The bright colors of some insects are often the ones that capture our attention. Black can be dull and overlooked. However, black can be a sign of important biological processes. The color we see as black is caused by the absence of reflected photons. Black can indicate an area that is used for converting photons from the sun into heat. The most efficient photon collection areas will absorb the highest fraction of photons and reflect the least photons and the areas will appear black.
Insects that must thermoregulate can have basking behaviors that position their body to get the maximum heat from the sun. Basking replaces energy from stored food with solar energy. Butterfly species with large black areas on their wings may face the sun in the cool morning air to warm their bodies. In the afternoon when temperatures are warmer, the butterfly can adjust its wing position to minimize the heat gain.
The swallowtail butterfly, Papilio nireus, has a beautiful green fluorescent wing band. The wing fluoresces at a wavelength that is in tune with the principle light receptor pigment in the compound eyes of adult butterflies. Producing an intense fluorescence is key to successful mate attraction.
The green color is produced by a pigment that is maximally stimulated by light at 420 nm and fluoresces at 505 nm. The pigment is arranged in a photonic crystal slab below the outer cuticle of the scale. The arrangement of the slab is such that light emitted is directional, both above and below the plane of the scale surface.
Below the photonic crystal slab is a reflector (Distributed Bragg Reflector or DBR) produced from several layers of cuticle with an appropriate gap in between. This reflector operates on similar principles to the DBRs used in fiber optics. The swallowtail reflector preferentially reflects light at 505 nm and allows other wavelengths to pass or be absorbed. This arrangement is such that little light is lost giving a bright signal. The fluorescent pigment is oriented to emit in only two directions, up and down. Horizontal emission is suppressed. The light emitted in the down direction is reflected upward and joins the light reflected upward to efficiently emit as much of the available light energy as possible.
Directionally Controlled Fluorescence Emission in Butterflies Pete Vukusic and Ian Hooper SCIENCE VOL 310 p. 1150.
Butterflies typically produce black wings using the pigment melanin. The melanin is incorporated into the cuticle secreted to form the scale. Scales are not solid structures; they have nano-structure. Scales have airspaces, and internal features such as ridges and spikes at the nano-level. When a photon leaves a cuticular area and enters an airspace, it does so at an angle, rather than perpendicular. This makes the photon path more tortuous increasing its chances of absorption by contacting a pigment molecule.
This can be demonstrated by comparing the absorbance of a black scale in air and a black scale dipped in a solvent that fills the airspace and has a similar refractive index to the cuticle. The scale in air is more absorbent than the scale in solvent because the solvent disrupts the internal structure. These types of structures can have useful engineering applications.
P.Vukusic, J.R.Sambles & C.R.Lawrence. Structurally assisted blackness in butterfly scales. Proc. R. Soc. Lond. B (Suppl.) 271, S237–S239 (2004)
Artist: Lisa Palombo
The wings of butterflies, like the wings of flies and bees are clear. The chitin/protein composite secreted to make the wing membrane is transparent. The color of butterfly wings is due entirely to the presence of scales. Rub the scales off a butterfly wing and only the veins and clear membrane will be left.
Scales are produced by cells in the wing. Each hair cell produces only a single scale. Each scale is a single color. Butterflies produce two types of scales: Ground scales and cover scales. The cover scales are transparent. They cover the ground scales and allow light to pass. Ground scales are colored. The color may be due to a pigment or to reflection due to the way physical structure of the the scale interacts with the spectrum of light at different wavelengths.
When pigments are used to produce colors, each cell produces a single pigment, and that pigment is incorporated into the scale. The three basic pigment colors are black, yellow and red. The color patters on the butterfly wing reflect the distribution and location of the pigment producing cells. Why do we see more than three colors on some butterflies? Some of the colors may be structural and greens and blues are often produced by reflectance. However, scales are so tiny that producing different colored scales close together will be perceived as a blend of the two or more colors. Subtle differences in shades can be created by altering the density of a pigment type within and area. Each pattern that we on a butterfly wing is not solid color, but the product of tiny dots of color, too small to be seen individually, but as a whole that give the impression of solid colors and patterns similar to those seen in impressionist art.
Scales on Cecropia Moth Wing
The wings of butterflies and moths are covered with scales. Scales are modified hairs. A hair is formed by a hair cell that secretes a linear strand of cuticle. Scales form when secreted cuticle instead of growing as a linear hair, forms a flat plate.
Scales are tiny but visible under a light microscope. A typical scale would measure 100 microns long by 50 microns across. A square millimeter of butterfly wing typically contains several hundred scales.
A single cell in the butterfly wing produces a single scale. When the wings are fully formed, the cells die. If a scale is lost, there is no cell to replace it.
Handling butterflies by the wings will dislodge some of the scales and leave a powder of scales on your fingers. Scales are useful to butterflies and moths that fly into spider webs. The web will stick to the scales but not the wing itself. This allows moths to escape the web leaving a few scales stuck to the web and little worse for the wear.
Comma butterflies. “I am a dead leaf.
Butterflies use their wings as colorful billboards to communicate to other animals.
Aposematic colors may say, “I am toxic.”.
Cryptic colors may say, “I am not a butterfly, I am a dead leaf.”
A mimic may use color to say, “I am not a Viceroy, I am a Monarch”.
Colors are used by butterflies to identify potential mates. Coloration in butterfly species usually remains consistent over time and location, with some exceptions.
Consistency in coloration leads to consistency in message. Once a color pattern is selected for its ability to promote survival and reproductive fitness, the pattern and the message it communicates becomes fixed in the population. Color patterns in butterflies are controlled by genes that fix the color pattern. If genetic changes occur that cause the color pattern to stray too far from the most adaptive, those individuals do not reproduce and the genes are not passed to the next generation. The next generation will continue to have only the genes that produce a consistent pattern. In this way, color patterns, like many other butterfly traits become fixed.
Venezuelan artist, Rafael Araujo, has an art series called Calculation. Araujo uses mathematical calculations for geometric patterns to determine the coordinates for his art objects. Art objects are drawn on a background with lines connecting the coordinates. His art subjects are derived from nature and in insects such as butterflies. Calculation reflects the geometry underlying many behaviors and forms found in nature.
Visit the Rafael Araujo website to view more insect inspired art.