Many types of insects, spiders, worms, snails and even flies can survive in a closed terrarium. Springtails, earthworms and woodlice are found in most closed terrariums. All three species provide a stable ecosystem and not for you to wory about. In general, insects also make a closed terrarium look much more lively, of course.
Spiders can also be interesting, but may trap many of your insects and, in the worst case, may exterminate them. Which insects and other animals are suitable for your closed terrarium, you can find out here.
If you have other general questions, read the answers to the 10 most common questions about closed terrariums.
Which bugs are suitable for closed terrariums?
Strictly speaking, insects are only animals with exactly six legs. Worms, spiders and snails are therefore not insects. But I'll get to them in a moment. Insects are the most diverse class of all animals with almost one million described species. They really occur almost everywhere where there are plants and other animals.
If you use soil from your garden for your closed terrarium, you will definitely already have small insects in it, even if you don't see any initially. If you use soil from the forest, you will get many more little animals. You will probably immediately notice that there are some crawling around in it. Exactly what bugs are in it?
By the way: There are other animals with six legs. Springtails, for example.
Exactly, springtails aren't classified as insects. Instead they belong to the Entognatha. It won't make any difference for your closed terrarium if they're insects or Entognatha though. Springtails come in many forms and colors (more than 6000 species are known). They are mostly known for their incredible jumping ability in relation to their body size. They're usually smaller than 1 cm but can jump up to 30 cm. This corresponds to a human being able to jump over 50 m.
Springtails will appear in almost any closed terrarium made with garden or forest soil. At first they're so small that you can't see them. Only inside your closed terrarium will they grow big enough for you to notice. Springtails are good for your ecosystem and as described in my article about substrates you should always enrich your soil with garden or forest soil.
Springtails multiplay all by themselves and eat all the dead or sick parts of your plants so the rest can stay healthy. This plant waste is also called detritus. Springtails thus have a detritivorous diet. They help your ecosystem and are an important part of the nutrient cycle. It's a good sign if there are many, healthy springtails in your closed terrarium.
Here you can find a detailed article about springtails.
Woodlice, a kind of isopods, are small armored insects that can curl up. Strictly speaking, they belong to the terrestrial isopods. Just like springtails, they are handy helpers to keep your closed terrarium clean. Just like springtails, woodlice feed mostly detritivorously, making them the ground crew of your cleanup team.
Isopods are very resilient and thrive in many closed terrarium conditions. You won't have to worry about their population. It will reach a fitting amount all by itself. You'll probably have to manually add some isopods at first to start the colony.
Of course, you can buy woodlice online. But it will be much easier to find them outside or in your basement. They usually hide under stones or wood. Generally, damp, dark places are a good place to start.
By the way, woodlice cannot roll up completely. Only the common pill-bug can do that.
If you want to find out more about them, you can read the whole article on woodlice here.
#3 Earth Worms
Often you will also find worms in your soil. This is something good for your soil, as they loosen up your substrate. With the daily water cycle, small substrate particles are washed down, compacting the soil over time.
If you didn't have worms in your closed terrarium, your floor would eventually be a solid block with a puddle of standing water on top. Your plants would die and your closed terrarium would be reduced to a brown wasteland. The worms' small tunnels, on the other hand, create an entrance for air, allowing the roots of your plants to get more oxygen.
Worms multiply very easily by themselves. You don't really have to encourage them to do it. In fact, if you really need a lot of worms, there are also special breeding containers. These are also called vermicomposts.
From time to time you will find a small snail in your closed terrarium. They usually don't grow too big and therefore don't eat all your leaves. Personally, I find it exciting to see them gliding over the glass pane.
Just like woodlice, snails and slugs also like to eat dead leaves. However, they also feed on fungal spores that could later become a mold problem. Sometimes snails just eat soil, additionally loosening your substrate. They need a lot of calcium to keep their snail shell stable. If you want snails in your closed terrarium, you can help them by enriching your substrate with calcium beforehand. This can be done for example with crushed mussel shells, hard water or herbal soil from the hardware store.
In my first terrarium, one snail was neatly snacking on a strawberry leaf, as you can see above. If you notice that the snails are getting too big in your closed terrarium, please be kind enough to release them. They should not suffer from getting too big for their ecosystem. Usually, after all, an air-tight terrarium can be easily opened again. Translated with www.DeepL.com/Translator (free version)
Spiders are actually rather a rarity unless you intentionally put them in. Unless you necessarily have a large source of insects for the spiders to eat, I would also advise against it. Most spiders are carnivores and spin webs. Your insects will quickly get caught in them. Whether the spider is hungry or not makes no difference. The web is always sticky. This is a similar problem to the sundew.
At worst, you'll have a net full of insects rotting away. That leaves you with more organic waste and you lose your little cleanup helpers. But if you're a total spider fan, you can specifically create a spider terrarium.
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By the way, you can easily breed springtails by keeping them in a dark container with water and activated carbon. There they multiply magnificently. You can either buy colonies or breed them yourself. You can learn more about this in the article about springtails.
A closed terrarium is full of insects and other bugs. Those are worms, snails and spiders, for example. Although springtails may look like insects, they actually aren't. All of these bugs serve their role inside your ecosystem and help to keep it stable. Usually you won't have to think about insects since they're already inside the soil from your garden or a forest. The amount of insects self-regulates due to the fixed amount of resources.
If you like this type of article, I recommend your read 11 types of moss for your closed terrarium next. You'd be amazed at how different types of moss look and what great decorative elements they can be!