Coloration

Often the result of various pigments present in granules in the epidermal cells, the colors of insects can also be due to colored material in the cuticle, diffraction or interference of light caused by special cuticular structures, or the Tyndall effect.

A brown coloration in the cuticle develops often during sclerotization of the exocuticle, especially when NBAD is used as precursor for the sclerotization agents, whereas uncolored and transparent cuticles results when NADA is the sole sclerotization precursor. The intensity of the color varies from very light brown over tan to a very dark brown, which can be difficult to discern from the genuinely black cuticles that contain melanins. Melanins are formed when free tyrosine or dopamine is oxidized to orthoquinones, which readily polymerize to complex, black, intractable materials. Melanins either are diffusely distributed in the cuticle or occur in discrete, membrane-bounded granules.

Structural colors of the cuticle from interference of light can be caused by regularly spaced layers in the cuticle in, for example, the cornea of the compound eyes in many flies. Light reflected from the individual layers will interfere to give colors varying with the angle of reflection. Structural colors may also be produced by diffraction of light by regularly spaced microscopic structures on the cuticular surface. The brilliant colors of many beetle species are due to such surface diffraction.

Light scattered by sufficiently small particles (< 0.7 |lm in diameter) looks blue because of the Tyndall effect, as in the blue colors of many dragonflies. The light-scattering particles may be located in the epidermal cells underlying a transparent cuticle, or the light may be scattered by a very fine bloom of wax filaments deposited on the cuticular surface after emergence.

0 0

Post a comment