How do butterflies produce their colors? And why are many of them not actually colors at all?

How do butterflies create their colors?

Anyone who works with focus stacking will, sooner or later, end up using butterfly wings. Hardly any other subject combines such a fine, regularly organized microstructure with such intense—and at the same time puzzling—color effects. Yet these colors are often produced in a very different way than one might expect. This article explains how butterflies generate their blue, green, and metallic sheen.

For many years, Chrysiridia rhipheus and Papilio ulysses have been among my personal favorite butterflies. As so often, my fascination went beyond the visual impact alone. I wanted to understand the physical processes behind their colors. This article is the result of that exploration.

Under the microscope, butterfly wings reveal themselves not as decorative surfaces but as highly complex optical systems that shape light in very different ways. Anyone who takes a deeper interest in the microscopic photography of butterflies will almost inevitably arrive at these two species. Both are readily accessible under microscope objectives, both yield spectacular images—and both confront the photographer with the same question: Why do their colors look the way they do, and why do they often behave so differently in focus stacking than one would expect?

Single wing scale of Papilio ulysses, width approx. nine hundredths of a millimeter (HLB Plan Apo 50x)

The closer one looks, the clearer it becomes that these colors are not merely surface properties, but the result of precisely organized structures. This article is an attempt to get to the bottom of these colors—not aesthetically, but physically.

These butterflies display colors that are already impressive in conventional macro photography—yet under the microscope they suddenly raise questions. The colors seem unstable, change with the angle of illumination, and appear harder, flatter, or “more unreal” in a stack than to the naked eye. Sometimes color almost disappears; sometimes it seems to explode.

By this point it becomes clear: these are not ordinary surface colors.

What we see in these butterflies is, in part, not pigments in the classical sense, but optical effects produced by extremely fine structures on the nanometer scale. The wings of these animals do not function like painted surfaces, but like microscopic light components—reflectors, filters, interference systems.

Wing scales of Morpho menelaus on a micrometer; the scale width is about seven hundredths of a millimeter; the transparent cover scales are even narrower

That is precisely why they are so appealing for photomicrography—and at the same time so tricky. Focus stacking makes these structures visible, but it also changes their optical effect. What we photograph is not simply “the color,” but a result of structure, illumination, viewing angle, and image processing.

For that reason, this article is not meant to be a gallery of beautiful butterflies. It aims to explainhow these colors arise physically, why different butterflies have evolved completely different color strategies, and why these strategies respond so differently in photomicrography.

How Do Butterflies Produce Color?

When we talk about the colors of butterflies, we usually assume quite naturally that color is a property of the surface. Something appears blue because it is colored blue; green because a corresponding pigment is present. For many materials that is true—but for butterfly wings only to a limited extent.

The most striking colors of many butterflies are not based on pigments, but on the interaction of light with microscopically fine structures. To understand how these colors arise, one has to distinguish between two fundamentally different principles of color production.

Colors from dyes: pigment color

The more familiar way of producing color is pigment color. A pigment appears colored because it absorbs certain wavelengths of incoming light and reflects others. A red pigment absorbs mainly blue and green light; a yellow pigment mainly the blue portion. The perceived color results from the spectrum of the remaining reflected light.

The color diversity of certain scale arrangements in Chrysiridia rhipheus is likely nearly unparalleled in the animal kingdom in this form—here evolution pulls out all the stops (Mitutoyo M Plan Apo 50x, LED)

Pigment colors are largely independent of viewing angle. They remain stable even in diffuse light and change little with movement or changing illumination. Many dark and warm tones in butterflies—such as black, brown, red, or yellow—arise in this way and therefore appear uniform and reliable.

Colors without dye: structural color

In addition, there is a second, physically much subtler way of producing color: structural color. Here light is not “colored” by absorption—i.e., not by “removing” certain components of the radiation—but is shaped by targeted redirection and superposition.

The wing scales of many butterflies possess extremely fine, regularly or semi-regularly arranged structures on the scale of the wavelength of light. When light hits such structures, it is reflected multiple times, diffracted, and phase-shifted. Certain wavelengths then add together and are strengthened (constructive interference), while others cancel each other out (destructive interference). The resulting color is therefore not a material property, but the outcome of an interference phenomenon.

