Birthworts are a family of shrubs and vines with about 600 members. One kind of European birthwort has the habit of “kidnapping” insects that visit its flowers in search of nectar. The “ransom” that the plant seeks is bits of birthwort pollen on the insect’s body.
The birthwort’s flower forms a long curved tube, in the shape of a saxophone. When an insect enters the tube, it finds that stiff hairs pointing down into the flower prevent its escape. So the captive insect journeys deeper into the flower, toward a transparent area in the wall of the tube that looks like an exit.
But it’s a dead end. If the insect doesn’t brush against the inside parts of the flower and pollinate it, then the visitor will never escape. Once the insect has pollinated the plant, though, the flower opens up and relaxes the stiff hairs, permitting the “kidnapped” creature to finally escape.
Sometimes a pollinated birthwort even bends downward and drops the insect out of its temporary prison!