Since the time of Darwin, biologists have considered the floral nectar spur to be an adaptation representing a high degree of plant specialization. Nevertheless, some researchers suggest that nature is more complex and that even morphologically specialized plants attract a wide spectrum of visitors. We observed visitors on Impatiens burtonii (Balsaminaceae) and measured the depth of the proboscis insertion into the spur, the distance of the nectar surface from the spur entrance and the visitor's effectiveness. The hoverfly Melanostoma sp., with the shortest proboscis, was most active early in the morning and fed on pollen and nectar near the spur entrance. The honeybee Apis mellifera and the hoverfly Rhingia mecyana were the most frequent visitors before and after noon, respectively. Although R. mecyana, the only visitor able to reach the end of the spur, was the most frequent, it did not deposit the largest number of pollen grains per visit. Nectar spurs may function as complex structures allowing pollination by both short- and long-proboscid visitors and separating their spatial and temporal niches. Spurred plants should be considered as more generalized and exposed to more diverse selection pressures than previously believed.

Biology Centre Institute of Entomology The Czech Academy of Sciences Branišovská 31 CZ 370 05 České Budějovice Czech Republic

Department of Ecology Faculty of Science Charles University Viničná 7 CZ 128 44 Praha 2 Czech Republic

Faculty of Science University of South Bohemia Branišovská 31 CZ 370 05 České Budějovice Czech Republic

Institute of Botany The Czech Academy of Sciences Dukelská 135 CZ 379 82 Třeboň Czech Republic

Zoologisches Forschungsmuseum Alexander Koenig Leibniz Institut für Biodiversität der Tiere Adenauerallee 160 53113 Bonn Germany

References provided by Crossref.org

Flower visitation by hoverflies (Diptera: Syrphidae) in a temperate plant-pollinator network