Cortical-like colour-encoding neurons in the mushroom body of a butterfly.

Colour is an important visual modality for many animals including insects. The flower-foraging swallowtail butterfly Papilio xuthus has spectrally acute chromatic vision using UV-, blue-, green- and red-sensitive photoreceptors. The spectral organization of Papilio's retina is well understood but, as for other insects, how chromatic information is processed in higher order brain regions remains unclear. To identify neurons underlying color perception in Papilio, we have investigated the spectral properties of the visual inputs to the mushroom body (MB), a brain region implicated in learning and memory. By recording intracellular responses to a series of monochromatic lights, combined with dye injection, we have revealed a wide variety of spectral responses in three morphologically distinct neuron types. These heterogeneous responses are characterized by colour opponency and sharp tuning to particular wavelengths, which do not in general align with the sensitivities of the retinal photoreceptors, and presumably contribute to Papilio's acute wavelength discrimination. This finding provides new insights into the processing underlying insect colour vision.