THE MOEGISTORHYNCHUS-LONGIROSTRIS (DIPTERA, NEMESTRINIDAE) POLLINATION GUILD - LONG-TUBED FLOWERS AND A SPECIALIZED LONG-PROBOSCID FLY POLLINATION SYSTEM IN SOUTHERN AFRICA

A guild of 20 late spring- and early summer-flowering species of Irida ceae, Geraniaceae and Orchidaceae is pollinated partly or exclusively by the long-proboscid fly Moegistorhynchus longirostris (Nemestrinidae ). This large-bodied fly, active in late spring and early summer, has mouthparts 40-70 (-90) mm long and forages for nectar from a variety o f species. These plants share a suite of convergent floral features in cluding a straight or weakly curved floral tube usually 50-70 mm long but sometimes to 90 mm, relatively short petals or tepal lobes coloure d white, cream or salmon with reddish nectar guides, and often violet or red anthers and pollen. Flowers of most species with these characte ristics are zygomorphic with the stamens either arcuate (most Iridacea e) or declinate (Geraniaceae and some Iridaceae). The flowers are odou rless and typically secrete large amounts of nectar of relatively cons tant sugar concentration, mostly 24-29%, with a high sucrose:hexose ra tio. Guild members utilize five separate sites of pollen deposition on the body of the fly, typically utilizing different deposition sites w hen two or more co-occur, indicating strong selection to aviod pollen contamination. M. longirostris is restricted to the west coast of sout hern Africa and at least 8 species appear to depend exclusively on the insect for pollination. The remaining species in the guild are pollin ated by one or both of the long-proboscid flies Philoliche gulosa and P. rostrata (Tabanidae) over other parts of their range. Species and r aces pollinated entirely by M. longirostris have longer floral tubes w hich makes nectar unavailable to other insects, including other specie s of long-proboscid fly. The only insect with mouthparts long enough t o forage effectively on these long-tubed flowers is M. longirostris an d this fly must be considered a keystone species in the ecosystems in which it occurs.