Wing scales of Papilio ulysses; alongside the blue scales with structural colors, there are also scales of other colors with pigment colors

What matters here is that color is not a fixed property of the scale. It only arises through the interplay of structure, illumination angle, and viewing direction. That is why structural colors are often especially brilliant, yet at the same time strongly angle-dependent. One and the same scale can appear intensely blue from one angle and nearly colorless from another.

The true carriers of these effects are the wing scales themselves. They are by no means simple, flat plates, but highly differentiated micro-optical components. Depending on the species, they can function as diffraction gratings, scattering structures, or optical filters. Some scales are strongly periodic and produce sharply defined interference colors; others are deliberately structured more irregularly to create broadband effects that are less direction-dependent. Often, multiple scale types occur side by side or layered on top of one another, with each layer fulfilling its own optical task.

What is crucial is that the same visible colors do not necessarily rest on the same physical mechanisms. A blue can be produced by pigment, created structurally through interference, or be the result of a combination of both. To the naked eye this difference does not matter—but for photomicrography it very much does.

Wing scales of Papilio ulysses on the tip of a medical injection cannula

It is precisely in focus stacking that this difference becomes visible. Structural colors react sensitively to the illumination angle and to the slightest changes in geometry. When stacking, images from different focal planes are combined—often under slightly varying reflection conditions as well. The resulting image not only renders the scale structure sharply, but also overlays different interference states into a new, computational result. Colors can therefore appear harder, flatter, or more unstable than in direct observation.

Pigment color is a property of the object—a comparatively stable state. Structural color, by contrast, is not a fixed feature, but an optical event that arises only in the interplay of object, illumination, and observer. This physical foundation is crucial for everything that follows. Only against this background does it become understandable why Chrysiridia rhipheus, Papilio ulysses, and Morpho pursue completely different optical strategies despite similar color effects—and why they behave so differently in front of the microscope objective.

Chrysiridia rhipheus — Green from structure and filter

Chrysiridia rhipheus, often still known by the older genus name Urania, belongs to the butterflies that can initially mislead even experienced observers. Its wings display intense green, turquoise, and blue, combined with bright red, yellow, orange, white, and black. At first glance, the green seems self-evident—as if it were simply “colored green.” Under the microscope, however, that apparent obviousness falls apart very quickly.

Warning colors of a toxic butterfly: Chrysiridia rhipheus from Madagascar

The green on the wings of Chrysiridia rhipheus does not arise from green pigments. However, this butterfly does not generate its green exclusively as a structural color produced solely by physical structures, either. In other words, it is not simply a green interference color with a clear reflectance maximum in the green spectral range. Instead, this green is based on a combination of the structural color blue and filtering by pigments.

The wing scales of Chrysiridia possess fine, lamellated nanostructures that generate a broadband interference spectrum. This spectrum lies predominantly in the blue range, but it is less tightly confined than in “pure” structural colors such as Morpho blue. If one were to view these scales in isolation, without other optical influences, their reflection would appear bluish-white and strongly iridescent, but by no means stably green.

The decisive second element is yellow pigments embedded in the same scales. These pigments absorb primarily the short-wavelength portion of the reflected light—violet and blue. What remains is a spectral residue that the human eye perceives as green. Physically speaking, this is therefore a “blue made green”: the structure generates the light, the pigment shapes its spectrum.

Only on a few millimeter-sized areas of the wing are the extremely colorful compositions found, always at the transition between the different base colors green (structural color with pigment filter) and red/orange/yellow (pigment colors)

This coupling of structural color and pigment filter has far-reaching consequences. For one thing, the extreme angle-dependence of pure structural colors—such as Morpho blue—is clearly reduced. The green of Chrysiridia rhipheusremains visible from many viewing directions and changes less dramatically with movement than a pure interference blue. For another, the color effect is preserved even in diffuse light, for example in shade or under cloudy skies—conditions under which pure structural colors often lose intensity.

Biologically, this strategy is no accident. Chrysiridia rhipheus is diurnal and conspicuous, moves openly in the light, and is among the toxic species (as a caterpillar it feeds on Omphalaea leaves and thereby takes up toxic diterpenes that remain effective later in the adult). The coloration serves as a warning signal, not a fleeting effect. A warning signal must be reliable: it must not depend on the observer happening to be at the “right” angle. The combination of structural color and pigment filter provides exactly that reliability.

Under the microscope, this strategy can be followed quite well. In reflected-light images, the scales show an iridescent play of blue and green tones, while transmitted light or altered illumination makes the pigmentary component clearly apparent. If one mentally—or experimentally—removes the pigment filter, the green disappears immediately. What remains is an unspecific blue or a bright sheen without a stable color assignment.

For photomicrography, and especially for focus stacking, this construction has a practical advantage: Chrysiridia rhipheus responds comparatively robustly to changing illumination conditions. The colors remain relatively stable even in a stack, because the pigmentary filtering damps extreme interference peaks. At the same time, the structural details of the scales remain visible and give the images their characteristic depth.

The metallic effect of the wing scales is not caused by interference, but has a different origin (see below)

Chrysiridia rhipheus thus exemplifies a hybrid color strategy: neither pure pigment color nor pure structural color, but a finely tuned optical system that combines physical brilliance with biological reliability.

In this butterfly, however, only the green is a hybrid structure/pigment color. The blue areas of Chrysiridia rhipheusshow essentially the same structural color source as the green ones, only without the yellow pigment filter.

Red, orange, and yellow in this butterfly are true pigment colors, whose effect may at most be enhanced by underlying structures, but not produced by them. The metallic sheen of the scales, however, does not arise from the color itself, but from the highly ordered surface architecture of the scales (see further below).

Papilio ulysses — Color as information

Compared with Chrysiridia rhipheus, Papilio ulysses looks almost classic at first glance. Large, clearly delineated blue wing fields, framed by deep black, without the layered play of green, red, yellow, orange, and white. Yet this seemingly simple impression is deceptive. The coloration of Papilio ulysses follows a fundamentally different logic than that of Chrysiridia—physically and biologically.

Papilio ulysses produces blue color fields framed by black areas

The blue of Papilio ulysses is a structural color, but not one designed for maximum brilliance or long-range impact. The underlying scales possess lamellated microstructures that allow light to interfere, but they are built far less strictly periodically than in Morpho butterflies. The resulting interference spectrum is broader, less sharply defined, and more strongly angle-dependent.

Accordingly, the blue changes visibly with the butterfly’s movement—it “lives” with perspective. This angle dependence is not a drawback here, but part of the function. Papilio ulysses does not use its color as a warning signal or as a far-visible long-range signal, but as an information signal. The blue coloration plays a role in species and sex recognition, in territorial behavior, and in visual communication at short to medium distance. In such contexts, color may be variable—indeed, it may even stand out precisely because it changes.

Unlike Chrysiridia rhipheus, Papilio ulysses does not employ a pigment filter to stabilize its structural color. Pigments are present, especially in the black wing areas, but the blue fields themselves are produced predominantly by structural color. The dark surroundings of these fields do, however, provide optical support: they absorb stray light and increase the contrast of the interference color, without substantially reducing its angle dependence.

Papilio ulysses does not use pigment filters to stabilize its structural color

Pigment-colored scales occur in many places between the blue structural colors to produce certain effects. Each scale primarily serves one function—reflection, absorption, or structuring.

Microscopically, Papilio ulysses shows a different organizational form of scales. Different scale types usually lie side by side, not layered on top of one another. Each scale primarily serves one task—reflection, absorption, or structuring—without the complex layer coupling found in Morpho butterflies. The optical effect arises from the interplay of these modules, not from a hierarchically stacked architecture.

This modular construction has consequences for photographic depiction. Under the microscope, the blue areas of Papilio ulysses react more sensitively to illumination angle than the colors of Chrysiridia. Even small changes in lighting can noticeably darken the blue or shift it toward violet. In focus stacking, this effect is often amplified: different interference states from different focal planes are combined, which can produce a slightly restless or inhomogeneous color impression.

At the same time, the structural details of the scales remain clearly visible. The color does not “break” as abruptly as in extremely directional structural colors (e.g., Morpho), but shows gradual transitions. For photomicrography, that means: Papilio ulysses is less forgiving than Chrysiridia with respect to

The regularly arranged concave micro-depressions of the scale cause direction-dependent optical effects through light interference (HLB Plan Apo 50x, LED)

Papilio ulysses thus represents a color strategy in which changeability is desired. Its color is not meant to warn stably (Chrysiridia) or flash at long range (Morpho), but to communicate—situation-dependent, angle-dependent, and context-sensitive. Physically, this is a deliberately less extreme solution; biologically, it is exactly the right one.

