Apocynaceae have a precise pollination mechanism. However, broad pollinator assemblages, including several insect orders, have often been recorded. We test how the corolla tube length and operative width required for effective release of the pollination mechanism could restrict the pollinator assemblage in putatively hawkmoth-pollinated Apocynaceae that differ in flower depth. If corolla tube length were effective in filtering pollinators, a broader pollen vector assemblage would be expected in short-flowered species. For this to occur, the operative width of the pollinating apparatus would be additionally expected not to be specific to any particular pollinator. Alternatively, if the pollination mechanism were to have any effect in the exclusion of pollinators, access to short flowers would be expected to be limited by proboscis width rather than length. Pollination of three species in the Apocynaceae was studied in seven populations through direct observations and palynological analysis of hawkmoths. Proboscis widths of hawkmoths carrying and not carrying pollen were compared with operative floral width as measured by proboscis analogues of different diameters that could successfully release the pollination mechanism. Species studied were exclusively hawkmoth pollinated. Pollen was always attached to pollinator proboscides, either near the base or on the apical half. The long-tongued hawkmoth species Manduca sexta (L.) was the major pollinator of Mandevilla longiflora (Desf.) Pichon and Mandevilla petraea (A. St.-Hil.) Pichon. Surprisingly, another long-tongued species, Manduca tucumana (Rothschild & Jordan), was the main pollinator of the short-flowered Mandevilla laxa (Ruiz & Pav.) Woodson. Here, the operative flower width was a decisive factor restricting the pollinator spectrum to hawkmoths with proboscides narrow enough to release the pollination apparatus. Short-tongued hawkmoths, which also have wider proboscides, cannot release the pollination mechanism. In M. petraea, the operative length, and not the operative width, restricts the pollinator assemblage. Thus, two different plant strategies were observed to restrict the pollinator spectrum: floral tube length and the operative width of the pollination mechanism.
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Vol. 94 • No. 2