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Monday, February 20, 2017

Spurs: Hard to Get Nectar

Violet.
© Beatriz Moisset
 Many flowers are very particular about their clientele. They have developed ways to make themselves attractive to certain flower visitors while discouraging others. They do so by adopting a certain shape. The spurs of many flowers serve this purpose. A petal or a sepal develops an elongated hollow spike. This is what botanists call a spur.

Violet (Viola mirabilis). Note the spur
Wikicommons. © Antti Bilund

Violets have a spur. Nectar is collected at its bottom. A pollinator needs a tongue long enough to reach the hidden nectar waiting in this special vessel. The rest of the flower is so designed that the insect gets covered with pollen while performing this task. If it is carrying pollen from a previous flower visit, it is likely to deposit it on the stigma. Only long tongued insects can take advantage of this flower and they need to develop an efficient way to approach this specially shaped blossom. After a few times they get better and better and proceed faster. The most common pollinators of violets are small, solitary bees.

Bumble bee visiting a jewelweed blossom
© Beatriz Moisset
Jewelweed (Impatiens) also possess a spur. The flower itself is a small chamber just a little larger than a bumble bee. The petals form a curtain that slightly blocks the entrance to this chamber. Bumble bees are pros at collecting nectar from these flowers. Their plump bodies fit inside the chamber like a finger in a glove and their long and flexible tongues are suited to the curved spur.

Columbine.
© Beatriz Moisset

Columbines have not one but five spurs. Each petal is shaped like a long hollow horn ending on a knob. This is where the nectar gathers. Long tongued insects and hummingbirds pollinate these flowers. In most species, the flower nods or points downward, and the spurs point to the sky. This arrangement seems to be agreeable to their most common pollinators, hummingbirds.

Columbine.
© Beatriz Moisset
By contrast, the flowers of the alpine columbine remain erect, don't nod. They are also creamy white, a color favored by moths. And thus, they are preferably pollinated by large moths, the so called hawkmoths. A few other species of columbines, including the Colorado blue columbine, also face upwards and are also pollinated by moths.

Columbine and carpenter bee
stealing nectar
© Beatriz Moisset
Jewelweed and ants
stealing nectar
© Beatriz Moisset
This specialization has advantages but it also has some disadvantages. Not all flower visitors behave like honest pollinators. These difficult flowers are an invitation to cheating. Some visitors learn to take a shortcut, especially if their tongues aren't long enough to reach the bottom of the spur. They approach the flower from the side. Perhaps, they can smell the sweets through the walls of the spur. A quick bite or a stab through the delicate petal may be all it takes to reach the hidden food. Carpenter bees, bumble bees and even ants have been seen performing this robbery.

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