Batesian mimicry in the non-rewarding orchid Disa pulchra, and its consequences for pollinator behaviour

The non-rewarding flowers of the South African orchid Disa pulchra are remarkably similar in morphology and spectral reflectance to the flowers of a sympatric nectar-producing iris, Watsonia lepida. Field observations indicated that both D. pulchra and W. lepida are pollinated by the long-tongued fly, Philoliche aethiopica (Tabanidae). The hypothesis that D. pulchra is a floral mimic of W. lepida was supported by choice experiments, which showed that, in terms of visits to inflorescences, flies do not discriminate between the two species. However, flies probed fewer flowers and spent less time on D. pulchra inflorescences than on Watsonia inflorescences. Thus the absence of nectar in the flowers of D. pulchra may reduce the risk of geitonogamy and pollen discounting. A breeding system experiment showed that fruits of D. pulchra that arise from self-pollination contain relatively few viable seeds compared with outcrossed fruits. These findings support the hypothesis that deception in orchids evolved under selection for more efficient mating systems. (C) 2000 The Linnean Society of London.