TheMoegistorhynchus longirostris (Diptera: Nemestrinidae) pollination guild: long-tubed flowers and a specialized long-proboscid fly pollination system in southern Africa

Abstract

A guild of 20 late spring- and early summer-flowering species ofIridaceae, Geraniaceae andOrchidaceae is pollinated partly or exclusively by the long-proboscid flyMoegistorhynchus 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 of species. These plants share a suite of convergent floral features including a straight or weakly curved floral tube usually 50–70 mm long but sometimes to 90 mm, relatively short petals or tepal lobes coloured white, cream or salmon with reddish nectar guides, and often violet or red anthers and pollen. Flowers of most species with these characteristics are zygomorphic with the stamens either arcuate (mostIridaceae) or declinate (Geraniaceae and someIridaceae). The flowers are odourless and typically secrete large amounts of nectar of relatively constant sugar concentration, mostly 24–29%, with a high sucrose:hexose ratio. Guild members utilize five separate sites of pollen deposition on the body of the fly, typically utilizing different deposition sites when two or more co-occur, indicating strong selection to aviod pollen contamination.M. longirostris is restricted to the west coast of southern Africa and at least 8 species appear to depend exclusively on the insect for pollination. The remaining species in the guild are pollinated by one or both of the long-proboscid fliesPhiloliche gulosa andP. rostrata (Tabanidae) over other parts of their range. Species and races pollinated entirely byM. longirostris have longer floral tubes which makes nectar unavailable to other insects, including other species of long-proboscid fly. The only insect with mouthparts long enough to forage effectively on these long-tubed flowers isM. longirostris and this fly must be considered a keystone species in the ecosystems in which it occurs.