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The Prosoeca peringueyi (Diptera: Nemestrinidae) pollination guild in southern Africa: long-tongued flies and their tubular flowers

Abstract | Materials and Methods | Results | Discussion | Literature


Plant Characteristics

A total of 28 plant species occurring along the west coast and near interior of southern Africa and mostly endemic there, were identified as converging on Lapeirousia silenoides in floral morphology Table 3. These include 22 species of Iridaceae in the genera Babiana, Geissorhiza, Hesperantha, Lapeirousia, Romulea, Sparaxis, Tritonia, and Xenoscapa, and six species of Geraniaceae, all in the genus Pelargonium. These include seasonal geophytes (Iridaceae and Pelargonium incrassatum) and small to moderate-sized shrubs (Pelargonium cortusifolium, P. crassicaule, P. echinatum, P. magenteum, and P. sericifolium). While the habits and growth forms of the various species vary, their flowers share several unusual properties and may be considered to constitute a distinct floral syndrome Figs. 7-9. The floral tube is straight or slightly curved to sigmoid, very narrow (1.5-2.5 mm in diameter), and (18-)30-70 mm long, and the petals or tepals are shorter than the tube. The flowers are typically zygomorphic with stamens and styles unilateral but are actinomorphic in five species in which the stamens are symmetrically arranged around a central style Table 3. In all species except Pelargonium sericifolium (which does not produce nectar - Goldblatt et al., 1995) nectar accumulates at the base of the floral tube and fills its lower third. Nectar is thus accessible only to insects with tongues long enough to reach at least into the lower third of the tube.

The flowers are mostly intensely pigmented in colors ranging from dark blue-purple and violet to bright red-purple or cerise but in taxa from the Richtersveld (northern Namaqualand) and southern Namibia are pale mauve. Contrasting markings in white or cream are almost always present usually accompanied by additional darker areas of pigmentation Figs. 7-9. The markings, which may take the form of streaks or spots near the tepal bases in the case of species of Iridaceae, are confined to the lower tepals in species with zygomorphic flowers but are present on all the tepals in species with actinomorphic flowers. In Pelargonium Fig. 7 the pale color signal is to be provided by the white filaments which are unilateral and declinate, rather than by tepal coloring. Whatever the shape and color of the flowers, the anthers and stigmatic surfaces are always held outside the mouth of the floral tube in a position where they will be brushed by the body of an insect probing the tube. Pollen is often inconspicuous and of the same color as the tepals, especially in Lapeirousia, or may be white, possibly adding to the signal provided by the contrasting color of the perianth. In P. incrassatum and P. magenteum the pollen is bright orange. We have no data on ultra-violet light reflectance in any of the species under consideration and it is possible that differential reflectance in the UV light range may add to the visual signals evident in the visible range.

Flowers open during the day and often close partially or fully at night. They are typically unscented, at least to the human nose both in the open air and when several flowers are held in a warm confined space. Some species of Babiana, however, have a light to moderately strong scent Table 3 . The flowering season in the guild ranges from late May (one species) with a marked rise in July and a peak in early September, and continues until early October Fig. 10. Individual species and populations remain in flower for at least two weeks, or for a considerably longer time in the case of the shrubby Pelargonium species. Individual flowers usually last three to four days, and longer in species of Iridaceae when not pollinated. Species of Iridaceae are protandrous. The pollen is shed half a day to one day before the stigmas unfold and become available for pollen deposition. Unless removed by some agent, the pollen remains in place in the anther thecae. Species of Pelargonium are also protandrous. The deciduous anthers are shed the same day that the flower opens whereas the stigmas only unfold the following day. Flowers of the species of the guild are almost all herkogamous (and self-incompatible at least in L. dolomitica and L. silenoides) and thus require and require insect mediated pollination. The only known exception is L. jacquinii which is self-compatible and autogamous (Goldblatt et al., 1995).

