The spectral sensitivity of single photoreceptors of Osmia rufa was determined by a fast voltage-clamp technique. Three receptor types were found whose spectral sensitivity functions followed a rhodopsin-like photopigment absorption function with λmax values at 348nm (ultraviolet receptor), 436nm (blue receptor) and 572nm (green receptor). The λmax of the green receptor in Osmia rufa is shifted to much longer wavelengths compared with other insect species. Discrimination of colour signals was tested after training a bee at the entrance to its nest. The colour signals were filter discs (70 mm in diameter) with a hole (10 mm in diameter) in the centre and the bees quickly learned to use the coloured disc as a marker of the nest entrance. Tests were dual forced-choice tests with two coloured discs closely positioned next to each other. 94 different tests were each repeated 5–15 times and were performed after training to 12 different colour signals.
A photoreceptor model was used to calculate the loci of the colour signals in a three-dimensional colour space and in a chromaticity diagram. The perceptual distance between the colour loci was calculated as line elements (minimum number of just noticeable difference, jnd-steps), which were based on the noiseproperties of the photoreceptors. The discrimination determined by the behavioural tests correlated very well with the jnd-steps. The correlation was better for the line elements in the colour plane than in the colour space. Osmia rufa was compared with the honeybee Apis mellifera and the stingless bee Melipona quadrifasciata. There is no difference in colour selection between Osmia and Apis, whereas Melipona discriminates less well in the violet-blue region. The model calculation was used to compare the chromaticity diagrams and the spectral discrimination functions of the three species. It is concluded that the receptor model used in this study predicts the discrimination behaviour of the three bee species very well. Therefore, comparative studies on colour vision in flowervisiting insects may be based on spectral measurements of the photoreceptors, and in many cases this reduces the extent of laborious behavioural studies.