Cellular innovations and diversity in the lepidopteran compound eye.

Lepidoptera, having co-diversified with flowering plants and adapted to various diel niches, present a remarkable system for studying compound eye cell type diversity. Here we synthesize the latest research regarding Lepidopteran eye evolution across different timescales, from species-level variation to family-level changes, and mechanistic levels, from broad anatomical variation to molecular mechanisms responsible for spectral tuning. Opsin duplication, differential expression, and co-expression, combined with lateral filtering pigments, generate diverse spectral sensitivities in photoreceptors. Lateral filtering is particularly important for the convergent evolution of red vision. These diverse photoreceptors combine to form a handful of ommatidial types distributed differentially across eye regions, potentially specializing for distinct behavioral tasks. The coordinated development of these complex retinal mosaics requires precise regulatory mechanisms that we are only beginning to understand. Notably, only a subset of these ommatidial types contribute to color vision, highlighting the need for more research on their roles in motion and polarization vision. We also review support cells providing essential functions such as light insulation or reflection. Future research should focus on identifying ecological pressures driving visual system evolution, genetic bases of diverse retinal mosaics, and neural integration of visual information in Lepidoptera.