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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 Africa.
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

ABSTRACT

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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