Floral biology of Hesperantha (Iridaceae: Crocoideae):: How minor shifts in floral presentation change the pollination system

Filed observations, floral dissections of a representative range of Hesperantha species, and pollen load analyses of insects captured on many of them indicate that flowers of this African genus are cross pollinated by a relatively broad range of insects. The pollination ecology of Hesperantha can be divided into four overlapping systems that exploit insects of four orders (Coleoptera, Diptera, Hymenoptera, and Lepidoptera). Species of the H. falcata type have erect or nodding, salver-shaped, strongly fragrant, white flowers that open in the mid to late afternoon and evening and are pollinated by long-tongued apid bees and/or noctuid moths. Species of the H. pauciflora type have a virtually identical floral morphology, but the perianth is yellow or pink to mauve or blue and the flowers are usually unscented and are open during the day, closing between midday and late afternoon, ca. 16:30 H. Flowers of this type are also pollinated by apid bees, but in the southern African winter-rainfall zone other effective pollinators include nemestrinid flies (Prosoeca) with relatively short probosces and hopliine scarab beetles. In H. latifolia type flowers the perianth is pink to magenta or red (rarely pale yellow), odorless, opens during the day but has an elongate perianth tube exceeding 18 mm in length. These flowers are pollinated mainly by long-proboscid flies in the genera Prosoeca and Stenobasipteron (Nemestrinidae) or Philoliche (Tabanidae), but the red flowers of H. coccinea are pollinated by a guild of large butterflies including Papilio and the satyrid Aeropetes. Lastly, H. vaginata has odorless and nectarless, short-tubed yellow flowers, usually with contrasting dark markings, that open only during the day and are pollinated exclusively by the hopliine scarab beetle, Clania glenlyonensis. The taxonomic distribution of plant species with these pollination systems makes it clear that shifts in pollination systems have occurred repeatedly across Hesperantha, although floral morphology and nectar biochemistry are relatively conservative. Whether flowers are nocturnal, crepuscular, or diurnal, only four variables affect the floral ecology: length of the perianth tube, presence or absence of floral fragrance, timing of anthesis, and the closely associated trait of perianth color. Thus, species with pink, magenta, red, or yellow flowers close at night and are rarely fragrant, whereas those with white or pale yellow flowers are nearly always fragrant and either open late in the day or only at sunset and remain open for most of the night. Species show considerable variation in nectar volume and sugar concentration, closely correlated with pollination system, while two long-tubed species with floral characters typical of long-proboscid fly pollination produce no nectar and are inferred to be Batesian or guild mimics that achieve pollination by deception.