Natural photonics

Iridescence in Lepidoptera

The wings of butterflies and moths are adorned with a wealth of pattern and colour unrivalled in the living world. Some have uniformly-coloured, unpatterned wings of white, some have deep iridescent hues of blues and greens, while others single shades of yellow, orange, red, green, blue, violet, brown or black.

The complex patterning is genetically determined and developmentally controlled. The colour and pattern on a butterfly's front and rear wing-surfaces may be the same or different. The colour of the two sexes may be identical, slightly different, or in some cases so distinctive that they were originally described in different genera. Wing colour may be seasonal, geographical or locally variable, or polymorphic. Furthermore, closely related species may appear extremely different, while unrelated species can display nearly identical colour patterns.

The phenomenon of iridescence

The occurrence and distribution of iridescent colours and the various theories regarding their manifestation in the natural world were for a long time discussed by scientists. Even Sir Isaac Newton in his book Opticks, published in1704, put forward a reason for the iridescent nature of the colour from the feathers of peacock tails. The word iridescence is itself defined as the change in hue of the colour of an object as the observer changes viewing position.

An example of this iridescent colour is the appearance of light reflected from an oil film on the surface of water. If the oil film is viewed from different angles the colours appear to shift and change. Iridescent colours are particularly striking for the observer. They can be among the purest and most brilliant and cannot be matched by even the brightest pigmental colours in their depth and intensity. In addition, the glittering change of hue that accompanies any change of light angle or observer position lends these colours a magic and beauty that is unmatched by any others.

Butterflies and Moths

Lepidoptera, the generic name for butterflies and moths, are one type of insect which can exhibit brilliant iridescence on its wings. They are an extensive family of nearly 150,000 insects that are found in most countries in the world. The word Lepidoptera is derived from the Greek and means "scaly winged". These scales are detached easily when you touch one of their wings, and are so small that they feel like a fine powder on your fingers.

Butterflies differ from moths structurally in that their antennae are clubbed at the end and they lack a frenulum, a spine-like device which connects the front and hind wings of most moths. The general understanding of the taxonomic difference between them, is that butterflies have slender bodies, are brightly coloured and fly in the daytime, whereas moths have stout bodies, are dull coloured and usually fly at dusk or at night.

The complexity of Lepidopteran wings

A framework of tubular struts called veins supports the wings of these insects. In addition to providing crucial aerodynamic mechanical support for the wings as the butterfly or moth flutters in the air, the veins are also used by entomologists as a valuable aid in studying the relationship between various families and species.

The surface of the wings of these insects is composed of thousands of very small scales and it is from these that the wing colours and patterns originate. Examination of these individual scales yields the information as to the mechanisms by which the wing colour is produced.

Although individual scale shape, size and colour do vary from species to species, there is commonality across Lepidoptera as to the function of scales. Their presence acts as an aid to the aerodynamics of the wing, they are involved in the process of temperature control of the butterfly, and of course they are the centres from which wing colour and patterns are generated. This colour and patterning on the wing acts as a means of defence against predators and is also used for inter- and intra-sexual signalling.

Colour production on the wing

There are two fundamental mechanisms by which colour is produced on butterfly wings. One leads to what we would call ordinary colour, and the second leads to spectacular iridescent colour. The ordinary colour is entirely due to the presence of chemical pigments, which absorb certain wavelengths and transmit or reflect others. Different pigments are coloured differently. For example; plant leaves are green due to the presence of chlorophyll pigment; carrots are coloured orange due to the presence of the pigment that is called carotene; and the different shades of brown in human skin and hair are due to the relative concentration in the skin and hair follicles of the pigment called melanin. Red-haired people are thought to have an additional iron-based pigment present in their hair.

 Iridescent colour is produced not by pigmentation but by the interference of light due to multiple reflection within the physical structure of a material. For this reason it is sometimes referred to as structural colour.

Structural colour and pigmental colour can sometimes occur on the same object and leads to colour addition in the normal way. For instance, if a blue iridescent colour was produced from the structure of a surface that also contained yellow pigment, the resulting colour would be green (since blue+yellow = green).

A theory for iridescence; interference in a thin film

The credit for formulating the principle of iridescence goes to Robert Boyle who was a contemporary of Isaac Newton in the seventeenth century. More than a hundred years later, interference was explained in detail by the English physicist Thomas Young.

According to his definition, iridescent colour such as that produced from oil on water or from the skin of a soap bubble works in the following way. Very thin plates or films reflect some of the incident light waves from their mirror-like top surface. The unreflected light enters the film and travels on until it meets the lower surface where again some of it is reflected.

This reflected light wave from the bottom surface travels in the same direction as that reflected from the top surface and it eventually rejoins it. However, due to its journey within the oil film and to the reflection by the bottom surface, it may be out of phase with respect to the light wave reflected from the upper surface. The extent to which it is out of phase depends on the thickness and refractive index of the film, the angle at which the light strikes the film surface and also the wavelength of the incident light.

If the phase difference between the two waves is a multiple of exactly one full wavelength then the two waves will constructively interfere with each other and there will be a strong reflection of light at that wavelength.

If, however, the phase of the reflected waves differs by half a wavelength, or an odd multiple of half wavelengths, then the reflected waves are said to be completely out of phase and destructive interference will occur at that wavelength. This is manifested by a weak or absent reflection of light at that wavelength.

