2.1.1. Color production
The diverse colors of insects are produced by the interaction of light with cuticle and/or underlying cells or fluid by two different mechanisms. Physical (structural) colors result from light scattering, interference, and diffraction, whereas pigmentary colors are due to the absorption of visible light by a range of chemicals. Often both mechanisms occur together to produce a color different from either alone.
All physical colors derive from the cuticle and its protuberances. Interference colors, such as iridescence and ultraviolet, are produced by refraction from varyingly spaced, close reflective layers produced by microfibrillar orientation within the exocuticle, or, in some beetles, the epicuticle, and by diffraction from regularly textured surfaces such as on many scales. Colors produced by light scattering depend on the size of surface irregularities relative to the wavelength of light. Thus, whites are produced by structures larger than the wavelength of light, such that all light is reflected, whereas blues are produced by irregularities that reflect only short wavelengths.
Insect pigments are produced in three ways:
- by the insect’s own metabolism;
- by sequestering from a plant source;
- rarely, by microbial endosymbionts.
Pigments may be located in the cuticle, epidermis, hemolymph, or fat body. Cuticular darkening is the most ubiquitous insect color. This may be due to either sclerotization (unrelated to pigmentation) or the exocuticular deposition of melanins, a heterogeneous group of polymers that may give a black, brown, yellow, or red color. Carotenoids, ommochromes, papiliochromes, and pteridines (pterins) mostly produce yellows to reds, flavonoids give yellow, and tetrapyrroles (including breakdown products of porphyrins such as chlorophyll and hemoglobin) create reds, blues, and greens. Quinone pigments occur in scale insects as red and yellow anthraquinones (e.g. carmine from cochineal insects), and in aphids as yellow to red to dark blue—green aphins.
Colors have an array of functions in addition to the obvious roles of color patterns in sexual and defensive display. For example, the ommochromes are the main visual pigments of insect eyes, whereas black melanin, an effective screen for possibly harmful light rays, can convert light energy into heat, and may act as a sink for free radicals that could otherwise damage cells. The red hemoglobins which are widespread respiratory pigments in vertebrates occur in a few insects, notably in some midge larvae and a few aquatic bugs, in which they have a similar respiratory function.