Random array of colour filters in the eyes of butterflies

The compound eye of the Japanese yellow swallowtail butterfly Papilio xuthus is not uniform. In a combined histological, electrophysiological and optical study, we found that the eye of P. xuthus has at least three different types of ommatidia, in a random distribution. In each ommatidium, nine photoreceptors contribute microvilli to the rhabdom. The distal two-thirds of the rhabdom length is taken up by the rhabdomeres of photoreceptors R1­R4. The proximal third consists of rhabdomeres of photoreceptors R5­R8, except for the very basal part, to which photoreceptor R9 contributes. In all ommatidia, the R1 and R2 photoreceptors have a purple pigmentation positioned at the distal tip of the ommatidia. The R3­R8 photoreceptors in any one ommatidium all have either yellow or red pigmentation in the cell body, concentrated near the edge of the rhabdom. The ommatidia with red-pigmented R3­R8 are divided into two classes: one class contains an ultraviolet-fluorescing pigment. The different pigmentations are presumably intimately related to the various spectral types found previously in electrophysiological studies.