Color is an important mode of communication in arthropods, and its origins are multiple. Whether pigmentary or structural in origin, many organisms can change their colored appearance either actively or passively in very short times. And we also see, within the same species, seasonal or environmental chromatic variations extending over much longer periods. As we have seen, the interference phenomenon can produce impressive color effects. It is a passive phenomenon that depends only on the relative situation of the insect, the light source, and the observer. But other processes of color changes exist, involving other phenomena, which we group under the term x-chromia, x being the stress that causes the color change. Some of these processes depend only on the external climatic conditions, others on the participation of the insects. In this context, the phenomenon of iridescence will be called goniochromy. They are mainly due to photonic structures. They are quick and reversible (Fig. 19.1).