The structure now presents a periodicity in the three spatial directions, and it is under this form that the analogy with classical photonic crystals is the most obvious. The study of three-dimensional photonic crystals can then benefit from the formalism of classical crystallography, from the notion of reciprocal network, some of which are recalled in the appendix. Such photonic crystals are naturally present in the mineral world (they are opals), and in the animal world among some butterflies, but especially among certain Coleoptera, the Curculionidae Eupholus and potentially Cerambycidae, among others. Those Coleoptera possess thick resisting scales—3 to 3.5 μm—that are irregularly arranged on the elytron, limbs, and head. When individually examined with optical microscopes, these scales present an extraordinary iridescence, producing bright green or blue colors, sometimes with extensions towards yellow, orange, and red. A section or some broken scales show a very regular three-dimensional alveolar structure with an approximately 200 nm-long period. As a crystal diffracts X-rays, this structure diffracts certain visible wavelengths in different spatial directions, resulting in the iridescence of the insect.