Parallel Pathways for Visual and Olfactory Information in the Mushroom Bodies of the Swallowtail Butterfly Brain

The mushroom bodies (MBs), prominent paired neuropils of the insect brain, are multimodal sensory processing centers. In the MBs of Papilio xuthus, a flower-foraging swallowtail butterfly with sophisticated color vision, the inner zone and the rim of the outer zone of the primary calyx receive prominent visual input. These two visual zones are spatially segregated from the outer zone that receives olfactory input. Sensory information is transmitted to the MB output sites (the lobes) via the pedunculus, but it remains unknown how the concentric modality organization of the calyx is transformed and represented in the spheroidal lobe neuropils in P. xuthus. To address this question, we investigated the architecture of the Papilio MBs in detail. Immunofluorescent staining and tracer injections into the MBs revealed the branching patterns of Kenyon cells (KCs), the MB intrinsic neurons, and several output pathways to other neuropils of the central brain. We newly defined the α/β, α′/β′, and γ lobes of the Papilio MBs following the nomenclature system commonly used in other insects. KCs conveying visual information extend axon-like fibers to the α/β lobe (a large ventral division), while KCs conveying olfactory information send fibers to the α′/β′ lobe located between the α/β and γ lobes. These parallel sensory pathways in the MBs of P. xuthus were summarized as a three-dimensional reconstruction of the MB subdivisions. Our study facilitates future physiological and functional studies of multisensory processing in the swallowtail butterfly brain.