Carnivorous Plants
A carnivorous plant is a plant that catches, kills and digests small animals (usually insects) to get extra nutrients. There are around 800 species of carnivorous plant in the world, including the famous Venus flytrap, pitcher plants and sundews. They still do photosynthesis like other plants, but they grow in places where the soil is so poor in nitrogen that they have evolved to get the missing nutrients by trapping animals instead.
- Species worldwideapprox. 800In 12 different plant families
- Most famousVenus flytrapNative to North and South Carolina, USA
- BiggestPitcher plantsSome can hold over 1 litre and even catch small mammals
- Five main trap typesSnap, pitfall, sticky, suction, lobsterAll evolved separately
- Where they liveBogs, swamps, poor soilsWherever soil nutrients are scarce
- Time to digest a fly5 to 12 daysIn a Venus flytrap
Why do plants eat animals?
Most plants get the minerals they need (especially nitrogen) from the soil. But some places have soil that is naturally very low in nitrogen: bogs, swamps, sandy heaths, rocky cliffs and the like. Plants that grow in those places either need very low requirements, or they need an extra source of nitrogen. Carnivorous plants evolved the second strategy: they trap insects (which are full of nitrogen-rich protein) and digest them to top up their diet.
Plant carnivory has evolved at least 9 separate times in plant history, in completely different families. Almost any time plants have ended up in a low-nitrogen habitat, sooner or later some of them have started catching insects. This is a great example of convergent evolution: different lineages independently arriving at the same solution.
The five main types of trap
- Snap traps: like the Venus flytrap. Two hinged leaf halves snap shut when triggered by tiny hairs.
- Pitfall traps: like pitcher plants. Insects fall into a deep slippery cup full of digestive liquid and cannot climb out.
- Sticky (flypaper) traps: like sundews and butterworts. Leaves are covered in sticky droplets that trap insects on contact.
- Suction traps: like bladderworts. Tiny underwater bladders suddenly suck in passing prey when their trigger hairs are touched.
- Lobster-pot traps: like the corkscrew plant. Inward-pointing hairs force prey deeper and deeper, like a lobster trap.
The Venus flytrap
The Venus flytrap is the most famous carnivorous plant. It is actually a small bog plant native to a tiny area of the swamps of North and South Carolina in the United States. Each leaf is split into two hinged halves that open like a book, with stiff trigger hairs on the inside surface and spike-like teeth around the edge.
If an insect touches one trigger hair, nothing happens. If it touches a second trigger hair within about 20 seconds (or the same hair twice), the trap snaps shut in just 0.1 seconds: one of the fastest movements in the plant world. The spike-like teeth close together to form a cage, the insect is trapped, and over the next 5 to 12 days the plant secretes digestive juices that dissolve the soft parts of the prey. When digestion is finished, the trap opens up again, ready for the next victim.
Pitcher plants: deep liquid traps
Pitcher plants have leaves shaped like deep cups (the "pitchers") partly filled with watery digestive liquid. Insects are attracted by colour, scent or sweet nectar around the rim. The rim is so slippery that insects lose their grip and fall straight into the liquid. They cannot climb out because the inside walls are also slippery and often covered in downward-pointing hairs.
The biggest pitcher plants live in Borneo. Nepenthes rajah can hold over a litre of liquid in a single pitcher and is known to occasionally trap small frogs, lizards and even small mammals like rats and shrews, which have been found drowned and slowly digested at the bottom.
Sundews and butterworts: sticky death
Sundews have leaves covered in tiny stalks topped with shimmering drops of sticky glue. An insect that lands on a sundew thinking the droplets are dew or nectar gets stuck immediately. The leaf then slowly curls around the prey over several hours, bringing more glands into contact and pouring digestive enzymes onto the trapped insect. Within days the soft parts of the insect have dissolved away.
Butterworts work similarly but with sticky leaves that lie flat on the ground. They tend to catch smaller prey like gnats and fruit flies that get stuck and die when they cannot move.
Where to see them
Carnivorous plants are surprisingly common in the wild, although you have to know where to look. In the UK, you can find:
- Round-leaved sundew on many wet heathlands and bogs.
- Common butterwort in damp upland areas of Scotland, Wales and northern England.
- Bladderworts in lakes and slow rivers across the country.
Many are also widely grown as houseplants. Venus flytraps and small pitcher plants can be bought in any good garden centre and are surprisingly easy to keep, although they need rainwater (not tap water) and lots of sunshine.
Deeper dive: how the Venus flytrap counts
The Venus flytrap is one of the most studied plants on Earth, partly because it does something nothing else in the plant world really does: it counts.
Each leaf trap has three to six tiny trigger hairs on its inner surface. The plant has no nerves or brain, but it has a clever system that uses tiny electrical signals to track how often a trigger hair has been touched. Each touch produces a brief electrical pulse that travels through the leaf.
- 1 touch: nothing happens. This avoids wasting energy on raindrops or windblown debris.
- 2 touches within approx. 20 seconds: the trap snaps shut on the prey.
- 3 touches: the plant starts producing digestive enzymes.
- 5 or more touches: full digestion begins and the plant prepares to absorb the nutrients.
This counting system is one of the simplest known biological "memories" outside of an animal nervous system. Scientists are still trying to work out exactly how the plant stores the count, but it appears to use a build-up of calcium ions inside cells, which gradually fades away over a few minutes if no more touches occur. It is a remarkable trick for a creature with no brain.
For more general info, see what is a plant and flowering plants. For the strangest plant relationships, see pollination.