Tuning of photoreceptor spectral sensitivities by red and yellow pigments in the butterfly Papilio xuthus

The compound eye of the Japanese yellow swallowtail butterfly, Papilio xuth us, consists of different types of ommatidia characterized by the pigmentat ion around the rhabdom. About 75% of the ommatidia harbor red pigment, wher eas the other 25% contain yellow pigment. We find that the pigments functio n as spectral filters for the proximal photoreceptor cells. Intracellular r ecordings of the proximal cells yielded spectral sensitivities peaking in t he red (lambda(max) = 600 nm) and in the green (lambda(max) = 520 nm), resp ectively. Staining of the recorded cells and subsequent histology demonstra ted 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 pigme nt peaking at 600 nm. The sensitivity spectrum can be calculated with an op tical model for the butterfly rhabdom by incorporating measured absorbance spectra of the red pigments, yielding that the cell contains a visual pigme nt peaking at about 575 nm. The model also indicated that the spectral sens itivity of the green receptors is produced by the combination of the yellow lateral filter and a visual pigment peaking at 515 nm.