The swallowtail butterfly, Papilio nireus, has a beautiful green fluorescent wing band. The wing fluoresces at a wavelength that is in tune with the principle light receptor pigment in the compound eyes of adult butterflies. Producing an intense fluorescence is key to successful mate attraction.
The green color is produced by a pigment that is maximally stimulated by light at 420 nm and fluoresces at 505 nm. The pigment is arranged in a photonic crystal slab below the outer cuticle of the scale. The arrangement of the slab is such that light emitted is directional, both above and below the plane of the scale surface.
Below the photonic crystal slab is a reflector (Distributed Bragg Reflector or DBR) produced from several layers of cuticle with an appropriate gap in between. This reflector operates on similar principles to the DBRs used in fiber optics. The swallowtail reflector preferentially reflects light at 505 nm and allows other wavelengths to pass or be absorbed. This arrangement is such that little light is lost giving a bright signal. The fluorescent pigment is oriented to emit in only two directions, up and down. Horizontal emission is suppressed. The light emitted in the down direction is reflected upward and joins the light reflected upward to efficiently emit as much of the available light energy as possible.
Directionally Controlled Fluorescence Emission in Butterflies Pete Vukusic and Ian Hooper SCIENCE VOL 310 p. 1150.
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