Plants That Eat Animals

1 · Set

Open and waiting

Two lobes, bowed outward, with about three trigger hairs standing on each. An ant is on its way up the leaf stalk.

2 · First touch

It notices, and waits

The ant brushes a hair. The hair fires an electrical signal across the leaf, much like a nerve impulse.

The trap does not move. It only remembers, for about 20 to 30 seconds. A raindrop or a blown grain of sand touches once and is ignored.

3 · Second touch

Snap

A second touch inside that time, and the lobes shut in about a tenth of a second.

No muscle does this. The open lobes are bowed outward, like a bent tape measure. A small change in the leaf tips them past their limit and they flip inward all at once. Exactly what gives that last push is still being worked out.

4 · Caged

Bars first, not a seal

The teeth lace together like the bars of a cage, with gaps between them. Darwin thought the gaps were there to let very small insects walk out: a meal too small to be worth a week of digesting.

5 · Counting

Three, four, five

The ant struggles and keeps hitting hairs. The plant keeps count. After about five signals it treats the catch as food: the lobes press together into a sealed pouch, and tens of thousands of glands start releasing digestive fluid.

More struggling means more fluid.

6 · Digesting

A meal takes a week

For 5 to 12 days the closed trap works as a stomach. The soft parts of the ant dissolve and are taken in through the leaf.

7 · Reset

Open again

The trap reopens with only the empty shell left, and wind or rain clears it away.

A trap catches just a few meals in its life. Then it blackens, and the plant grows another.

1 · Set

A leaf full of drops

The leaf seen from above. Long stalks round the edge, shorter ones toward the middle, a drop of glue on every one.

2 · Caught

Stuck

A gnat lands. The glue is not like water: it pulls out into long threads and does not let go. Every kick puts another leg in another drop.

3 · Moving

The leaf closes its fingers

The stalks that were touched begin to bend, and carry the gnat toward the middle of the leaf. Darwin timed one stalk: moving after 10 seconds, at the centre in half an hour.

Then the neighbours bend in to join them.

4 · Digesting

A stomach on the outside

The same glands that made the glue now release digestive juice, then soak up what dissolves. There is no mouth and no stomach. There is only a wet leaf, slightly cupped.

5 · Reset

Days later

The stalks straighten and new drops form. The dry shell stays stuck to the leaf until rain or wind takes it.

1 · Set

Lid up, rim wet

A pitcher about 9 cm tall. An ant is climbing the outside, following a trail of sugar.

2 · Bait

Sugar on the rim

It steps onto the rim to drink the nectar there.

3 · The slip

A rim like a wet floor

The rim is covered in fine grooves that hold a film of water. On a wet rim the sticky pads of an insect's feet cannot grip. It aquaplanes, the way a car tyre does on a flooded road, and slides in.

4 · No way up

A wall that comes off on your feet

Below the rim, the inside wall of many species is coated in loose flakes of wax. They break away and clog the ant's feet, so it cannot climb back out.

5 · The pool

Acid, and sometimes slime

At the bottom is a fluid the plant makes itself. It is acidic and contains digestive enzymes. In some species it is also stretchy, like saliva, and an insect that tries to swim only tangles itself.

6 · Lodgers

A pond in a leaf

Not everything in the pool is food. Mosquito larvae and other small animals live there unharmed, eating scraps of the catch. What they leave behind feeds the plant too.

1 · Set

Sides sucked in, door shut

The bladder is set: most of its water pumped out, its walls bowed inward like a squeezed bottle. The specks are bits drifting in the pond water. Watch them.

2 · Trigger

A touch on the door

Stiff bristles stick out from the door. A water flea grazing past bumps one.

3 · Half a millisecond

The door gives way

The door bulges outward. Now it buckles inward and swings open. Water rushes in at up to 1.5 metres a second, and the flea goes with it.

Opening takes about half a thousandth of a second. A screen showing 60 pictures a second would miss it between two frames.

4 · Shut

And closed again

About 2.5 thousandths of a second later the door has swung back and sealed. The walls have sprung out, the flea is inside, and the door cannot be pushed open from within.

5 · Reset

Pump, and wait

Glands inside digest the catch and pump the water out again. In about an hour the trap is set once more.

If nothing touches it, it fires by itself anyway, every 5 to 20 hours.

The trick

The same idea as the flytrap

A plant has no muscles. To be fast it stores energy slowly in a curved, springy part, then lets the curve flip. The flytrap flips a whole lobe. The bladderwort flips a door smaller than a pinhead, and is two hundred times quicker.

1 · Above and below

A plant with no roots

Above ground there is a small rosette of green leaves and a flower. Below ground, what look like white roots are leaves as well. This plant has no roots at all.

2 · The way in

Through the slit

In the wet sand live animals made of a single cell. One of them, a ciliate a tenth of a millimetre long, finds the slit in a corkscrew arm and swims in.

3 · One way only

Hairs that all point inward

Inside, ring after ring of stiff hairs point up the tube. A swimming animal can push past them going in. It cannot turn back against them.

Eel traps and lobster pots work the same way.

4 · The chamber

Where the tube leads

The only way on is up, to a swelling in the tube. Glands in its wall digest whatever arrives and absorb it.

5 · Found out

Guessed in 1875, shown in 1998

There is no snap, no glue and no suction here. The prey does all the moving. Darwin suspected these tubes were traps. It took until 1998 to show it, and to find that much of the catch is single-celled.