Canadian botanist Margaret Helder explains about bladderwort (Utricularia, 200 species),
… some plants … attract, catch and eat aquatic insects, water fleas and young tadpoles, fish fry, tiny worms and very young insect larvae including mosquito wrigglers.
We knew they needed nitrogen, and that animals are a good supply, but we didn’t know they were into fusion food. So they even eat small vertebrates … using suction power.
Just how plants evolved to do this has been a 200-year headache. Even Darwin and Wallace knew that their natural selection theory would not really explain it.
More from Helder,
… all Utricularia have a highly unusual structure. They have absolutely no roots, not even as an embryo in the seed. Furthermore it is difficult to distinguish between stem and leaf. Masses of small flowers – either purple or yellow – extend above the surface of the water. It is usually the flowers that one notices first.
And the traps?
he traps are usually oval or egg-shaped bladders. At the front end, a trapdoor guards the entrance. Special cells on the trapdoor surface release nectar and mucilage. These smell so good. Numerous small animals make a point to investigate the source of these pleasant aromas. In so doing, they unintentionally brush one of the prominent trigger hairs projecting from the threshold into their pathway.
After this, things happen awfully quickly. The curious intruder wishes he had stayed far away. A doorstop inside shifts, allowing the trapdoor to swing up and inward. Water gushes inward to fill a vacuum in the trap’s interior. The watery inrushing flood sweeps the hapless victim along into the trap. And the view inside is certainly discouraging. The trapdoor swings firmly shut behind the victim, who cannot now escape. To add insult to injury, the victim’s pitiful struggles stimulate special gland cells on the interior wall. These release digestive juices into the cavity and the victim is digested.
When only the skeletal parts of the meal remain, the trap is re-set.
More.
Either plants are smarter than animals or something is smarter than both. It would be interesting to know if any animal ever evolved a defence against the trap.
Helder also writes,
The time to find these plants in Canada is usually in July when one sees clusters of yellow flowers (like snapdragon blossoms) extending just above the water. Even in marshes near here (not acid at all, actually) one sees these plants in shallow water. When you see the blossoms, you can reach in and grab a nice big clump of the plant. The bladders are obvious.
Bladderwort at work, high speed: