Butterflies are colorful insects with diverse color visual systems. The diversity found in this group relies primarily on opsin proteins, which together with the chromophore 11-cis 3-hydroxy retinal form the visual pigments, and are responsible for converting light into a bioelectric signal. Three opsin genes are found in most insects, and except for Drosophila, duplications of UV opsin genes are relatively rare. Heliconius butterflies have a UV opsin duplication that arose in the ancestor of the genus just before the genus underwent an adaptive radiation. However it is unclear whether the two UV opsin proteins, UVRh1 and UVRh2, are expressed in different R1 and R2 photoreceptors or co-expressed in the adult compound eye. We aimed to characterize the retinal mosaic of adult H. erato using antibodies against the ultraviolet, blue and long wavelength opsins and to identify individual photoreceptor cell spectral sensitivities with intracellular recordings. We unexpectedly discovered that the H. erato compound eye is sexually dimorphic with respect to opsin protein expression. Males do not express UVRh1 while females express both UVRh1 and UVRh2 in tightly regulated cell combinations. This sex difference extends to the types and proportions of photoreceptor cells and ommatidia in the compound eye. Electrophysiological recordings confirm that the female has three spectrally distinct short-wavelength photoreceptor cells while the male has two types. Both males and females also have two types of LW photoreceptor cell, a green receptor and a red-shifted LW sensitive photoreceptor cell, which is responsible for red color discrimination previously found in this species. This is the first evidence of sexual dimorphism with respect to UV expression in the Heliconius eye, and the first evidence of five distinct cell types in females, with four types in males.