Increase of pollinator attraction by means of a visual signal in the sexually deceptive orchid, Ophrys heldreichii (Orchidaceae)

Orchids of the genus Ophrys are pollinated by males of solitary bees and wasps through sexual deception. Flowers mimic the odor of a receptive female and thus attract males that seek to copulate. Visual stimuli have been assumed so far to play only a minor role in male attraction. We investigated the role of the perigon as a potential visual signal in attracting pollinators in the orchid Ophrys heldreichii and its pollinator, the males of the long-horned bee Tetralonia berlandi (Apidae). In contrast to many other Ophrys species, O. heldreichii exhibits a large and bright pinkish perigon that appears visually conspicuous to a human observer. In a dual choice test we presented two flowers from a single plant and counted visitation rates. We then removed the perigon of one flower and retested the relative attractiveness of both flowers. For 292 male visits in ten trials we found a significant decrease of visitation rate for flowers with the perigon removed. In a second experiment we repeated the dual choice test using photos of the flowers. Males also significantly chose the picture of an intact flower over the picture of a modified flower where the perigon was digitally removed. From our data, we conclude that T. berlandi males respond to and are attracted by the bright pink perigon of the orchid in addition to other stimuli. A bright colorful perigon occurs almost only in the Ophrys holoserica-oestrifera group, a large sub-group of the genus. We hypothesize that this kind of visual signal is adaptive particularly in those Ophrys species where the targeted males patrol resourced-based encounter sites and strongly rely on their visual system while searching for their females.