Each colour is a specific signal. The spectral sensitivity of insect eyes (especially that of the bee’s eye) differs from that of the human eye. Bees distinguish ultra-violet light, but they do not perceive the long-wave light of the red part of the spectrum (that is perceived by humans). The flowers of many plant species are characterized by UV-marks that are recognized by bees though not by humans. Special optical tricks help to picture even such patterns.

To the left: Flower head of the North American Compositae Rudbeckia bicolor (cone flower; also cultivated in European gardens, occasionally grown wild). Photographed with normal material and standard objective.

To the right: The same object photographed with quartz optic, a ruby filter that blocks visible light, and an UV-sensitive film (W. BARTHLOTT, 1986)