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 rema rkably similar in morphology and spectral reflectance to the flowers of a s ympatric nectar-producing iris, Watsonia lepida. Field observations indicat ed that both D. pulchra and W. lepida are pollinated by the long-tongued fl y, Philoliche aethiopica (Tabanidae). The hypothesis that D. pulchra is a f loral mimic of W. lepida was supported by choice experiments, which showed that, in terms of visits to inflorescences, flies do not discriminate betwe en the two species. However, flies probed fewer flowers and spent less time on D. pulchra inflorescences than on Watsonia inflorescences. Thus the abs ence of nectar in the flowers of D. pulchra may reduce the risk of geitonog amy and pollen discounting. A breeding system experiment showed that fruits of D. pulchra that arise from self-pollination contain relatively few viab le seeds compared with outcrossed fruits. These findings support the hypoth esis that deception in orchids evolved under selection for more efficient m ating systems. (C) 2000 The Linnean Society of London.