Morpho menelaus — Color as an extreme case

In the context of photomicrography, Morpho menelaus and other butterflies of the genus occupy a special role. While Chrysiridia rhipheus and Papilio ulysses are often deliberately chosen for focus-stacking images, Morpho is less suitable for that purpose. Its colors react too sensitively to angle, illumination, and perspective to be reliably reproducible in a stack. In addition, these scales are significantly smaller. Precisely for that reason, it is worth looking at Morpho: not as a practical subject, but as a functional extreme case against which the other color strategies can be especially well interpreted.

The blue structural coloration of Morpho menelaus is strongly direction-dependent; even slight changes in viewing angle lead to clearly different color impressions, as the comparison of the two images shows.

The intense blue of Morpho butterflies is often regarded as the epitome of structural color. In fact, this is one of the most radical implementations of this principle in the animal kingdom. Pigments play virtually no role; the color effect arises almost entirely from highly periodic nanostructures in the wing scales.

The ground scales possess strongly developed, regularly arranged lamellae that act like precisely tuned interference gratings. They produce a relatively narrowband reflectance maximum in the blue spectral region. When light strikes these structures at the appropriate angle, blue is reflected with extraordinary intensity. Outside that angular range, however, the color effect collapses rapidly—the blue almost completely disappears.

A decisive difference from Papilio and Chrysiridia lies in the double scaling of Morpho wings. Above the color-producing ground scales lies a second layer of largely transparent cover scales. These cover scales do not themselves generate color, but they influence the light distribution: they slightly scatter the reflected light, damp extreme glare peaks, and at the same time mechanically protect the delicate nanostructure of the underlying scales. Without this cover layer, Morpho blue would be even more directional, but also mechanically and optically more unstable.

Morpho menelaus has an additional layer of transparent scales above its ground scales that slightly scatters the light

Biologically, this system serves a different purpose than in the previously discussed species. Morpho butterflies live in shaded rainforest and fly comparatively fast. Their color does not primarily serve communication at short distance (Papilio) or continuous warning (Chrysiridia), but long-range impact through movement. The characteristic flashing of blue during wingbeats acts like an optical blink signal that can be perceived over great distances—and then disappears again in the next moment. This extreme direction-dependence is not a drawback here, but an integral part of the function.

For photomicrography, a clear consequence follows. Morpho blue is not a stable object, but a strongly direction-dependent event. In focus stacking, different illumination and reflection states are inevitably combined computationally. The result is often an image that is physically sharply resolved, but in terms of color shows something that never exists in nature: either overemphasized, blotchy, or unnaturally homogeneous. What works as a dynamic effect in flight cannot be translated without loss into a static, computationally assembled image.

It is precisely in comparison that it becomes clear why Morpho serves here as a contrast: Chrysiridia rhipheus stabilizes structural color through pigment filtering; Papilio ulysses accepts and uses angle dependence for information signals. Morpho, by contrast, pushes structural color to its physical limit and deliberately accepts its instability. All three strategies rest on the same foundations, but pursue completely different goals.

In this sense, Morpho is less a practical focus-stacking subject than a reference model: it shows what happens when color is thought of almost exclusively as a physical phenomenon—and why the other two genera make more complex, photographically more accommodating compromises. In focus stacking, the blue is best captured with good light diffusion.

Three strategies — one physical problem

At first glance, Chrysiridia rhipheus, Papilio ulysses, and Morpho seem hardly comparable. Their colors look different, their habitats differ, and their photographic behavior could hardly be more opposite. Yet viewed from a physical perspective, it becomes clear: all three solve the same basic problem—they make light visible. The difference lies not in the goal, but in the chosen strategy.

What they share is that their striking colors are not primarily made of dyes, but of micro- and nanostructural interventions in the path of light. None of the three genera “paints” its wings in the conventional sense. But how strongly structural color dominates, how it is controlled, and what it is used for biologically differs fundamentally.

Left: Cross section of a wing scale of Morpho menelaus. The rib-like structures with lateral cross lamellae form a Christmas-tree-like micro-architecture that produces a strongly direction- and wavelength-dependent blue structural coloration (AI simulation of an electron micrograph). Right: Schematic representation of the Christmas-tree-like rib structure. Variations in the length and spacing of the cross lamellae influence the spectral and angle-dependent expression of the structural color.

