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Pollination Biology of Lapeirousia subgenus Lapeirousia (Iridaceae) in southern Africa; floral divergence and adaptation for long-tongued fly-pollination


Abstract | Materials and Methods | Results | Discussion | Literature

METHODS

Lapeirousia species grow in a variety of habitats ranging from montane areas to coastal flats, and in various soils including coarse sands of various origins, shales and clays, and granites. Plants are often fairly common locally, but species have distribution patterns ranging from extremely local to widespread. Species of subgenus Lapeirousia include local endemics such as L. verecunda and L. simulans, both known from single extended populations in Namaqualand, and widespread species such as L. anceps, L. fabricii, and L. pyramidalis, that extend over half the entire southern African range of the subgenus. The species studied, study sites, and voucher information are listed in Table 1. Voucher specimens were made for all populations of Lapeirousia studied, as for species of other plant taxa observed to be visited by the same insect taxa collected on Lapeirousia. Plant vouchers are deposited at the Missouri Botanical Garden Herbarium, St. Louis (MO), and the Compton Herbarium, Cape Town (NBG).

Observation of insect foraging involved 4-20 hours per plant species from 1992 to 1994, and included aspects such as the frequency (number of visits per unit time) and taxonomic diversity of floral foragers, and how they removed rewards from flowers. When observed to probe the floral tube or contact the anthers or stigma, insects were captured and killed in a jar using ethyl acetate fumes. Location of pollen deposits was based on the examination of pinned insects. Pollen was removed from individual insects after pinning by placing the insect on a glass slide and gently rinsing the whole body in 100% ethanol while gently dislodging pollen loads on the frons, thorax, and in the case of bees, hind legs with a dissecting needle. When the ethanol had evaporated the pollen residue was stained and mounted in 1­2 drops of Calberla's fluid (Ogden et al., 1974). To prevent contamination of the body of an insect with pollen carried by another in the same killing jar, the bodies of insect specimens were isolated from each other by wrapping them in tissue. The pollen of a plant species was scored as present on the body of an insect if more than 10 individual grains (or polyads) were counted on the slide (Table 2).

Field estimates of nectar volume were made from unbagged flowers, thus nectar volumes (Table 4) represent the result of secretion and exploitation. To extract nectar, flowers were picked and nectar was withdrawn from the base of the floral tube with 3-µl capillary tubes after separating the ovary from the perianth. Nectar samples were dried on Whatmans filter paper no. 1 and sent to B.­E. van Wyk, Rand Afrikaans University, Johannesburg, for analysis (Table 3). The percentage of sugars dissolved in fresh nectar (Table 4) was recorded on a Bellingham & Stanley hand­held refractometer (0­50%) using nectar extracted from flowers in the manner described above. Most nectar sugars were analyzed in the field, but a few samples were taken from flowers of cut stems placed in water. The chemistry of nectar sugars was evidently not affected by the latter treatment, but nectar volumes were found to be higher than in specimens examined in the field, presumably because visitors were excluded, and water availability was not limiting.

Identification of all insects collected (Tables 2, 5), excluding bees, was made by comparing our specimens with existing collections. Bee identifications except for Apis, Parafidelia, and Tetraloniella (provided by V. Whitehead, South African Museum, Cape Town) were made by C. D. Michener and R. W. Brooks, Snow Entomological Museum, Lawrence, Kansas. Insect voucher specimens are housed at the Snow Entomological Museum, Lawrence, Kansas and the Natal Museum, Pietermaritzburg, South Africa.

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