The Prosoeca peringueyi (Diptera: Nemestrinidae) pollination guild in southern Africa: long-tongued flies and their tubular flowers 1
John C. Manning 2 & Peter Goldblatt 3
1. This research was supported by National Geographic Society Grant 4816-92. We gratefully
acknowledge the work of B.-E. van Wyk, Rand Afrikaans University, Johannesburg, who
provided the analyses of sugar nectars. We also thank Peter Bernhardt, Dee Paterson-Jones, and
Kim Steiner for helpful comments during the preparation of this paper.
2. Compton Herbarium, National Botanical Institute, Private Bag X7, Claremont 7735, South
3. B. A. Krukoff Curator of African Botany, Missouri Botanical Garden, P. O. Box 299, St.
Louis, Missouri 63166-0229, U.S.A.
Originally published in the Annals of the Missouri Botanical
Garden 82: 517-534. 1995. Copyright Missouri Botanical Garden 1995.
Abstract | Materials and Methods | Results | Discussion | Literature
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. nov., are active in the late winter and spring and
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 that 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 as 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 and 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 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.
A close association between the form and color of flowers and pollination by a particular
pollinator is well known. Convergence in floral morphology among species that rely on the same
pollinator class led to the recognition of floral syndromes (Faegri & van der Pijl, 1979; Grant,
1981; Vogel, 1954). Those species with morphologically similar flowers that share the same
pollinator species constitute a particular pollination guild, an extension of the term (Root, 1967)
describing a group of species that exploits the same class of resources in a similar way. A guild is
thus a functional unit independent of taxonomic considerations. Although a number of pollination
syndromes have been identified in the southern African flora (Vogel, 1954), very few guilds have
been described. The most striking of those that have been documented is the association between
the butterfly, Aeropetes (Meneris) tulbaghia, and late summer-flowering species with large bright red blossoms (Johnson & Bond, 1994).
Pollination by long-tongued flies is a relatively unusual phenomenon, first documented in
southern Africa by Rudolf Marloth (1908) and later in somewhat more detail by Stefan Vogel
(1954). Although pollination by long-tongued flies has also been reported in India (Fletcher &
Son, 1931) and California (Grant & Grant, 1965) it appears to be particularly well developed
only in southern Africa.
In the western part of southern Africa 28 plant species of Iridaceae and Geraniaceae have
intensely colored purple to crimson flowers with extremely long floral tubes. These species all
occur in a restricted geographic area, flower between July and September, and often occupy
similar habitats. The convergent floral morphology in this group of spring-flowering geophytes
and small shrubs constitutes a distinct floral syndrome and their coincident geography and
phenology suggests that they are members of a specific pollination guild. Some of these species
belong to the genus Lapeirousia (Iridaceae) and have already been found to be pollinated by one
or both of two species of long-tongued flies in the genus Prosoeca (Diptera: Nemestrinidae)
(Goldblatt et al., 1995). The purpose of this investigation is to extend our observations to
determine whether the convergence in floral morphology to the L. silenoides-type flower in these
additional species coincides with pollination by the same fly species. Our results support the
recognition of a distinct pollination guild. We discuss some of the implications of such a
specialized pollination system on plant ecology and evolution and consider its possible origin.
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