If the full spectrum of colours in the form of white light is incident, then for a given film thickness and refractive index only one colour is of the correct wavelength to satisfy the conditions for constructive interference. In other words, when white light is directed at the thin film, only one colour will be strongly reflected at a particular angle.

If on the other hand it is monochromatic light that is incident then the interference pattern will consist of a series of bright and dark bands of light.

Constructive and destructive interference will be strongest, and the reflected colour purest, if the waves reflected from each surface have the same amplitude. This in turn relies on the refractive indices of, in this case, the air and the film and on the angle of incidence of the light onto the surface.

Multilayer interference

It turns out that if instead of only one thin film of reflecting material there existed an ordered series of parallel thin films, then even stronger constructive interference could occur for the correct thickness, refractive index and incident angle conditions. Subsequently even purer and more intense colours would be reflected.

Such multiple thin layers would seem to exist only very rarely on first consideration. Can you think of an example from nature in which several thin solid layers of equal refractive index are spaced by an equal number of thin layers of a different refractive index?

The answer to this can be found in the skin, feathers or scales of almost every animal, bird or insect that displays iridescence.

The structure of iridescent scales

The scales of butterflies and moths which are iridescent show, with only few exceptions, spectacular multilayer structures. The diversity and complexity of these tiny structures places Lepidopteran scales among the most complicated extra-cellular structures that are manufactured by a single cell.

Two excellent examples of iridescence of butterflies and moths are the wing scales of most of the Morpho family of butterflies and the Urania family of moths. These were among the first to be examined because of their brilliant iridescence. Unfortunately we cannot see them alive in this country, as they are only limited to tropical regions of the world.

The wings of all adult butterflies and moths are covered with a dense array of partially overlapping scales. Each scale is a long and flattened extension of a material called cuticle that originates from a single epidermal cell. The scales are arranged in highly ordered rows in the same fashion as slate tiles on a roof. In most cases there are two distinct layers of these scales, and in some cases even three layers. The scales in the bottom layer are in every species different in structure and function to those in the second layer.

Close examination of the cross-section of a single Urania moth wing scale shows clear evidence of a thin film multilayer structure. The scale is comprised of four or five layers of cuticle each around 40nm thick and each separated by a hollow region filled with air around 100nm thick. These thin layers are held apart by tiny vertical rods of cuticle. By altering the spacing and thickness of the cuticle layers and also incorporating different coloured pigments within the scale the varied series of colours seen on the wings of the Urania moths may be obtained.

The structure that causes the iridescence on the scales of the Morpho family of butterflies is even more impressive. Rather than having thin multilayer films built within the body of their scales, Morpho butterfly scales have Christmas tree-like structures on their surfaces. These tiny structures form miniature multilayer films that support the interference that comes from multiple reflections of light within them. It is clearly possible to see up to eight or nine of these mini-thin films within each tree in transmission electron microscope images of the cross sections of these scales.

The intricacy of these tiny structures is difficult to comprehend. They are so minute that even the most powerful optical microscope would not be able to image them. This is because the size of the smallest object you can see is limited by the wavelength of the radiation used (an object smaller than about l/2 cannot be imaged).

The dimensions of the thin layers on these Morpho trees are around ten times smaller than the wavelength of light. It is only with electron microscopes (since the electron wavelength is of the order of 0.1nm) that an image of the complex scale structure can be obtained.

Most of the members of the Morpho butterfly family show stunning bright blue iridescence. As is predicted by thin film reflection theory, the colour of the iridescence moves towards the violet end of the spectrum as the viewing angle increases. In fact when the wing is viewed at close to grazing angle the interference generated reflection wavelength becomes ultra-violet and only the dull brown natural colour of the melanin cuticle is visible.

Altering the interference conditions

A very interesting iridescent colour change is observed on dropping some low index solvent such as acetone onto the wing surface. The act of doing this changes the interference conditions within the multilayer system since the acetone seeps into all the spaces in the structure and replaces the layers of air with layers of acetone. This change in refractive index from 1.0 of air to 1.38 of acetone alters the phase change undergone by light as it travels through it, which ultimately changes the peak wavelength for which strong interference can occur. A drop of acetone on an iridescent blue Morpho wing immediately changes the iridescence to a bright green colour. Acetone, being volatile, soon evaporates away and the green iridescence returns to the original deep blue as the solvent is replaced by air and the former interference conditions established again.

If, in place of acetone, a solvent is used with a refractive index that matches exactly that of the cuticle (which is about 1.56) then the incident light will not meet a series of thin films but it will instead meet a homogeneous thick film. As a result of this constructive interference can no longer occur and iridescence is not displayed, leaving the wing colour as the dull brown colour of melanin.

Iridescence in British butterflies and moths

Although the colours of native British Lepidoptera are largely pigmental in origin, there are a few species that do display iridescence. Some of the Lycenid family of butterflies that are generally found on the South Coast, for example the Adonis Blue and the Long-Tailed Blue are noticeably iridescent on almost all of the top surface of their wings. The more common and familiar Peacock butterfly, from the Nymphalid family, shows iridescent patches in each of the four eye-spots on its wings. The Common Burnished Brass moth displays two wonderfully iridescent gold patches on the top surface of its wings but is much harder to spot due to its nocturnality.

 Although iridescence has been described here using only certain butterflies and moths as examples, there are a wealth of other insects and animals on which it is beautifully displayed. From the scales of fish, to the feathers of birds; from the scales of beetles to the skin of snakes, structural colour is around us in the living world wherever we turn.

This text was published in " Physics Review", Sept 1998, by Philip Allan Publishers

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