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 R1-R4, The proximal third consists of rhabdomeres of photoreceptors R5-R8, 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 R3-R8 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 R3-R8 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.