Fig. 4. Reflectance spectra measured from single-facet lenses in the eye of the satyrine Bicyclus anynana. The spectra fall into two classes, yellow and red. The reflectance spectrum of the yellow class is broad, extends into the ultraviolet, is very minor above 600nm and peaks at around 580nm; the reflectance spectrum of the red class is a restricted band around 650nm and is negligible below 560nm. The spectra are normalized to the peak value of the yellow class.