Visual discrimination between two sexually deceptive Ophrys species by a bee pollinator

Abstract

Almost all species of the orchid genus Ophrys are pollinated by sexual deception. The orchids mimic the sex pheromone of receptive female insects, mainly hymenopterans, in order to attract males seeking to copulate. Most Ophrys species have achromatic flowers, but some exhibit a coloured perianth and a bright, conspicuous labellum pattern. We recently showed that the pink perianth of Ophrys heldreichii flowers increases detectability by its pollinator, males of the long-horned bee Eucera berlandi. Here we tested the hypothesis that the bright, complex labellum pattern mimics the female of the pollinator to increase attractiveness toward males. In a dual-choice test we offered E. berlandi males an O. heldreichii flower and a flower from O. dictynnae, which also exhibits a pinkish perianth but no conspicuous labellum pattern. Both flowers were housed in UV-transmitting acrylic glass boxes to exclude olfactory signals. Males significantly preferred O. heldreichii to O. dictynnae flowers. In a second experiment, we replaced the perianth of both flowers with identical artificial perianths made from pink card, so that only the labellum differed between the two flower stimuli. Males then chose between both stimuli at random, suggesting that the presence of a labellum pattern does not affect their choice. Spectral measurements revealed higher colour contrast with the background of the perianth of O. heldreichii compared to O. dictynnae, but no difference in green receptor-specific contrast or brightness. Our results show that male choice is guided by the chromatic contrast of the perianth during the initial flower approach but is not affected by the presence of a labellum pattern. Instead, we hypothesise that the labellum pattern is involved in aversive learning during post-copulatory behaviour and used by the orchid as a strategy to increase outcrossing.

Fig. 1

Male choices for a intact flowers of Ophrys heldreichii (left column) and O. dictynnae (right column) (73 approaches in total), and for b flowers where the original perianth was replaced by an artificial one made from pink card (58 approaches). Number of approaches are given within the column. *** P < 0.001, n.s., P > 0.05; χ²-test

Fig. 2

Flower morphology of the perianth (a) and the labellum (b) of O. heldreichii (left column) and O. dictynnae (right column). Perianth size does not differ between the two species. However, O. dictynnae has a significantly larger labellum compared to O. heldreichii. N_{O. heldreichii} = 15, N_{O. dictynnae} = 27; means ± SEM; *** P < 0.001, n.s., P > 0.05; Mann–Whitney U-test

Fig. 3

a–c Spectral reflectance curves of the perianth of Ophrys heldreichii (dark violet), Ophrys dictynnae (pale pink), the artificial perianth (dark pink) and the background (green) as a function of wavelength. In a only mean reflectance of the flower perianth are given (N_{O. heldreichii} = 15, N_{O. dictynnae} = 27, N_Background = 5, see text). b and c show the mean reflectance of O. heldreichii and O. dictynnae, respectively, presented with the standard deviation of the mean to visualize the variation within the species. d The colour loci of Ophrys heldreichii (dark violet), O. dictynnae (light pink), the artificial perianth and two major food plants (Vicia cracca and Salvia fructicosa) represented in the colour hexagon model (Chittka 1992). The large circle denotes 0.1 hexagon units around the centre. All loci within this circle are assumed to appear achromatic to the bee (Chittka and Kevan 2005). The grey line indicates the loci of pure spectral lights at background intensity. Colour distances in the hexagon space are calculated as the Euclidean distance between two loci (Chittka 1992)