Simple and complex, sexually dimorphic retinal mosaic of fritillary butterflies

Butterflies have variable sets of spectral photoreceptors that underlie colour vision. The photoreceptor organization may be optimised for the detection of body colouration. Fritillaries (Argynnini) are nymphalid butterflies exhibiting varying degrees of sexual dimorphism in wing colouration. In two sister species, the females have orange (Argynnis paphia) and dark wings (A. sagana), respectively, while the males of both species have orange wings with large patches of pheromone-producing androconia. In spite of the differences in female colouration, the eyes of both species exhibit an identical sexual dimorphism. The female eyeshine is uniform yellow, while the males have a complex retinal mosaic with yellow and red-reflecting ommatidia. We found the basic set of UV-, blue- and green-peaking photoreceptors in both sexes. Males additionally have three more photoreceptor classes, peaking in the green, yellow and red, respectively. The latter is the basal R9, indirectly measured through hyperpolarisations in the green-peaking R1-2. In many nymphalid tribes, including the closely related Heliconiini, the retinal mosaic is complex in both sexes. We hypothesise that the simple mosaic of female Argynnini is a secondary reduction, possibly driven by the use of olfaction for intraspecific recognition, whereas vision remains the primary sense for the task in the males.

Methods
Electrophysiology, single cell recording. 

Imaging spectrophotometry.

Histology, light microscopy.