I Know Why She Swallowed The Fly

Biology concepts – carnivorous plants, minerals in biology, symbiosis, cryptids,

The Thing From Another Planet was a 1951 B-horror movie.

Arctic researchers find a space ship in the ice and thaw out

the pilot. He turns out to be a walking plant that needs

blood to feed his little seedlings. Never minds that the plant

is growling, feels just fine at -60 degrees, and is wearing

clothes. They finally kill him with electricity.

The man-eating tree is a cryptid (hidden) organism. Cryptid means there is no scientific proof for its existence, but for some reason there are people that say it exists. In this case, a German explorer named Carl Liche trekked through the Madagascar jungle and described the natives forcing a girl to climb the trunk of a tree. The branches (arms) grabbed her and lifted her to the top where she was crushed and absorbed. There was no explorer Carl Liche and the story was an utter fiction.

Myths, hoaxes, misidentifications, misunderstandings, they all have accounted for various cryptids, but every once in a while a cryptid turns out to be real. Gorillas? Once thought to be fictitious monsters. But a man-eating plant? Would you settle for small animal-eating plants? Those we have. The question is why?

Question of the Day – Why do some plants eat bugs?

Venus flytraps are active trap plants. They have a movement that requires energy, and the movement of the trap is one stage - the prey is digested by the part that moves. Much research has been devoted to the mechanism of its fast trap closure, and many hypotheses are still floating about.

We do know that it takes about 1.5 milliseconds to transmit the signal from the trigger hairs in the trap to the motor cells that close it. The signal is electrochemical, very similar to an action potential in an animal neuron. Channels pump ions across membranes, and the difference in the charges of each type of ion (sodium and potassium) cause an electrical impulse.

It seems that the electrical impulse causes water channels to open across various cells near the base of the trap. Water pressure is quickly changed from high to low and low to high in different layers of cells and this cause shape changes. Different shapes cause different stresses, and this closes the trap.

A relatively new hypothesis is that the open configuration is full of elastic stress, so that when water pressures are changed between layers of cells, there is an elastic snap to the closed state. The closing only takes a 0.2 seconds. After that there is a slower portion that brings more complete closing and the start of digestive enzyme secretion.

This is a visual representation of the electrical signal produced

 by triggering the venus fly trap. The red line is the first touch

to a trigger hair. It is not enough to reach threshold and close

the trap. If a second touch occurs before the first has dissipated

to much (green line) the threshold is crossed and the trap

closes. If the second signal is too late (blue line), the

threshold won’t be reached.

To reduce false or unproductive closures, the each trigger hair in the trap produces a sub-threshold action potential. One trigger hair being touched won’t close the trap. As the first signal dissipates, if a second signal is generated by a second hair being touched, the sum of the dying first signal and the second signal can raise the charge above the threshold level and the trap will close (see picture at left). However, in very warm weather (above 36˚C/97˚F) it only takes one trigger hair signal to close the trap – it has something to do with the molecules moving faster in higher temperature environments so that one signal can reach the threshold level.

Other carnivorous plants have semi-active, two-stage traps. The aquatic bladderwort is an interesting example. It is one of the smallest carnivorous plants, with a trap that is just 10 mm wide at its opening. In order to eat, bladderworts create a negative pressure inside the trap by pumping out the water. A trap door maintains the negative pressure inside, but if the trigger hairs outside the trap are touched, the door collapses and water + prey are sucked inside. The trap door then assumes its original shape and the prey is caught inside the trap (see video here).

Sundews are two-stage trap plants as well, having sticky liquid drops perched atop small pedestals. The prey, maybe a fly, gets stuck in the gummy drops. Only then does the tentacle slowly curl around the fly, becoming an “outer stomach” as termed by Charles Darwin (see picture). The digestive enzymes are secreted and the fly is no more.

A different species of sundew, Drosera glanduligera, has a different kind of trap. A new study from Germany shows that it has brittle hairs (called snap tentacles) at the edge of the trap that when triggered, catapult the prey into the resin glue. The catapult is quicker than the venus flytrap, occurring in less than 75 milliseconds. Only then will the prey be slowly pull down toward the portion of the plant that secretes enzymes.

On the left is a typical sundew, D. capensis. When a fly is stuck, a

slow curl of the tentacle will finally do him in and trigger digestion.

On the right is a rarer sundew, D. glanduligera. The number steps

show the catapulting of prey from the trigger hair into the glue in

the middle of the plant. It all occurs in just milliseconds.

There are also passively carnivorous plants, those that allow the prey to do the work. Pitcher plants are slippery on their edges; prey fall into the pitcher and can’t escape. Amazingly, the seeds of the Shepherd’s Purse are carnivorous, but the plants themselves are not. The seeds lie on the ground and exude toxins that attract and poison insects that pass by. The seeds also secrete enzymes that then digest the insects. This leaves a circular ring of very rich soil, giving the germinating plant an advantage.

In many of the plants, the digestive enzymes have started to be identified. A 2012 paper from Germany has looked the protein portions of the venus flytrap digestive fluid. It contains nucleases (digest DNA and RNA), phosphatases (remove phosphate groups), phospholipases (break down fats), chitinases (to digest the insect exoskeleton), and proteolytic enzymes (to break down proteins). Most of these are derived from pathogenesis proteins, so it is believed that digestion evolved from several self-defense processes.

