The long-proboscid fly pollination system in southern Africa

Some 14 species of long-proboscid flies (Diptera) in two families, Nemestrinidae and Tabanidae, with elongated mouth parts are known to pollinate species in several plant families, most importantly Geraniaceae, Iridaceae, and Orchidaceae, across southern Africa. Long-proboscid fly pollination appears to comprise three discrete guilds of flies in the genera Prosoeca, Moegistorhynchus, and Stenobasipteron (Nemestrinidae) and Philoliche (Tabanidue). Flies in the three guilds are on the wing at different times of the year and pollinate different suites of plant species, sometimes with different floral characteristics. The three pollinator guilds operate for the most part in different parts of the subcontinent. Where there is geographical overlap, the periods of activity differ. Plants pollinated by long-proboscid flies have flowers with an elongate, cylindrical floral tube, mostly 35-60 cm long, a perianth of specific colors and marking, a floral reward of nectar, and lack floral fragrance. Pollen is not eaten by these flies, and anthers and pollen are often cryptically colored. Flowers are usually zygomorphic and bilabiate, and the petal or tepal lobes have characteristically shaped nectar guides. With few exceptions, the flowers offer ample nectar of sugar concentration mainly in the 20-30% range. Several orchids and one Pelargonium using long-proloscid flies for pollination offer no nectar, and pollination is accomplished through floral mimicry and deception. In long-proboscid fly pollination systems, plasement of pollen on the insect's body is highly specific, and there are at least six mutually exclusive sites of pollen deposition on an insect's body. When two or more long-proboscid-fly-pollinated plant species co-occur, each typically utilizes a different pollen deposition site. This suggests that pollen contamination is detrimental to reproductive success and that differential pollen deposition sites are important for plants pollinated by long-proboscid flies. Since these flies are the sole or main pollinators of at least 120 plant species and the inferred pollinators of at least 80 more species in southern Africa, they must be considered keystone species in the ecosystems where they occur.