Table 1.

Chromatic contrast, green contrast and brightness of the natural O. heldreichii perianths and the artificial perianth in relation to the background

	Ophrs heldreichii (n = 19)		Artificial perianth
	Mean	Total range
Chromatic contrast (hu)	0.22	0.16–0.30	0.29
Green contrast	0.09	0.02–0.20	0.06
Brightness	1.78	1.39–2.11	1.88
Chromatic difference to artificial perianth (hu)	0.16	0.07–0.23

Values were calculated using the colour hexagon model (Chittka 1992). For the calculation, we used the spectral sensitivity curves of Apis mellifera (Peitsch et al. 1992). Note that for the green-receptor contrast the absolute values are given (Spaethe et al. 2001). Chromatic contrast is given in hexagon units (hu). Green contrast and brightness are dimensionless