The Prosoeca peringueyi (Diptera: Nemestrinidae) pollination guild in southern Africa: Long-tongued flies and their tubular flowers

A guild of 28 winter- and spring-flowering species of two plant families, Iridaceae and Geraniaceae, with intense purple to crimson flowers and extremely long and slender perianth tubes, is pollinated exclusively by two long-tongued flies of the family Nemestrinidae. The two species of flies, Prosoeca peringueyi and P. sp. no., are active in the late winter and spring, have large bodies, mouthparts 20-50 mm long, and forage for nectar while hovering. Plants pollinated by these two flies share a suite of convergent floral characteristics, including a straight or slightly curved floral tube at least 20 mm and up to 70 mm long, relatively short petals or tepals colored predominantly dark blue- or red-purple with pale nectar guides, and anthers and stigmas exserted from the tube and usually unilateral in orientation. with one exception, the flowers of all species secrete large amounts of nectar of relatively constant total sugar concentration, mostly 24-29%, and high sucrose:hexose ratio. Most members of the guild have odorless flowers. The long floral tube makes nectar unavailable to most insects. including a variety of bees, wasps, and other flies that pollinate plants which co-occur with members of the long-tubed flower guild. The two Prosoeca species have mouthparts long enough to forage effectively on these long-tubed flowers, and they are also effective pollinators because pollen adheres to their bodies and is transported from flower to flower. The flies visit a wide range of plants but are effective pollinators only of those with tube lengths greater that their proboscis lengths. We have identified four mutually exclusive sites of pollen deposition on the insects' bodies: when two or more members of the guild co-occur, each species typically utilizes a different pollen deposition site. This suggests that pollen contamination is detrimental to reproductive success. Differential pollen deposition sites may have evolved in response to selection for reduced pollen contamination. Since 27 of the 28 plant species appear to depend exclusively on these two species of Prosoeca for pollination, these flies must be considered keystone species in the ecosystems where they occur.