There are 600 known species of terrestrial carnivorous plants and 50 in the water, but scientists are now realizing that many more plants use a mechanism similar to the Shepherd’s purse and can be considered at least semi-carnivorous. Would you believe that tomato and potato plants have sticky hairs that may trap aphids and other insects. They die and drop to the ground around the stem. This enriches the soil and the plant absorbs the nutrients.

Tomato vines have sticky hairs on their stems. It turns

out that they can trap bugs, hold them until they die,

drop them to the ground, and let their carcasses

fertilize the soil around the plant. Now that’s

miracle grow!

No matter the method of the trapping, the reason is the same; the plants need nutrients. Not glucose, proteins or lipids – they're photosynthetic for gosh sakes. They can make their own proteins, nucleic acids and fats from the carbohydrates they produce during photosynthesis. That is, they can if they have the correct additional materials.

Proteins are made of amino acids, and amino acids contain a lot of nitrogen. Nucleic acids (DNA, RNA) are made from nucleotides, and these include a lot of phosphorous. Many biomolecules and physiologic processes use minerals like nitrogen, potassium, and phosphorous. These are the amin constituents of the fertilizers humans add to the soil to help crops, flowers, and in my case - weeds, grow.

Carnivorous plants often live in nutrient poor soil. Sandy soil (flytraps), tropical jungle soils (sundews), and Andean mountain tops (bromeliad described below) are all mineral poor. In jungles, for instance, most of the minerals are tied up in the huge trees, and such little sunlight penetrates to the ground that few plants can live there; therefore, there is little recycling of nitrogen and phosphorous in the topsoil. Eating insects is just an adaptation to allow them to live where other plants can’t.

Many minerals are made available by digestion of insects.  Carnivorous plants get 5-100 % of their seasonal nitrogen and/or phosphorous gain, but only 1-16% of their potassium uptake. If there is one nutrient these plants covet more than the others, it's the nitrogen.

Many plants acquire nitrogen from symbiotic bacteria around their roots that fix nitrogen gas in the soil. Fixing means converting from gas to a solid. Carnivorous plants do not have these advantages so they had to come up with another strategy. However, help doesn’t always come from digestion of insect prey.

High in the Andes Mountains grows the world’s largest

bromeliad. It can be alive for 140 years before it flowers

the first time. The tall stalk is what holds the flowers.

The business is lower, rounder, and full of sharp spines.

Birds live there, but can also be skewered there. They

say most fatal accidents do occur close to home.

The Roridula genus of South African plants acquire minerals from prey with a little help. It has sticky leaves but no digestive enzymes. Its sticky fluid is resin, not mucus; therefore, enzymes can’t be included because they are not soluble in resin. 

When prey insects get stuck in the resin, they are consumed by another animal. This consumer defecates either before or after its meal. The feces are nitrogen rich remnants from a previous bug meal, so this is an indirect mechanism for profiting from killing for a meal.

Another example of this is Puya raimondii, the world’s largest bromeliad. This plant is about 2-3 m tall, but when it flowers, the stalk may rise as much as 12 m! Birds can live in its foliage and when they defecate, they provide nitrogen to the plant. But P. raimondii has huge sharp spines that can actually kill some of the birds. As the birds rot, they also release minerals to be used by the tree; therefore, P. raimondii is semi-carnivorous.

It isn’t all death and destruction. Take the nepenthes pitcher plants for example. These are the largest of the pitchers, holding more than 2.5 liters of digestive fluids. Their pitchers are little ecosystems. Some larvae, particularly a couple of species of mosquito, can survive ONLY inside the pitcher liquid.

I don’t think I can do this picture justice. The only

things this tree shrew lacks are a magazine and a

can of air freshener.

On the other hand, a 2013 paper shows that the nepenthes pitchers also secrete antimicrobial agents, making them very sterile environments… except for all the digesting corpses. The pitchers also house assassin insects, such as swimming ants that can live in the pitchers without being harmed. Their leftovers and feces help to feed the pitcher plant.

One last exceptional case of acquiring nitrogen. Borneo tree shrews trade their own feces for nectar from three species of nepenthes pitcher plants. The shrews sit on the edge of the pitcher facing the outside of the plant, like on a little toilet (see picture on left). They lick the nectar from the edges of the trap and then make a deposit of feces into the pitcher. The nitrogen rich feces will sustain the plant in times of low insect number. Is it really worth it? 






Poppinga, S., Hartmeyer, S., Seidel, R., Masselter, T., Hartmeyer, I., & Speck, T. (2012). Catapulting Tentacles in a Sticky Carnivorous Plant PLoS ONE, 7 (9) DOI: 10.1371/journal.pone.0045735  

Buch, F., Rott, M., Rottloff, S., Paetz, C., Hilke, I., Raessler, M., & Mithofer, A. (2012). Secreted pitfall-trap fluid of carnivorous Nepenthes plants is unsuitable for microbial growth Annals of Botany, 111 (3), 375-383 DOI: 10.1093/aob/mcs287  

Schulze, W., Sanggaard, K., Kreuzer, I., Knudsen, A., Bemm, F., Thogersen, I., Brautigam, A., Thomsen, L., Schliesky, S., Dyrlund, T., Escalante-Perez, M., Becker, D., Schultz, J., Karring, H., Weber, A., Hojrup, P., Hedrich, R., & Enghild, J. (2012). The Protein Composition of the Digestive Fluid from the Venus Flytrap Sheds Light on Prey Digestion Mechanisms Molecular & Cellular Proteomics, 11 (11), 1306-1319 DOI: 10.1074/mcp.M112.021006