Stamen movements in flowers of Opuntia (Cactaceae) favour oligolectic pollinators

Opuntia brunneogemmia and O. viridirubra occur sympatrically in the Serra do Sudeste, Rio Grande do Sul, Brazil. Their flowers have 450—600 thigmonastic stamens and provide large amounts of pollen and nectar for bees. Bees of 41 species were registered at the flowers of O. brunneogemmia and 30 at the flowers of O. viridirubra. Females of three oligolectic species are the only effective pollinators: Ptilothrix fructifera (Anthophoridae), Lithurgus rufiventris (Megachilidae), and Cephalocolletes 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 presented in an easily accessible upper layer of anthers and various, nearly inaccessible lower layers. The lower layers contain about 80% of the pollen reward. Only females of the three oligolectic pollinators exploit the pollen from the lower layers and reach the nectar furrow. Therefore, through their stamen movements, Opuntia flowers hide most of their pollen from flower visitors but favour effectively pollinating, oligolectic bees.