Nectar quantities are ample, and the upper range of nectar volumes for species in the guild is 1.1 ml to 9.6 ml Table 2. Nectar sugar concentrations are mostly 24-30%, exceptionally as low as 20% in Romulea hantamensis and 22% in Babiana dregei Table 2. Nectar sugar analyses, available for 17 species Table 2, show a characteristic sucrose-rich to sucrose-dominant pattern in the 15 species of Iridaceae examined. Sucrose:hexose sugar ratios range from a high of 4.13 in Lapeirousia dolomitica subsp. dolomitica to 1.20 and 0.94 in the two species of Hesperantha for which we have data, and 1.00 in Romulea hantamensis, the only species of that genus belonging to the guild. Most species have sucrose:hexose ratios in the 2 to 3.5 range. The pattern in two species of Pelargonium, however, shows hexose dominance with sucrose:hexose ratios of 0.28 and 0.33. This is a marked contrast to the spectrum for Iridaceae.

Pollinator Identity

Pollinator observations were obtained for 17 out of the total listing of 28 plant species Table 4. These species are from throughout the range occupied by members of the guild. In all of these instances pollination was carried out by either Prosoeca peringueyi or P. sp. or rarely both Figs. 1, 2 . No other insects were seen to visit any of the species of the guild during a total of over 200 hours of observation time except for three anthophorod bees which visited but did not forage on individuals of L. silenoides. It is almost certain that the plant species for which we do not have pollinator observations will prove to be one or both of these fly species. Because the fly species are allopatric we have inferred pollinator indentity on the basis of the range of the plant species for which we have no observations. Prosoeca peringueyi is confirmed as the primary pollinator of three species of Geraniaceae and eleven species of Iridaceae and P. sp. of four species of Iridaceae Table 4.


The plant species with the Lapeirousia silenoides-type flowers are restricted to coastal and near interior southern Africa, a semi-arid region of low, predominantly winter rainfall. The 28 species have a collective range that extends from extreme southwestern Namibia through the western part of Northern Cape Province, South Africa, an area known as Namaqualand, and reaches the northwestern portion of Western Cape Province Fig. 11. The distribution of individual species within this area is often highly local (Goldblatt, 1972, 1984, 1985; Lewis, 1959; van der Walt & Vorster, 1988). At most seven species of the guild are present in any quarter degree square and no more than four guild members co-occur locally. Diversity is greatest in the Kamieskroon area of Namaqualand and in the Pakhuis Mountains of Western Cape Province. Usually the ranges of two or more species overlap. Up to seven species have been recorded in a quarter degree square of geographical latitude and longitude. Species are infrequent in the north of the range, where only L. dolomitica subsp. dolomitica, Tritonia marlothii, Pelargonium cortusifolium and P. crassicaule occur, and along the coast where the putatively autogamous L. jacquinii is the only representative. The combined ranges of Prosoeca peringueyi and P. sp. Fig.12 fall entirely within the main range of the plants species and accord almost exactly with the area within which two or more plant species occur. Prosoeca peringueyi has the wider distribution, extending from northern Namaqualand to the Pakhuis Mountains and P. sp. has a localized range along a corridor of high country in Northern Cape Province between Nieuwoudtville and the Hantam Mountains.

Pollen Placement

Observation of living and pinned insects, corroborated by pollen washes, confirmed that pollen of a particular plant species is consistently deposited on a limited part of the insect's body Table 4 . We have identified four mutually exclusive sites of deposition:

  • top of the thorax or dorsum;
  • top of the head or frons;
  • the base of the proboscis or face;
  • and underside of the thorax or sternum and abdomen Fig. 13.

Pelargonium flowers have declinate stamens so that the filaments and anthers are situated below the mouth of the floral tube Fig. 7 and pollen deposition is invariably sternotribic. In P. incrassatum Fig. 7C, which has long filaments, pollen is deposited on the underside of the sternum and thorax Fig. 13D but in P. sericifolium and P. magenteum Fig. 7A-B, 13C, which have very short filaments, deposition is on the face. Species of Iridaceae belonging to the guild have either actinomorphic flowers with symmetrically disposed stamens Fig. 9B, D, E or zygomorphic flowers with the stamens unilateral and arcuate Fig. 8, 9A, C . The anthers are then either held above the mouth of the floral tube or are dorsal to it and pollen deposition is nototribic Fig. 13A, B. Filament length is also variable, being short in species of Lapeirousia and Romulea but relatively long in Babiana and the site of pollen deposition varies accordingly. In species of Babiana pollen deposition is mostly on the top of the thorax (in one species on the top and sides), in Hesperantha on the ventral head, whereas in Lapeirousia and Romulea it is on the frons.

