How Do Bees (and Humans) See Grey Levels? : Northern Bee Books : 9781914934544 : 1914934547 : 15 Mar 2023 : There are several sources of serious confusion in the investigations of how bees and humans see grey and black. First, von Frisch trained bees to go to a coloured paper, and then tested whether they could distinguish that colour from a palette of 15 shades of grey placed together on a test board. Unfortunately, he used papers made from wood pulp, which do not reflect ultraviolet, so the UV receptors were excluded. Secondly,16 years later it was shown that bees require a 25% difference in brightness to discriminate grey levels, so his test was uncertain. Thirdly, bees are dichromats, and detect only green contrast and the fraction of light that stimulates the blue receptors. The most interesting confusion is that grey photons do not exist, but that does not affect bees because they are functional dichromats and treat grey like any other colour. No problem. The UV receptors in the