The spectral input systems of hymenopteran insects and their receptor-based colour vision

Abstract

Spectral sensitivity functions S(lambda) of single photoreceptor cells in 43 different hymenopteran species were measured intracellularly with the fast spectral scan method. The distribution of maximal sensitivity values (lambda max) shows 3 major peaks at 340 nm, 430 nm and 535 nm and a small peak at 600 nm. Predictions about the colour vision systems of the different hymenopteran species are derived from the spectral sensitivities by application of a receptor model of colour vision and a model of two colour opponent channels. Most of the species have a trichromatic colour vision system. Although the S(lambda) functions are quite similar, the predicted colour discriminability curves differ in their relative height of best discriminability in the UV-blue or blue-green area of the spectrum, indicating that relatively small differences in the S(lambda) functions may have considerable effects on colour discriminability. Four of the hymenopteran insects tested contain an additional R-receptor with maximal sensitivity around 600 nm. The R-receptor of the solitary bee Callonychium petuniae is based on a pigment (P596) with a long lambda max, whereas in the sawfly Tenthredo campestris the G-receptor appears to act as filter to a pigment (P570), shifting its lambda max value to a longer wavelength and narrowing its bandwidth. Evolutionary and life history constraints (e.g. phylogenetic relatedness, social or solitary life, general or specialized feeding behaviour) appear to have no effect on the S(lambda) functions. The only effect is found in UV receptors, for which lambda max values at longer wavelengths are found in bees flying predominantly within the forest.