The lycaenid butterfly Polyommatus icarus uses a duplicated blue opsin to see green.

The functional significance of gene duplication is rarely addressed at the level of animal behavior. Butterflies are excellent models in this regard because they can be trained and the use of their opsin-based visual pigments in color vision can be assessed. In the present study, we demonstrate that the lycaenid Polyommatus icarus uses its duplicate blue (B2) opsin, BRh2, in conjunction with its long-wavelength (LW) opsin, LWRh, to see color in the green part of the light spectrum extending up to 560 nm. This is in contrast to butterflies in the genus Papilio, which use duplicate LW opsins to discriminate colors in the long-wavelength range. We also found that P. icarus has a heterogeneously expressed red filtering pigment and red-reflecting ommatidia in the ventral eye region. In behavioural tests, the butterflies could not discriminate colors in the red range (570-640 nm). This finding is significant because we have previously found that the nymphalid butterfly Heliconius erato has filter-pigment mediated color vision in the long wavelength range. Our results suggest that lateral filtering pigments may not always influence color vision in insects.