The compound eye of the Japanese yellow swallowtail butterfly, Papilio xuthus, consists of different types of ommatidia characterized by the pigmentation around the rhabdom. About 75% of the ommatidia harbor red pigment, whereas the other 25% contain yellow pigment. We find that the pigments function as spectral filters for the proximal photoreceptor cells. Intracellular recordings of the proximal cells yielded spectral sensitivities peaking in the red (lambda(max) = 600 nm) and in the green (lambda(max) = 520 nm), respectively. Staining of the recorded cells and subsequent histology demonstrated that the red receptors contain red pigment and that the green receptors contain yellow pigment. The sensitivity spectrum of the red receptors was considerably narrower compared to the absorption spectrum of a visual pigment peaking at 600 nm. The sensitivity spectrum can be calculated with an optical model for the butterfly rhabdom by incorporating measured absorbance spectra of the red pigments, yielding that the cell contains a visual pigment peaking at about 575 nm. The model also indicated that the spectral sensitivity of the green receptors is produced by the combination of the yellow lateral filter and a visual pigment peaking at 515 nm.
