Colour saturation triggers innate reactions to flower signals: Flower dummy experiments with bumblebees

Summary

Innate behavioural reactions, i.e. reactions of untrained, flower-naive bumblebees (Bombus terrestris L., B. lucorum L.; Apidae) were observed in flower dummy experiments. It was proven that an innate releasing mechanism responds to optical flower signals: the spectral purity of ‘corolla colour’ was found to be crucial for far attraction toward flower dummies. During the subsequent near orientation, that is when a bumblebee finally reaches a flower dummy, the bumblebee's antennae contact the part of highest spectral purity while the bee is still in flight. ‘Guides’ such as ‘stamen patches’ present in the center of flower dummies are used only for near orientation. Flower dummies receiving the greatest number of antennae reactions at the guide were always those with low spectral purity in the surrounding background colour, high spectral purity at the corolla colour and highest spectral purity at the guide colour. In contrast, dominant wavelength and intensity of flower dummy colours had no detectable influence on innate behavioural reactions, while colour contrast had some. These results are interpreted as follows: orientation toward guides is based upon a gradient of centripetally increasing, bee-subjective colour saturation which directs the bumblebee's approach toward the center of the flower dummy where additional factors may contribute to stimulating the landing reaction.