We study a variety of colors produced in the biological world focusing on a variety of interesting objects that include butterfly wings, bird feathers (humming birds, peacock feathers, duck feathers), all of whose colors are due to interference, the Hercules beetle (use of color for camouflage), moths which produce color both by interference and diffraction, interference filters in the compound eyes of butterflies, and biological analogues of cholesteric liquid crystals (optically active scarab beetle cuticles). The case of cholesteric liquid crystals will serve to illustrate the elegance with which metallic colors are produced by the scarab beetles. A common unifying theme, in considering this diverse set of materials, is the nature of color generation. In all of these materials, color is generated in the absence of chromophores, primarily by structural variations resulting in interference, diffraction, or scattering.

Interference
(Soap bubble, Humming bird)
Selective reflection

 

 
The finely colored feathers of some birds, and particularly of Peacock tails appear to possess different color by changing the position of eyes. This is remarkably similar to colors in light seen from thin films or plates and thus indeed result from slenderness of the very fine hairs or Capillamenta which grow out of the grosser lateral branches or fibers of those feathers.

 
When one looks at the wing, it becomes evident that on a given patch of wing there are typically two and sometimes three types of scales, which alternate positions on a roof-tiles-like arrangement. The larger "cover" scales and the smaller "ground" scales are arranged in an alternating fashion. In most cases, the cover scales tend to be architecturally more elaborate, although oftentimes the ground scales may show a similar architecture. The form of both cover and ground scales can change from a given patch of the wing to the next. The density of the scales varies from about 200 to 500 scales per square millimeter. The scales making up the color pattern that is seen on the wings are quite delicate. Anyone who has handled a butterfly by its wings would have noticed that the scales rub off easily along with the color pattern.

At high magnification it becomes evident that the color pattern is a result of a finely tiled mosaic, with each tile of the pattern being made up of a single wing scale containing structural features which is responsible for color. This is schematically shown in Figure below, where the butterfly and its wing structure are shown at increasing magnification. The entire color pattern is then made up of single colored tiles, often comprising between three and five colors, and the unique variations in color and hue are created just by varying the number and density of the different colored scales on the wings.

 

Structural variations that give rise to the beautiful colors of butterfly wings. In the center is a schematic cut-away view of a scale fragment showing the upper and lower layers, ridges, crossribs, ridge-lamellae, and microribs. (A) Ridges that produce thin-film reflectors giving rise to the colors. (B) Flats between the ridges may have an elaboration that gives rise to color due to scattering. (C) Lamella/microrib system now becomes the structure producing color. (D) Structure where the microribs fills the space and are the structural elements. (E) Flats may be filled with plates and pores pattern. (F) Interior of the scale may be filled with body-lamellae that now become the elements of a thin-film reflector. (G) Scales may be filled with a crystalline lattice that produce diffraction colors and may behave as zero-order gratings. (Reprinted with permission from Ghiradella, H. Microsc. Anat. Invet. 1998, 11A, 257. 1998, Wiley-Liss.)