Why are butterflies colored? (interference)

The morpho butterflies are among the largest in the world, with a wingspan of 7.5 to 20 cm. The males have the beautiful metallic blue upper wings. The females are drab. Their iridescence due to the microscopic strucure of their wings.


	Blue Morpho butterfly (Morpho menelaus) This brilliant blue butterfly can be found in the rainforests of South America (Brazil & Guyana).		Underside of wings. When the Morpho lands, it closes its wings tightly showing only their brown, camouflaged undersides so the bird that is looking for a blue butterfly can't see it.

Iridescent surfaces, such as butterfly wings, help animals to elude potential predators. When these insects fly, the upper surface of their wings continually changes from bright blue to dull brown because the angle of the light striking the wing changes. As the butterflies move their wings up and down during flight, they seem to disappear and they reappear a short distance away, looking like ethereal flashes of bright blue light. The dark undersides of their wings strengthen this effect. Combined with an undulating pattern of flight, this ability to quickly change color makes them difficult items for predators to pursue.

Why are their wing tops blue?

The blue color comes from tiny iridescent scales that cover the tops of the butterfly's wings. Pilots flying over the rainforest can occasionally see brief flashes of blue as these butterflies travel above the forest canopy.

The wings of butterflies and moths consist of a colorless translucent membrane covered by a layer of scales (the name of the order is Lepidoptera, meaning 'scaly wings'). Each scale is a flattened outgrowth of a single cell and is about 100 µm long and 50 µm wide. The scales overlap like roof tiles and completely cover the membrane, appearing as dust to the naked eye.

In butterflies, iridescence is caused by multiple slit interference. Sunlight contains a full range of light wavelengths. "Interference" occurs when light hitting the wing combines with light reflected off the wing. Light is a wave. If the crests and the troughs of the waves are aligned, or "in phase", they will cause constructive interference and iridescence. This happens when the one light wave hits the first groove, and a second light wave travels half of a wavelength to an other groove and is reflected back in phase with the first. If the crest of one wave meets the trough of an other wave ("out of phase"), they will cancel each other out, demising the overall light intensity.

Blue light has a wavelength range from 400-480 nm, and is the only wavelength that is interfered with constructively by the slits of the morpho, which are 200 nm apart. The slits are attached to a base of melanin, a material that absorbs other light, further strengthening the blue image.

The scales on the underside do not cause interference. It resembles foliage, with lackluster browns, greys, blacks and reds. Its coloration does not change with viewing angle as it is due to normal organic pigments, rather than a physical structure. The iridescence is more useful on the top side of the butterfly, where it can be used to elude their main predators; birds. These scales are also for protecting the wing from physical contact.

Detailed views of scales from the wing tops, and wing undersides. Notice the veins in the magnified views.

Top of wing (blue):

Underside of wing (brown):

Each scale is about 70x200 µm, and is covered with thin parallel veins. The veins are much like ridges on a vinyl record. There are about 1,800 veins per millimeter. The texture forms a reflection grating. The veins are made of stacked chitine-lamellas kept equidistant by fine braces. On the underside, the slits on the scales are 160 nm apart, which is not a multiple of half the wavelength of visible, and thus do not cause interference.