Morpho stands at one end of this spectrum. Here structural color is used almost without compromise. Highly periodic lamellae produce a narrowband, extremely intense blue whose visibility depends strongly on the angle of incidence. This color is not a lasting state, but a short-lived optical event that flashes during the wingbeat and disappears again in the next moment. Biologically, that is exactly what is desired: visibility over long distance through movement, not stability. Photographically, this strategy is problematic because it resists static depiction. Morpho serves here as a reference case—as a demonstration of what happens when structural color is pushed to its physical limit.

At the other end stands Chrysiridia rhipheus. Here, too, structural color plays a central role, but it is deliberately tamed. The interference structures primarily generate blue light, which is then spectrally filtered by pigments. The result is a green that does not exist as a sharp interference maximum, but as a stable color impression. The structure provides brilliance; the pigment provides reliability. This strategy reduces extreme angle dependence and makes the color visible under widely varying lighting conditions. Biologically, it meets the requirements of a warning signal: conspicuous, persistent, and unambiguous. Photographically, this stabilization proves advantageous—the colors of Chrysiridiaremain relatively consistent even in focus stacking.

Papilio ulysses occupies an intermediate position, but pursues a different goal. Its color is meant neither to flash extremely nor to warn continuously, but to convey information. The structural colors are distinctly angle-dependent, but less strict than in Morpho. Pigments play a supporting, not regulating, role. The color may change; it may react; it may vary in the interplay of movement and viewing angle. This variability is functional: it supports species and sex recognition as well as territorial signals at short to medium distance. For photomicrography, that means greater sensitivity to lighting and perspective, but also a wider range of depictable states.

Comparing the three strategies makes it clear that none of them is “better” or “more advanced” than the others. Each is a precise adaptation to ecological and communicative requirements. Morpho maximizes physical effect and accepts instability. Chrysiridia sacrifices some structural purity in favor of biological reliability. Papilio deliberately uses the changeability of structural color as an information carrier.

For photomicrography, an important insight follows: what we see and photograph is not simply color, but a snapshot of an optical system. Depending on whether this system is designed for event-like flashing, stabilization, or variability, the colors respond differently to illumination, focal plane, and image processing. The direct comparison of these three genera makes it clear that color is not decorative add-on, but a functional decision—physically precise and biologically grounded.

What Makes Chrysiridia rhipheus Scales Look So Metallic?

Anyone who looks at Chrysiridia rhipheus tends to describe the impression in almost the same way: the wings look like polished metal or chrome. This impression arises regardless of whether the scales appear green, blue, yellow, or red. That is exactly the key to understanding, because the metallic sheen has nothing to do with the color itself.

The metallic impression is not a color effect, but a reflection effect. It does not arise because particular wavelengths are reflected, but because of how the light leaves the surface.The visible basis of this effect is the fine, parallel ridging on the surface of the scales. It already gives the scale a strong directional order. Light that strikes such a surface is not scattered randomly, but is preferentially reflected along defined angles. This reduces the diffuse component of the reflected light—the surface looks smooth and glossy.

The wing scales of Chrysiridia rhipheus show clearly visible longitudinal ridging

But the true origin of the metallic character lies deeper. Beneath the visible ridges, the scale consists of a regularly stacked lamellar structure of chitin and air. These lamellae form a series of plane-parallel interfaces at which light is reflected multiple times. What matters is their high degree of order: the light components reflected at the lamellae do not combine randomly, but in an ordered way. Certain spatial directions are enhanced, others suppressed.

The result is reflection with a high directional component and minimal diffuse scattering. Exactly this combination is interpreted by our visual system as “metallic.” Polished metals do not look shiny because they are colored, but because they preferentially reflect light in a directional manner. Chrysiridia rhipheus achieves the same impression—not with free electrons like a metal, but with a precisely ordered architecture.

The color of the scale is secondary here. In the green and blue areas it arises through structural interference; in the red, orange, and yellow areas through pigments. Yet in all cases the same ordered surface architecture underlies the effect. That is why all color regions share the same metallic sheen, even though their color production is physically very different.

In pure structural color (blue), in the combination of structural color and pigment filter (green), and in pigment-based colors (red, orange, yellow), the metallic impression is enhanced by the surface architecture of the wing scales.

A revealing point is the sensitivity of this effect. If the lamellar structure is damaged or its order is disturbed, the metallic impression disappears immediately. The color may remain, but the sheen does not. Conversely, even pigment-colored scales can appear metallic as long as the surface order is intact.

