STAMEN MOVEMENTS IN FLOWERS OF OPUNTIA (CACTACEAE) FAVOR OLIGOLECTIC POLLINATORS

Opuntia brunneogemmia and O. viridirubra occur sympatrically in the Se rra do Sudeste, Rio Grande do Sul, Brazil. Their flowers have 450-600 thigmonastic stamens and provide large amounts of pollen and nectar fo r bees. Bees of 41 species were registered at the flowers of O. brunne ogemmia and 30 at the flowers of O. viridirubra. Females of three olig olectic species are the only effective pollinators: Ptilothrix fructif era (Anthophoridae), Lithurgus rufiventris (Megachilidae), and Cephalo colletes rugata (Colletidae). During their visits in Opuntia-flowers, bees touch the filaments and stimulate the movement of the stamens to the centre of the flower. At the end of this movement, the anthers are densely packed around the style. As a consequence the pollen is prese nted in an easily accessible upper layer of anthers and various, nearl y inaccessible lower layers. The lower layers contain about 80% of the pollen reward. Only females of the three oligolectic pollinators expl oit the pollen from the lower layers and reach the nectar furrow. Ther efore, through their stamen movements, Opuntia flowers hide most of th eir pollen from flower visitors but favour effectively pollinating, ol igolectic bees.