Often there are at least two species of the guild, and sometimes more, co-blooming locally and visited indescriminately by P. peringueyi. At such sites, particularly in Namaqualand, Lapeirousia silenoides, Pelargonium incrassatum, and one species of Babiana, either B. curviscapa, B. dregei, B. framesii, or B. pubescens flower together. In the Clanwilliam District L. jacquinii, sometimes L. violacea, Pelargonium magenteum, and a species of Babiana and or Sparaxis commonly form part of a local plant community. At sites in the Kamiesberg, central Namaqualand, as many as four co-blooming members of the guild were recorded. Lapeirousia silenoides, B. curviscapa, Hesperantha latifolia and Pelargonium incrassatum were noted near Leliefontein, and on Sneeukop we encountered Babiana dregei, H. latifolia, and two other presumed members of the guild, Xenoscapa uliginosa (endemic there) and Geissorhiza kamiesbergensis.

A similar situation prevails with species visited by Prosoeca sp. nov. At different sites Babiana framesii and either Lapeirousia oreogena or L. jacquinii, or L. jacquinii and B. sambucina, or B. flabellifolia, Hesperantha oligantha, and Romulea hantamensis flower concurrently and are visited indiscriminately by the same fly individual. In the above examples, pollen contamination of one species by that of another is minimized or prevented by the use of different deposition sites as outlined above. At any study site that included two or more species of the guild the pollen of each species was placed on a different part of an insect's body e.g., Fig. 13 . The only exception to this pattern was at Botterkloof Pass (and presumably other localities where these two species co-occur) where pollen deposition sites for Lapeirousia jacquinii and L. violacea are identical.

A crude estimate of potential pollen contamination was determined by comparing the number of guild members recorded from any quarter degree square with the number of pollen deposition sites utilized by these species. The number of placement positions exploited in any quarter degree square is positively correlated with the total number of guild members occurring in that grid Fig. 14 . A coefficient of pollen contamination was calculated for grids containing more than one species of the guild by dividing the number of species into the number of loading sites per quarter degree grid. For the species in the guild the mean coefficient was 0.77, suggesting selection for reduced pollen contamination. The index greatly underestimates the pollen contamination coefficient because not all of the species in a grid co-occur locally. The local co-occurrence of more than one species using the same pollen deposition site is rare and we only know of the single example mentioned above.

Foraging Patterns

Adult specimens of Prosoeca peringueyi have been collected from late July to late September with a peak during mid August to mid September. Specimens of P. sp. have only been recorded from mid August to mid September. Both species have a similar foraging behavior. The flies move rapidly between flowers and hover for two to three seconds while orienting and inserting their proboscis into the floral tube (Goldblatt et al., 1995) Figs. 3-6 . In species with zygomorphic flowers the fly always orients itself in the same way, approaching the flower directly from the front. The flies are unable to discriminate between flowers which have already been visited either by themselves by other individuals and have been seen to repeatedly visit the same flower even when all the nectar has been removed. When nectar is absent visits are abbreviated.

Both fly species are active on mild to warm days from mid-morning to early afternoon, and again in the late afternoon. Foraging is most active on warm days between 12:30 and 2:30 PM but some foraging occurs at almost any time of day. Density of visitors varies considerably, ranging from 4-5 flies present locally at the same time, or as few as one or two flies may be seen at sites over periods as long as an hour. Flies remain at each flower for 3-5 seconds and pollen is passively brushed onto various parts of the head, thorax, or abdomen, depending on the species visited. Pollen depositions are usually heavy enough to be visible to the naked eye against the dark bodies of the insects. Commonly pollen of two or three different species can be distinguished by color, that of each species located on a different part of the body.

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