Southern Africa, with 24000 plant species (10% of the world's total), has the richest flora for any equivalent-sized region in the world by far (Goldblatt et al. 2000).This is matched by a remarkable range of pollination systems, many of which have been discovered only in the past few decades. Pollination is important because it is a process that is vital for the successful reproduction of the majority of the world's plants (Goldblatt et al. 2000). Most plant species rely on the intervention of animal vectors to carry pollen from flower to flower, and any disruption of this process may lead to diminished seed production (Goldblatt et al. 2000). The Victorian era studies of pollination in South African plants were inspired by the publication in 1877 of Darwin's seminal book The Fertilization of Orchids (1877). Local naturalists attempted to imitate Darwin, but advancement was slow. For example, it took more than 30 years and the efforts of a succession of naturalists before Marloth in 1895 discovered that the spectacular flowers of the orchid Disa uniflora were pollinated by a satyrine butterfly.
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The definition of long-proboscid flies are those insects that have mouthparts at least 15 mm long and a body length of more than 15 mm. Fifteen species in two families, Nemestrinidae and Tabanidae, are known to have mouthparts this long, 14 of them restricted to the southern African region, Lesotho, Namibia, South Africa, and Swaziland, and one to the Himalayas (Goldblatt et al. 2000). Adult flies depend mainly on floral nectar for their nutrition and are devoted foragers of nectar rich flowers. Their visits to the flowers of some plants result in the accumulation of pollen or pollinaria as they brush against anthers, and in turn, the passive transference of pollen or pollinaria to stigmas during visits to other flowers of the same species. Most other Nemestrinidae and Tabanidae have substantially shorter mouthparts and, although they also feed on nectar and pollinate plants, they are not known to be the only pollinator of any plants. Long-proboscid flies are large-bodied insects, typically measuring 15-24 mm from the tip of the abdomen to the base of the proboscis. Moegistorhynchus has 6 species of which three are known to be significant pollinators. Moegistorhynchus is identifiable by its well developed proboscis. Mouthparts are as long as, or often substantially longer than, the insect's body, the most extreme example being Moegistorhynchus longirostris where individuals along the Cape West coast have been recorded with proboscis lengths of up to 100 mm (Goldblatt et al. 2000).
Moegistorhynchus as a pollinator
The behaviour of Moegistorhynchus Longirostaris during foraging is very similar to the foraging behaviour of Prosoeca species. Nemestrinid species fly with downward hanging mouth parts and forage for nectar by extending their flexible proboscis forward or downwards, depending on the particular flower (Goldblatt P et al. 1995). Male M. Longirostris flies defend small areas very aggressively which are located in dense nectar plants and mating pairs usually stick together when in these dense areas (Pauw A et al. 2008).
During foraging, pollen is deposited on the dorsal of some species with accurate stamens and ventral in species with declinate stamens. Pollen deposition is on the front / and / or the base of the probiscid when the anthers are held close to the mouth of the floral tube (Manning JC et al. 1996). In the long probiscid fly pollination system, the placement of pollen on the body of a fly is an important consideration. The frons and base of the proboscis, the dorsal part of the head and thorax, ventral part of the thorax and abdomen are used as pollen deposition sites for many plant species (Manning JC et al. 1996). Although some Orchidaceae pollinaria is deposited near the base of the proboscis (Johnson SD et al.1997).
Pollination guild of Moegistorhynchus longirostris
Three separate pollination systems or guilds occur within the long probiscid fly pollination strategy. Little or no overlap is shown for the fliers that belong to each system. Each system has well defined guilds of plant species that have one or occasionally 2 fly species as their sole pollinator (Manning JC et al. 1996).
1. The first guild is the Prosoeca peringuey which includes 2 fly species (Manning JC et al. 1996).
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2. The second guild is the guild of Moegistorhynchus Philoliche. This genera includes six or even seven different fly species. Two tabanid species have the widest ranges. Three Moegistorhynchus species and one or two prosoeca species of the system have narrow ranges. These flies are mostly active from late spring to early summer, mid September to November. Pollination depends on the geography of the area as long floral tubes are found on the West Coast and these flowers are associated exclusively with a single pollinator such as Moegistorhynchus Longirostris (Manning JC et al. 1996).
3. The third is the Prosoeca ganglbaueri guild named for the most widespread and common fly species in the system (Manning JC et al. 1996).
The pollination guild of M. longirostris consists of 20 endemic plant species that grow on the sand plain along the west coast of South Africa (Barraclough, 2006; Goldblatt and Manning, 2000;Johnson, 2003;2009; Pauw et al., 2008). These plants have common floral characteristics such as, pale-coloured flowers, elongate floral tube, and usually no scent (Barraclough, 2006a; Goldblatt and Manning, 2000; Pauw et al., 2008). Another common characteristic is the small amount of nectar in the flowers (Johnson and Steiner, 1997). Laperirousia anceps is the most common member of this plant guild, its leaves begin to emerge in late winter and plant flower in early summer (Pauw et al., 2008).
M. longirostris is important as a pollinator because there are particular plant species that appear to be pollinated exclusively by these flies. The L. Fibricii and L. Fibricii type flowers appear to be exclusively pollinated by M. longirostris and Philoliche gulosa. Among the species with this type of flower, L. Anceps sets apart, with its unusually long floral-tubes 20 -76 mm. Populations occurring at the west and north of its range have extremely long floral tubes (47 -76mm), this suggests that they are exclusively pollinated by M. longirostris, because only this fly can access nectar, given its proboscis length (Manning et al., 2006).
