THE PROSOECA-PERINGUEYI (DIPTERA, NEMESTRINIDAE) POLLINATION GUILD INSOUTHERN AFRICA - LONG-TONGUED FLIES AND THEIR TUBULAR FLOWERS

A guild of 28 winter- and spring-flowering species of two plant famili es, Iridaceae and Geraniaceae, with intense purple to crimson flowers and extremely long and slender perianth tubes, is pollinated exclusive ly by two long-tongued flies of the family Nemestrinidae. The two spec ies of flies, Prosoeca peringueyi and P. sp. no., are active in the la te winter and spring, have large bodies, mouthparts 20-50 mm long, and forage for nectar while hovering. Plants pollinated by these two flie s share a suite of convergent floral characteristics, including a stra ight or slightly curved floral tube at least 20 mm and up to 70 mm lon g, relatively short petals or tepals colored predominantly dark blue- or red-purple with pale nectar guides, and anthers and stigmas exserte d from the tube and usually unilateral in orientation. with one except ion, the flowers of all species secrete large amounts of nectar of rel atively constant total sugar concentration, mostly 24-29%, and high su crose:hexose ratio. Most members of the guild have odorless flowers. T he long floral tube makes nectar unavailable to most insects. includin g a variety of bees, wasps, and other flies that pollinate plants whic h co-occur with members of the long-tubed flower guild. The two Prosoe ca species have mouthparts long enough to forage effectively on these long-tubed flowers, and they are also effective pollinators because po llen 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 res ponse to selection for reduced pollen contamination. Since 27 of the 2 8 plant species appear to depend exclusively on these two species of P rosoeca for pollination, these flies must be considered keystone speci es in the ecosystems where they occur.