Good question. All living things need energy. They also need matter as the building blocks for making all the molecules that living things are made of. We call these building blocks nutrients. They include nitrogen (used to make protein), phosphorus (used in cell membranes), and many others.

Venus fly traps, pitcher plants, and sundews all consume insects, but thats not how they get their energy. They still get their energy from the sun and transform it into food (such as sugars) using photosynthesis. They use the dead insects for their nutrients. We find the carnivorous plants in environments where there are very low levels of nutrients in the soil.

One nutrient that they dont need to get from insects is carbon. All living things need carbon, but plants get it from carbon dioxide (CO₂) in the air. They absorb the CO₂ through holes in their leaves called stomata.

Nutrients get recycled. So a Venus fly trap takes in CO₂ from the air. It may turn it into sugar, using water from its roots and energy from the sun. It may use that sugar itself for energy, releasing the water and CO₂ out its leaves. Or maybe it puts a bunch of sugar molecules together to make a starch in its leaves. A caterpillar may eat the leaf and break down the sugar to get energy, releasing the water and breathing out the CO₂. Or it may transform the sugar to make fat. You can probably think of other journeys for a carbon atom to take. The atoms themselves never get created or destroyed (except in nuclear fission and fusion, but thats another story).

Energy gets lost every time that it is converted from one form to another, thats why ecosystems need to get a certain amount of light in order to survive.

So, to (finally) get back to your question, when were talking about producers, primary consumers (herbivores), secondary consumers (carnivores), etc., we like to pretend that the categories are very neat. In fact they can be complicated. When people eat apples, theyre primary consumers. But what if there are worms in the apple? Then they are being primary consumers and secondary consumers at the same time.

When Venus fly traps are doing photosynthesis, theyre producers. When theyre dissolving insects, Id say that theyre consumers.

Heres the big question that a lot of my college students have trouble with: As you go up each level (producer, primary consumer, secondary consumer), how much of the energy actually makes it to the next level? To put it another way: If you could weigh all of the producers in one community, then all of the primary consumers, then all of the secondary consumers, which would weigh more?

That is an excellent question and is one that also puzzled me when I was a kid!

Venus fly traps are green plants, as such they make their own carbon-based food by photosynthesizing. Like all green plants, Venus flytraps use their leaves to harness energy from the sun and make their own food; they use carbon dioxide (CO₂) to make energy-rich carbohydrates. So they are Producers.

But as you may have guessed, the story doesn't end there? Like us, plants need more than just carbohydrates to survive and grow. Plants also need nitrogen, an essential element that is a key component of amino acids, and thus many important proteins. Most plants get their nitrogen from soil in their habitats. But the Venus fly trap lives in bogs and grasslands where soils contain VERY, VERY little nitrogen. So these plants have evolved a way to acquire the nitrogen they need by capturing insects, killing them, and absorbing the nitrogen contained in insect bodies.

So in addition to undergoing photosynthesis to make carbohydrates, Venus fly-trap leaves serve as elaborate traps that capture insects, secrete digestive enzymes that break down their prey, and absorb this "bug juice" into their tissues. These carnivorous traps are a very unique and interesting adaptation that allows these plants to survive and prosper in their harsh, nitrogen-poor environment! To make a long story short, Venus fly traps are Producers that engage in carnivory to acquire nitrogen.
Isn't that cool?

Good question!

You just demonstrated that trophic levels (that's the general term for producers, herbivores, carnivores, etc. is) are not absolutes.

That said, most of the carbon that a Venus fly trap gets is fixed through photosynthesis as with most any other plant, so it's mainly a producer. The carnivorous aspect of its ecology is mainly to get things that it can't get through photosynthesis, such as nitrogen and phosphorous - the things in fertilizer, in other words.

A Venus Fly trap DOES make its own food through photosynthesis. It IS a producer. However, it tends to grow on very wet soils that are very poor in nitrogen, an important element in the processes of life. Without nitrogen, the plant will slowly fail. The solution to being able to survive in these soils is to trap animals, which are rich in nitrogen. This it does with its "trap" operated by different water pressures in the cells of the plant. We therefore refer to it as a Carnivorous plant. It is first & foremost a producer, but because it traps and digests insects, we secondarily (because this is a specific adaptation) can term it a carnivore. This is also true of Sundews (Drosera), Butterworts (Pinguicula) and pitcher plants (Nepenthes, Sarracenia, Darlingtonia, etc. )

A venus flytrap is both a producer and a carnivore. Despite their carnivorous ability to eat insects and arachnids, venus flytraps are still plants which means that they can still grow using photosynthesis, a process in which they take energy from sunlight and nutrients from air and soil. Then they use the food they trap to supplement the nutrients in their "diet."

If you want to know more, Professor Owens from Cornell University wrote a very informative answer to a similar question here:

You might call a venus flytrap a 'mixotroph'. Autotrophs make their own food (like most plants); heterotrophs get food from elsewhere (like animals, which eat other organisms), and mixotrophs can do both. There are actually a variety of creatures that are mixotrophs, in particular among the protists. Some dinoflagellates, for example, can be mixotrophic. They are able to do photosynthesis, but they also can gobble up small cells. (Dinoflagellates are single-celled organisms that live in the ocean. Some of them can fluoresce -- if you've ever splashed around at night in the ocean and seen 'sparks' it's probably dinoflagellates.)