Table 1.

Chromatic contrast, green contrast and brightness in relation to the background

	Ophrys heldreichii (n = 15)	Ophrys dictynnae (n = 27)	Mann–Whitney U-test
Chromatic contrast (Hexagon units)	0.23 ± 0.03	0.16 ± 0.04	P < 0.001
Green contrast (dimensionless)	0.09 ± 0.05	0.09 ± 0.05	NS
Brightness (dimensionless)	1.74 ± 0.21	1.62 ± 0.25	NS

The values represent means ± standard deviations. 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)