Pollination by long-proboscid flies in the endangered African orchid Disa scullyi

Abstract

Flowers specialized for pollination by long-proboscid flies are particularly prevalent in the southern African flora. Although many orchids in this region possess flowers with traits that are consistent with long-proboscid fly pollination, evidence from field observations is lacking for most of these species. Flowers of the critically endangered orchid Disa scullyi Bolus (Orchidaceae) were observed to be pollinated by the large nemestrinid fly species Prosoeca ganglbaueri Lichtwardt at a remote site in the Cape Drakensberg mountains, South Africa. The orchid's floral spur contains copious amounts of dilute sucrose-rich nectar and its length (c. 42 mm) closely matches that of the fly proboscis (c. 40 mm). Flies caught on the orchids carried pollinaria of D. scullyi on the basal portion of their proboscides. Although flies were not common at the study site, they showed considerable fidelity to D. scullyi, resulting in high levels of pollen removal and deposition in flowers in the population. Habitat loss and trampling by cattle, rather than pollination failure, appear to pose the greatest threat to the few remaining populations of this species.