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Spectrally opponent responses, that is, wavelength‐dependent inversions of response polarity, have been observed at the level of photoreceptors in butterflies. As inter‐photoreceptor connections have been found in the butterfly Papilio xuthus, and histamine is the only neurotransmitter so far identified in insect photoreceptors, we hypothesize that histaminergic sign‐inverting synapses exist in the lamina between different spectral receptors as a mechanism for spectral opponency as in the medulla of Drosophila. Here, we localized two histamine‐gated chloride channels, PxHCLA (Drosophila Ort homolog) and PxHCLB (Drosophila HisCl1 homolog), in the visual system of Papilio xuthus by using specific antisera. The antiPxHCLA labeling was associated with the membrane of nonphotoreceptor cells that are postsynaptic to photoreceptors, while the antiPxHCLB labeling overlapped with photoreceptor axons …