We saw in Chaps. 9 – 14 that many arthropods have developed complex photonic structures at the interface between their body and the outside world, participating in several vital functions. To optimize inter- and intraspecific communication functions and the management of energy flow, some have added constrained fluorescence to their light panel, generated within the photonic crystal itself. Photonic structures produce color effects that are usually intense and visible from a distance, which is clearly an evolutionary advantage, but also often directive, which can hamper the effectiveness of communication (Fig. 22.1). Some butterflies have addressed this problem by combining fluorescence emission with classical diffractive effects, which strongly modifies, both spatially and spectrally, the emitted color message. A guiding effect of the light emitted in Troides magellanus is described in the fifth section of Chap. 9 . Other effects of this constrained fluorescence have been observed in various butterflies, such as Papilionidae Papilio nireus (Fig. 22.2), and Morphidae Morpho sukowkyi.