This paper describes the action spectrum of foraging behavior of a butterfly, Papilio xuthus. We first established an experimental protocol to evaluate learning and discrimination of monochromatic light by the butterflies. We trained butterflies to feed on sucrose solution at the window illuminated with certain monochromatic light produced through a monochromator. After confirming that they learned the monochromatic light, after 10 days of training, we tested the butterflies one by one. We presented training wavelengths for each individual at different intensities, and recorded whether they perform foraging behavior under freely-flying as well as tethered conditions. Freely-flying butterflies responded to light by visiting the window and searching for nectar around it, whereas tethered butterflies responded by extending their proboscides towards the window. The light intensity required to elicit 50% response for each tested monochromatic light was plotted. The resulting action spectrum for the visit was rather flat with the maximum sensitivity a 420 nm, whereas the spectrum for the proboscis extension had prominent peaks at 380, 500 and 600 nm. The difference in action spectra indicates that the visit and the proboscis extension are controlled by two independent mechanisms at least in part.
