Carnivorous plants evolved to catch and digest insects because they grow in bogs and poor soils where nitrogen and phosphorus are scarce. They still make their food through photosynthesis; prey is a nutrient supplement, like fertiliser. Around 800 species use five main trap types – snap traps, pitfall traps, flypaper traps, bladder traps and lobster-pot traps.
Key takeaways
- There are roughly 800 known carnivorous plant species across about a dozen families.
- They live in nutrient-poor habitats such as bogs, heaths and sandy savannas.
- The Venus flytrap counts touches: two trigger hairs within ~30 seconds close the trap.
- Pitcher plants use slippery rims, nectar and digestive fluid to drown prey.
- Grow them with rainwater only, bright light and peat-free carnivorous compost.
Why would a plant eat animals?
Charles Darwin was fascinated by carnivorous plants; his 1875 book Insectivorous Plants proved that sundews digest meat. Carnivory evolved independently at least ten times, always in sunny, waterlogged, acidic places where soil nutrients are scarce. Catching insects gives these plants nitrogen, phosphorus and other minerals that their roots cannot get.
Because traps are costly to build and run, carnivory pays off only where there is plenty of sunlight and water but few nutrients – which is why you will not find carnivorous plants in rich woodland soils.
The five types of trap
| Trap type | How it works | Examples |
|---|---|---|
| Snap trap | Leaves snap shut in a fraction of a second | Venus flytrap, waterwheel plant |
| Pitfall (pitcher) | Prey slips into a fluid-filled leaf cup | Sarracenia, Nepenthes, Cephalotus |
| Flypaper | Sticky glandular hairs glue prey | Sundews (Drosera), butterworts (Pinguicula) |
| Bladder (suction) | Underwater bladders suck in tiny animals | Bladderworts (Utricularia) |
| Lobster-pot | Inward-pointing hairs force prey towards the stomach | Corkscrew plants (Genlisea) |
The Venus flytrap: a plant that counts
The Venus flytrap (Dionaea muscipula) is native only to a small area of wetlands around Wilmington in North and South Carolina. Each trap has three to four trigger hairs on each lobe.
- The first touch of a hair generates an electrical signal but does not close the trap – avoiding false alarms from raindrops.
- A second touch within about 30 seconds snaps the trap shut in around a tenth of a second, among the fastest movements in the plant kingdom.
- As the insect struggles, it touches more hairs. After about five signals, the trap seals tightly and floods with digestive enzymes.
- Digestion takes 5–12 days; the trap then reopens, leaving an empty exoskeleton.
A 2016 study confirmed the plant effectively counts these signals. The snap works by rapidly changing the curvature of the lobes, like flipping a contact lens inside out.
Pitcher plants: elegant drowning machines
North American trumpet pitchers (Sarracenia) and tropical monkey cups (Nepenthes) have leaves shaped into tubes. They lure insects with colour, UV patterns and nectar. The rim (peristome) of Nepenthes becomes ultra-slippery when wet – insects aquaplane into the fluid below. Some giant Nepenthes in Borneo have evolved partnerships with tree shrews, which sit on the rim to lick nectar and leave droppings in the pitcher – a toilet that feeds the plant.
Sundews and butterworts: living flypaper
Sundews’ leaves are covered in glistening tentacles tipped with sticky mucilage. When an insect lands, tentacles slowly bend inward to wrap it. Britain has three native sundew species on its heaths and bogs. Butterworts have greasy-feeling leaves that trap gnats – they are popular for controlling fungus gnats on windowsills.
Bladderworts: the fastest trap
Aquatic bladderworts have tiny bladders that pump out water to create a vacuum. When a water flea brushes a trigger hair, the trapdoor opens and the prey is sucked in in less than a millisecond – one of the fastest movements known in plants.
How to grow carnivorous plants at home
- Water: use rainwater, distilled or reverse-osmosis water only. Tap water minerals slowly kill them. Stand pots in a tray of water in summer.
- Compost: use a specialist peat-free carnivorous plant mix (often coir, perlite and sand). Never use regular compost or fertiliser.
- Light: as much as possible – a sunny windowsill, greenhouse or outdoor bog garden.
- Dormancy: Venus flytraps and Sarracenia need a cool winter rest (around 0–10 °C) for 3–4 months.
- Feeding: outdoors they catch their own. Indoors, an occasional live insect is enough; never feed meat.
- Do not trigger traps for fun – each trap can only close a handful of times before it dies.
Conservation
Many carnivorous plants are threatened by wetland drainage and illegal poaching. Venus flytraps are protected in North Carolina, where digging them from the wild is a felony. Always buy nursery-propagated plants. Curious about other plant behaviours? Read how plants communicate.
Frequently asked questions
Why do carnivorous plants eat insects?
Carnivorous plants grow in nutrient-poor soils such as bogs. Insects provide nitrogen, phosphorus and minerals that their roots cannot obtain, while sunlight still provides their energy.
How does a Venus flytrap know when to close?
A Venus flytrap closes when two trigger hairs are touched within about 30 seconds. This prevents it from wasting energy on raindrops or debris.
Can carnivorous plants hurt humans?
No. Even the largest pitcher plants only catch insects and occasionally small frogs or rodents. They are completely harmless to people.
What water do carnivorous plants need?
Use rainwater, distilled water or reverse-osmosis water. Minerals in tap water build up in the soil and gradually kill the plants.
Where do Venus flytraps grow in the wild?
Venus flytraps grow naturally only in a small region of wetlands within about 100 km of Wilmington, North Carolina, in the United States.
Sources and further reading
- Darwin C (1875) Insectivorous Plants. John Murray, London.
- Böhm J et al. (2016) The Venus flytrap Dionaea muscipula counts prey-induced action potentials to induce sodium uptake. Current Biology.
- International Carnivorous Plant Society
