Spectral sensitivities of elementary color-coded neurons in butterfly larva.

1. Response properties and spectral sensitivities of a functional class of medulla neurons that received dominant input from a single stemma in the swallowtail butterfly larva were examined with regard to neutral and chromatic backgrounds. 2. Eight types of neurons were dominated by a dichromatic stemma with blue and green receptors. Six showed specific color-opponent responses mediated by two receptors on different backgrounds; one showed strong color opponency because of fairly balanced antagonistic input on the neutral (dark and white) background; the other five usually had weak or concealed color opponency on a neutral background but became strongly color opponent when appropriate chromatic illumination was present on the background. The most complex neuron gave excitatory responses on a dark background, inhibitory responses on a white background, and color-opponent responses on a chromatic (yellow) background. 3. The remaining two types of neurons dominated by the dichromatic stemma showed non-color-opponent and broad action spectra ascribable to a synergistic effect of the two classes of photoreceptors. 4. All eight types of neurons dominated by a trichromatic stemma (with UV, blue, and green receptors) were color opponent. Five showed strong color-opponent responses with the mediation of the three classes of receptors or only two (UV and green) on a neutral background. The others revealed distinct color opponency in the presence of UV or green illumination on the background. 5. The larval medulla neurons described may form parallel, elementary color-coded channels in the medulla neuropile to combine the outputs of the different color receptors in a single visual unit (stemma) into specific color-opponent signals. They probably correspond to columnar relay neurons in the medulla of the adult insects with a compound eye.