
Fix: Photograph the same petal under open sky. If the sheen disappears, report it as visible under directional light only.
Why: What an animal can learn in an arena is taken for what it encounters.
Fix: Cite both the behavioural result and the optical challenge, and state that field relevance is unresolved.
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 rainbow scales of the peacock spiders Maratus robinsoni and M. chrysomelas carry two-dimensional nanogratings on curved microscale surfaces, giving at least twice the resolving power of a flat grating of the same period.
Source: Rainbow peacock spiders inspire miniature super-iridescent optics
FactModerate evidence
Bumblebees trained in the laboratory learned to use iridescence produced by a diffraction grating as a cue and to identify iridescent targets despite their changing appearance.
Caveat: Laboratory arenas with artificial targets; shows what bees can learn, not what flowers in the field present.
Source: Floral iridescence, produced by diffractive optics, acts as a cue for animal pollinators
FactModerate evidence
Imaging scatterometry of flower petals found that striated surfaces produce diffraction patterns when single cells are illuminated, but that the iridescent signal vanishes under illumination similar to natural conditions, leaving pigment as the determinant of floral appearance.
Caveat: The authors' conclusion that the pollinator-signal hypothesis is untenable is itself disputed.
Source: Iridescent flowers? Contribution of surface structures to optical signaling
FactStrong evidence
Beetle iridescence falls into three mechanistic groups — multilayer reflectors, three-dimensional photonic crystals and diffraction gratings — and the functions attributed to it are described in review as putative.
Colourwise interpretationModerate evidence
Floral iridescence is best treated as an open question: a function has been demonstrated under laboratory conditions and its relevance under field lighting has been credibly challenged.
Based on: Colourwise's reading of two peer-reviewed studies that reach opposite conclusions from different kinds of measurement — behavioural trials and petal optics.
Source: Floral iridescence, produced by diffractive optics, acts as a cue for animal pollinators; Iridescent flowers? Contribution of surface structures to optical signaling
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.