On the evolutionary significance of flower nectar spur

Howerfly Rhingia sp. © Thomas Bresson, CC BY 2.0

In 1862, Charles Darwin published a book called On the various contrivances by which British and foreign orchids are fertilised by insects. In this book, Darwin first assumed that the long nectar spur of some flowering plants, such as columbines and Star of Bethlehem orchid, is the result of coevolutionary race between plant and its specific insect pollinator whose long proboscis makes it possible to reach nectar at the bottom of the spur. An alternative hypothesis assumes that the plants with long nectar spurs shift between pollinators as they evolve. Although these hypotheses are widely accepted, the empirical data suggests that plant–visitor interactions are much more generalized than previously thought and even plants with highly specialized floral traits are visited by wide range insect species.

Anna Vlašánková at the Department of Zoology, Faculty of Science, together with her colleagues at the Entomological and Botanical Institute of the Czech Academy of Sciences, and the Department of Ecology of Charles University in Prague decided to find out how the morphology of the flower nectar spur and the amount of nectar in it are related to the morphology of its insect visitors. In order to find out, they investigated the Cameroon impatiens species Impatiens burtonii and insects that visit its flowers.

Researchers have found that all three most common species of insect visitors with different length of proboscis are able to pollinate the plant. The number of each visitor changed during the day as the amount of nectar in the spur changed. The hoverfly Melanostoma, with the shortest proboscis, is most active early in the morning when the spur is full of nectar. Around midday, the honeybees with a medium long proboscis are the most common visitors. In the afternoon, the hoverflies Rhingia, with the longest proboscis, sip the remaining nectar from almost empty spurs. Contrary to the expectations, the most successful pollinators were honeybees with a medium long proboscis.

It seems that a long nectar spur is not an adaptation to a single, narrowly specialized pollinator species. Rather, it may function as a complex structure allowing pollination by a wider range of pollinators, which, consequently, increases the likelihood of successful pollination.

Vlašánková, A., Padyšáková, E., Bartoš, M., Mengual, X., Janečková, P., & Janeček, Š. (2017). The nectar spur is not only a simple specialization for long‐proboscid pollinators. New Phytologist215(4), 1574-1581.