Why are some coleoptera beetles blue?
Because light striking the
external hard parts undergoes destructive interference. Precisely
how this happens is now being studied quantitatively by a team of
scientists in Namur, Belgium. Electron microscope pictures of the
beetle's scaly cuticle, online at Physics News Graphics,
help to explain that each scale is made of alternating layers of
pure chitin (high index of refraction) and mixtures of chitin and
air (low index of refraction).
The resulting structure is a
photonic crystal: because of wave interference, light of certain
frequencies are excluded. In this case blue light is forbidden from
being absorbed by the animal's shell; all blue light is reflected
while other frequencies are absorbed in the cuticle, and the
creature consequently has a blue appearance.
crystals have been studied for many years. Often featuring a
honeycomb structure, these materials are to light waves what
semiconductors are to electrons: transmission in certain energy
bands is permitted while other bands are forbidden. Lord Rayleigh
in 1918 was the first to suggest that the iridescence of some
insects arose from interference effects. And by now the
photonic-crystal effect is known to occur naturally in many places,
such as the opalescence of weevils and the striking colors in the
peacock's tail feathers.
According to Jean Pol Vigneron at the
University of Namur in Belgium (firstname.lastname@example.org), lessons learned
from the beetle scale's iridescence might be applied to the
manufacture of paint, clothes, paper and in simplifying the kinds of
windows and windshields that currently employ interference effects.
The beetle's optics might also help in designing micro-fabricated
displays in which different colors could be obtained through the
clever reflection, rather than by emission, of light.
Vigneron et al.,
Physical Review E, December 2005