Lying to Pinocchio: floral deception in an orchid pollinated by long-proboscid flies

Plants that lack floral rewards may nevertheless attract pollinators if their flowers sufficiently resemble those of rewarding plants. Flowers of the South African terrestrial orchid Disa nervosa are similar in floral dimensions and spectral reflectance to those of a sympatric nectar-producing irid (Watsonia densiflora s.l.). Observations showed that the orchid and Watsonia share the same pollinator, a long-proboscid tabanid fly Philoliche aethiopica. These flies visited inflorescences of both species during their foraging bouts and most (64%) observed or captured on Watsonia inflorescences carried pollinaria of the orchid on their proboscides. They probe an average of 6.3 flowers on Watsonia inflorescences, but just 1.9 flowers on the Disa inflorescences, a behaviour which would strongly promote cross-pollination in the self-compatible orchid. The orchid generally achieves high levels of pollination success, with approximately 50% of flowers receiving or exporting pollen at some sites. Pollination success was also high at one site that lacked Watsonia plants, suggesting that the orchid does not have an obligate dependence on Watsonia. Its pollination system may therefore be characterized as intermediate between generalized food deception and specific floral mimicry.