Morphology and spectral sensitivity of long visual fibers and lamina monopolar cells in the butterfly Papilio xuthus.

Extensive analysis of the flower-visiting behavior of a butterfly, Papilio xuthus, has indicated complex interaction between chromatic, achromatic, and motion cues. Their eyes are spectrally rich with six classes of photoreceptors, respectively sensitive in the ultraviolet, violet, blue, green, red, and broad-band wavelength regions. Here, we studied the anatomy and physiology of photoreceptors and second-order neurons of P. xuthus, focusing on their spectral sensitivities and projection terminals to address where the early visual integration takes place. We thus found the ultraviolet, violet, and blue photoreceptors and all second-order neurons terminate in the distal region of the second optic ganglion, the medulla. We identified five types of second-order neurons based on the arborization in the first optic ganglion, the lamina, and the shape of the medulla terminals. Their spectral sensitivity is independent of the morphological types but reflects the combination of pre-synaptic photoreceptors. The results indicate that the distal medulla is the most plausible region for early visual integration.