As shown in figure 1, the distribution of L. anceps extends in to the mountains (Uitkomsberge and Red Hill) where M. longirostris is replaced by shorter proboscid horse flies, Philoliche species (Pauw et al., 2008). The proboscis length of M. longirostris is not uniform over its pollination range; there is a notable trend of longer proboscis in the north to shortest in the south (Barraclough, 2006; Pauw et al., 2008). Because of the differences in peak flowering between the north and the southern populations, M. longirostris abundance in the north and southern limits occurs at different times of the year (Pauw et al., 2008). In the north peak flowering and fly abundance occurs in late September. In the south, peak flowering and fly activity occurs in early November (Pauw et al., 2008).
Specialized pollination systems in Southern Africa such as, the M. longirostris pollination system tend to have a large number of plants depending on the same pollinator (in this system M. longirostris) or a small set of pollinators with of the same type (Johnson, 2010). Thus, M. longirostris can be considered the keystone species-a species that influences a disproportionally large ways relative to its abundance (Starr and Taggart, 2006).
Co-evolution and Reciprocal selection
As suggested by Feinsinger in 1983 for sphinx moths, it seems that long-probiscid flies and their flowers have probably evolved through reciprocal selection. Visits from long-proboscid flies select long tube flowers, which in turn select long proboscid flies that are capable of reaching the nectar that lies down the tubes. The Red Queen effect (Manning JC et al. 1996) fits this pattern where species co-evolve indefinitely, some becoming extinct in the process or at a static non-evolving equilibrium. Evolution within the system may not be so simple as population densities of long-probiscid flies appear to be very erratic (Goldblatt et al., 1995).
Charles Darwin hypothesized that, the benefit to individual pollinator and plant species depends on the length of their interacting proboscis and floral tubes. He hypothesized that long-proboscid flies will forage effectively, when the proboscid length exceeds the floral tube, because flies with longer proboscis will drain almost all the nectar (Pauw et al., 2008). Effective pollination will occur if the floral tube, because the head or body of the insect make contact with the reproductive parts (located at the entrance of the flower) when they are forced to insert their entire proboscis to drain all the nectar in depths of the flower (Anderson and Johnson, 2007; Pauw et al., 2008). As a result pollinators should evolve longer proboscis to reach the nectar hidden in depths of floral tube.
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This positive feedback system can result in to a closely matched co-evolutionary arms race, capable of producing extraordinary long proboscis traits (Anderson and Johnson, 2007). However, Pauw et al (2008), states that it has not been shown that floral tube can act as a selective force on the proboscis length. Several researchers, however have found that individuals with the longer floral tubes do sometimes have higher reproductive success, these studies suggests that proboscis length can act as a selective agent on floral tube length (Pauw et al., 2008).
The mechanism of fluid feeding uses capillary forces for fluid uptake, and is widespread among insects, including those that specifically visit plants to consume floral nectars. The elongation of mouthparts enables insects to develop a pressure gradient along the food canal, allowing them to consume nectar from the concealed nectarines found in long, tubular corollas. This type of proboscis, termed a "concealed nectar extraction apparatus" by Jervis, often matches or exceeds the body length. At 280 mm, a tropical sphingid holds the record for mouthpart length in absolute terms. Relative to body length, however, record holders are South African nemestrinid flies, namely the Moegistorhynchus longirostris whose proboscid may be over four times the length of their bodies. A number of contrasting evolutionary pathways have preceded the development of these long, suctorial mouthparts in various taxa. Suctorial nectar feeding via an elongate proboscis has arisen multiple times in Diptera.
Recent studies have indicated that long tongues flies are important pollinators in the Cape region of Southern Africa (Johnson SD et al. 1997). On the sandplains on the West Coast of Southern Africa, Moegistorhynchus Longirostris was mainly observed visiting long tubed flowers of Pelargonium suburbanum. This species of flower looks much like an orchid (Johnson SD et al. 1997). Other flowers that were visited were Ixia Paniculata and Lapeiraisia anceps. Species with morphologically similar flowers that share the same pollinator species constitute a particular pollination guild. It is an extension of the term that describes a group of species that exploits the same class of resources. The definition of a guild is therefore a functional unit dependant of taxonomic considerations (Manning JC et al. 1996).
Threats and conservation
Africa, which has about 20,400 species of flowering plants (Goldblatt, 2000). Although, long-proboscid fly pollination assumes more than a marginal importance in at least two families in Southern Africa, Geraniaceae and Iridaceae. It is estimated that 25% of Southern African species of Pelargonium of the family Geraniaceae are pollinated by long proboscid flies (Goldblatt, 2000).
Southern Africa flora is rich and usually diverse for an area that falls under temperate latitudes (Goldblatt P, 2000). Thousands of species of native vascular plants occur in the region of about 50% are endemic. Factors such as climate, edaphic, topographic diversity and a history of paleoclimatic change in the late tertiary are thought to be an account for the species richness of the area (Goldblatt P, 2000).
Long-proboscid fly pollination poses important concerns for conservation. Plants pollinated by distinct insects or maximum two over their entire range are certainly more at risk that those that are pollinated by several different insects (Goldblatt P, 2000). Flies with long-proboscid may be regarded as keystone species for several plants as those plants rely on particular flies for their sexual reproduction and pollination.