Vision in flower foraging butterflies: from functional organization of the retina to encoding in the brain

The swallowtail butterfly Papilio xuthus is a model species in insect vision science, thanks to extensive studies over the past few decades. P. xuthus adaptively uses visual cues such as color, brightness, polarization and motion in various steps of flower-foraging behavior. We have explored these visual functions from both perceptual and physiological perspectives. This Review aims to summarize these studies by focusing on color vision as a prominent ability in foraging P. xuthus and on wide-field motion vision as a more-universal visual modality in insects. The compound eyes of P. xuthus consist of three types of ommatidia, each with a different combination of spectral receptor classes: sensitive to ultraviolet (UV), violet (V), blue (B), green (G), red (R) and broadband (BB) wavelength regions. Connectome analysis of the first optic ganglion, the lamina, reveals interphotoreceptor interaction causing spectral opponency and spectral integration in the second-order lamina monopolar cells (LMCs). These characteristics should be crucial in the initial processing underlying the acute color discrimination ability of tetrachromatic color vision based on UV, B, G and R receptors, as well as motion vision involving G, R and BB receptors. In addition, we have revealed that the spectral properties of interneurons connecting the optic lobe and the central brain well explain the behavioral properties of P. xuthus. By discussing the visual system of P. xuthus butterflies in conjunction with knowledge from honeybees, flies and other lepidopteran insects, we will provide valuable insights into the evolution of insect visual systems.