Fig. 1

The spectra of stimuli used in choice assays (black lines) and their natural equivalents (grey lines, from [35]); a white G. fornicata, b yellow G. fornicata, c backgrounds, both “average” (experiments 1–3, solid line) and “augmented” (experiment 4, dashed line) and d illuminant. Although normalized here for ease of comparison, all stimuli were matched to natural sources considering both spectral intensity (‘brightness’) and shape (‘colour’). Note that since flies are largely insensitive to wavelengths above 600 nm, all perceptually relevant spectral variation is limited to the 300–600 nm range