Spectral sensitivity of single photoreceptors and color vision in the stingless bee,Melipona quadrifasciata

1.	The spectral sensitivities of the photoreceptors in the compound eye of the stingless bee, Melipona quadrifasciata (Hymenoptera, Apoidea), was determined by the spectral scanning method. Three spectral receptor types were found with max at 356 nm, 424 nm, and 532 nm (Fig. 1). Intracellular markings confirmed one morphological type of green receptor (svf 1) and one type of UV receptor (1vf 1) whose axon morphology resembles that of the corresponding spectral receptor types in the honeybee, Apis mellifera (Fig. 2).
2.	Training experiments with a large number of color signals were performed at the hive entrance and the feeding place under natural daylight conditions (Figs. 4–6). The tests were either dual (2 alternative color signals) choice tests or multiple (12 simultaneously presented alternative color signals) choice tests. Melipona discriminates colors very well in both behavioral contexts, but discrimination is generally better at the feeding place (Fig. 7). A comparison with Apis shows that Melipona discriminates colors in the bluish green better than Apis, and that Apis discriminates all other colors better.
3.	The spectral properties of the receptor types were used to construct a color space in which all the color signals tested in the behavioral experiments are represented at particular loci (Fig. 3). A receptor model of color vision as proposed by Backhaus and Menzel (1987) for the honeybee is used to calculate the perceptual distance between the colors corresponding to the loci of the color stimuli. This model interprets the perceptual distance between two color stimuli as the number of just noticeable difference steps in the corresponding receptor voltage signals. The predicted distances are highly correlated with the discrimination values of the behavioral tests (Fig. 12).