The shape of the little sac fits the body of the bumblebee like a glove; the side petals open like a pair of curtains to allow its entrance; the length of its spur, full of nectar is just right for the tongue of the bumblebee. And, finally and most important to the plant, the anthers (that carry the pollen) and the stigma (which receives the pollen) are placed so that the hairy back of the bumblebee rubs against them when entering the flower. The pollen is deposited on the bee and later on it is transported to other flowers.
For pollination to take place the bee has to enter the front of the flower otherwise it would fail to touch the parts of the flower that matter.
Despite this marvelous system, some very nice pollinators can turn into robbers and cheat the flowers that they usually serve diligently. This happens when the pollinator chooses to take a shortcut and bypass the well planned scheme of the flower. Such is the case of this bumblebee. When it takes nectar by slashing the spur from the outside it doesn't come near the pollen carrying organs and doesn't perform pollination.
Pollinators: Robbers and Thieves
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Beginners Guide to Pollinators and Other Flower Visitors
© Beatriz Moisset. 2012