If you keep invertebrates in any kind of naturalistic or bioactive setup, you should probably be keeping springtails too. They are tiny, cheap, easy to culture, and they do the one job nobody else wants: eating mould. I started culturing them about two years ago and now I cannot imagine running a terrarium without them.
What are springtails?
Springtails are hexapods in the class Collembola. They used to be classified as insects, but they are now placed in their own group within the broader hexapod category. There are thousands of species worldwide, but the ones used in the hobby are mostly sold as "tropical white springtails" (the exact species is often unclear, though Sinella curviseta and similar tropical Collembola are commonly traded).
They are tiny. Most hobby springtails are 1-2 mm long. They get their common name from a forked appendage called a furcula on their underside, which they can release like a spring to jump several centimetres into the air. It is a predator escape mechanism, not a mode of regular locomotion.
In terms of development, springtails are ametabolous. They do not undergo metamorphosis. Juveniles look like small versions of adults and simply grow larger through successive molts.
Why they matter in a terrarium
Mould is the enemy of humid enclosures. You put in damp substrate, leaf litter, cork bark, and fruit for your beetles, and within a few days you have got white fuzzy growth on everything. In a sterile setup, you deal with this by removing affected items and improving ventilation. In a bioactive setup, springtails eat it.
Springtails feed on fungal hyphae, decaying organic matter, and bacteria. They do not eat living plants, they do not bother your animals, and they breed quickly enough to keep pace with mould growth in most enclosures. A healthy springtail population will visibly reduce mould within days of introduction.
They also break down frass, dead leaves, and uneaten food. Combined with isopods, they form a cleanup crew that processes waste and returns nutrients to the substrate. This is what makes a bioactive terrarium self-maintaining.
Culturing springtails
You can buy springtails from most invertebrate shops and many reptile suppliers. A starter culture costs a few pounds and will establish a self-sustaining colony within weeks.
Charcoal method
The most common approach. Take a plastic tub with a ventilated lid, fill the bottom with a layer of activated charcoal or horticultural charcoal (2-3 cm deep), and add dechlorinated water until the charcoal is damp but not submerged. Tip your starter culture in and feed with a sprinkle of brewer's yeast or a tiny piece of mushroom every few days.
The charcoal provides surface area for the springtails to live on, absorbs waste, and stays clean for months. When you need springtails for a terrarium, flood the tub with water. The springtails float to the surface and you pour them off into the enclosure. The colony recovers within a week or two.
Substrate method
Some keepers culture springtails on a thin layer of coco coir or damp sphagnum instead of charcoal. This works but it is harder to harvest the springtails cleanly because they hide in the substrate. The charcoal method is easier for most people.
Feeding the culture
Springtails eat very little. A pinch of brewer's yeast or nutritional yeast flakes once or twice a week is plenty. Rice grains also work, though they are slower to be consumed. Do not overfeed. Excess food in a warm, damp tub grows mould faster than the springtails can eat it, which defeats the purpose of having them.
Adding springtails to enclosures
Pour them directly onto the substrate surface. They will disperse on their own, finding the damp, mouldy spots that they prefer. For a new bioactive setup, add them at least two weeks before introducing your main animal. This gives them time to establish a breeding population.
How many to add depends on the enclosure size. For a small isopod tub, a tablespoon of culture is fine. For a large millipede terrarium, use a generous amount and top up again after a couple of weeks if you do not see many. They breed quickly, so even a modest starting population will grow if conditions are right.
Keeping them alive in your terrarium
Springtails need moisture. If your terrarium is well-maintained with regular misting and damp substrate, they will look after themselves. They struggle in dry enclosures, which is why they work well with tropical isopods and millipedes but may not persist in a dry jumping spider setup.
Temperature is less of a concern. Tropical springtails do well at 18-28C. Below 15C they slow down but rarely die off entirely. Above 30C they start to suffer. Normal UK room temperatures are fine year-round.
They can coexist with every commonly kept invertebrate. Isopods will eat springtails occasionally, but a healthy springtail population breeds fast enough to absorb the losses. Beetle larvae ignore them. Jumping spiders sometimes catch one, but springtails are too small to be a meaningful food source for adult spiders.
Troubleshooting
If your springtail culture crashes, the usual cause is either drought (the charcoal dried out), drowning (too much water with no dry surface), or overfeeding (food went mouldy and fouled the culture). Dump the culture, rinse the charcoal, and start again. They are resilient enough that even a nearly dead culture can bounce back if you fix the conditions.
If springtails disappear from an enclosure, check the moisture level. A terrarium that has dried out too much will lose its springtail population before you notice anything else wrong. Re-introduce them after bringing the humidity back up.
One thing worth mentioning: springtails sometimes climb out of enclosures and appear on shelves or windowsills nearby. They are harmless and cannot survive long outside a humid environment. If it bothers you, a thin line of vaseline around the rim of the enclosure acts as a barrier they cannot cross.