The metallic sheen of Chrysiridia rhipheus is therefore not a decorative side effect, but the visible result of highly precise light steering on the microscopic level. The scales look metallic not because they color light, but because they direct it. Color determines which light is reflected—the structure determines how it is reflected. And that increases the detectability of the colors and thus improves the warning system, making it more efficient.

Consequences for Photomicrography and Focus Stacking

Anyone who photographs butterfly scales microscopically is not simply documenting a surface. They are intervening in an optical system. That is the decisive point at which structural color and pigment color differ in practice—and where many misunderstandings arise.

Pigment colors behave in the microscope image largely as one would expect. They are location-bound, stable, and indifferent to small changes in illumination geometry. A yellow or red scale remains yellow or red, regardless of whether it is reconstructed from one focal plane or from multiple focal planes. Focus stacking changes chiefly sharpness here, not color.

Structural colors, by contrast, respond sensitively to any change in the geometric boundary conditions. They are not tied to a place, but to a configuration of structure, light, and viewing direction. Exactly that configuration is inevitably varied during stacking. Even minimal differences in illumination angle between individual frames are enough to produce different interference states that are then computationally merged.

Focus-stacked image of butterfly wing scales from around 200 individual frames to greatly increase depth of field.

The result is an image that is physically correctly sharp, but that—optically—shows something that never exists in nature in quite that form. This is especially obvious with strongly directional structural colors such as Morpho blue. Individual frames often show only partial areas of intense color, depending on the angle. The stack merges these states into a uniform, seemingly stable color—an artifact in the strict sense, even if it may look aesthetically convincing.

In Chrysiridia rhipheus, this effect is much weaker. The pigmentary filtering acts like an optical damper. It limits the extremes of interference and ensures that different individual states merge into a consistent color impression in the stack. That explains why Chrysiridia often appears more “forgiving” in focus stacking than other strongly structurally colored butterflies: the color has already been biologically stabilized by pigment filtering before it is processed photographically.

Papilio ulysses occupies an intermediate position. The structural color is variable, but not extremely directional. Accordingly, a certain inhomogeneity often appears in the stack: color fields look lively, sometimes restless, occasionally blotchy. This is not a sign of poor technique, but an expression of the fact that different optical states are being made visible side by side by means of the individual frames. Anyone who tries to smooth out this variability completely risks losing exactly what makes the color functional.

Wing scales of Chrysiridia rhipheus on the tip of a medical injection cannula

Another point that is easily underestimated at the microscopic scale is the role of illumination. Structural colors respond not only to the angle between light source and objective, but also to the spatial extent of the light source. Point-like, directional light strengthens interference effects and thus angle dependence. Diffuse light reduces them, but can also cost brilliance. Here, too, the rule is: there is no single “correct” illumination—only illumination that fits the particular color strategy.

It becomes especially critical when one tries to interpret structural color as an objective property. A microscopic image does not show “the color of the scale,” but one possible realization of this optical system under precisely defined conditions. The image is not a neutral finding, but a record of an event. This insight is uncomfortable, but necessary—especially when microscopic detail images are meant to be understood as documentary.

In practice, this does not mean giving up focus stacking or aesthetic optimization. It does mean, however, correctly classifying one’s own images. A stacked image of Morpho scales is not a depiction of natural appearance, but a synthesis. An image of Chrysiridia shows a biologically stabilized color, not a pure structural color. An image of Papilio documents variability, not error.

This is precisely where the special value of these objects lies for photomicrography with focus stacking. They force one not to treat color as a self-evident attribute, but as the outcome of a physical process. Anyone who knows that gains not only better images, but also a deeper understanding of what they are actually seeing and reproducing photographically.

Daniel Knop, www.knop.de

Literature sources:

Siddique, R. H., S. Diewald, J. Leuthold & H. Hölscher (2013): Theoretical and experimental analysis of the structural pattern responsible for the iridescence of Morpho butterflies. – PubMed, DOI:10.1364/OE.21.014351.

Vukusic, P., R. Sambles, C. Lawrence & G. Wakely (2001): Sculpted-multilayer optical effects in two species of Papilio butterfly. – Applied Optics, https://doi.org/10.1364/AO.40.001116

Yoshioka, S., T. Nakano, Y. Nozue & S. Kinoshita (2007): Coloration using higher order optical interference in the wing pattern of the Madagascan sunset moth. – PubMed Central, doi: 10.1098/rsif.2007.1268