Millipedes are one of the more straightforward exotic invertebrates to keep, and they're oddly pleasant to watch. They graze through leaf litter, burrow into substrate, and trundle over each other in a way that's somehow relaxing. If you're thinking about getting one, here's the full picture of what's involved.
What you're actually keeping
Millipedes belong to the class Diplopoda, meaning "double foot." Each apparent body segment is actually a fused pair of segments (called a diplosegment), and each diplosegment carries two pairs of legs. They're not insects, not crustaceans, and definitely not centipedes. Centipedes are fast predators with one pair of legs per segment and venomous forcipules. Millipedes are slow detritivores that eat rotting plant material and defend themselves with chemical secretions rather than biting.
One thing to get straight early: millipedes don't have a fixed number of legs. They add segments and legs with each moult as they grow. So quoting a specific leg count as a species characteristic doesn't really work. Most commonly kept species have somewhere between 100 and 400 legs as adults.
The enclosure
Glass terrariums and large plastic storage boxes both work. For a single large species like Archispirostreptus gigas (giant African millipede), aim for at least 60x40x40 cm. Smaller species need proportionally less space. Millipedes aren't climbers in any serious way, but they will push against a loose lid, so make sure it's secure.
Ventilation needs to be moderate. Too much airflow (full mesh lids, for instance) dries the enclosure out rapidly, and millipedes are very sensitive to low humidity. A few rows of small holes or a partially mesh lid works better than an open top.
Substrate
This is the most important part of millipede care, because the substrate is both habitat and food source. Millipedes eat decaying organic matter, and the substrate needs to provide it.
A good substrate mix is roughly 50% well-rotted deciduous hardwood (oak, beech, or birch that's been decomposing for at least a year), 30% coco coir or organic topsoil, and 20% leaf mould or dried leaves. On top of the substrate, scatter a generous layer of dried oak or beech leaves. The millipedes will eat these from the surface down.
Depth matters. For large species, 15-20 cm is the target. They burrow for moulting, humidity regulation, and to lay eggs. Shallow substrate limits all of these behaviours. Smaller species can manage with 10-12 cm.
Calcium is essential. Millipedes need it for their exoskeleton, and deficiency leads to failed moults and death. Scatter crushed cuttlefish bone or limestone pieces across the substrate surface. Replace as it gets consumed.
A critical rule: no softwoods. Pine, cedar, and spruce contain phenols and terpenes that are toxic to millipedes. Use only deciduous hardwood in the substrate and for any enclosure decor. This includes avoiding commercially treated wood of any kind.
Temperature and humidity
Most tropical millipede species do well at 22-26C. Room temperature in most UK homes sits around 18-21C in winter, which is a bit cool for tropical species. A heat mat on the side of the enclosure (not underneath) with a thermostat is the standard solution. Underneath heating can dry out the lower substrate layers where the millipedes spend much of their time.
Humidity should be 70-80% for most tropical species. Mist the enclosure regularly, and keep an eye on the substrate moisture. It should feel damp when you press it, like a wrung-out sponge. If it's visibly wet or water pools at the bottom, that's too much. If it feels dry and crumbly, mist more heavily. A digital hygrometer is useful for tracking this, especially while you're learning what your particular setup needs.
The exception is Orthoporus ornatus (the desert millipede from the southern United States), which does fine at 40-60% humidity and tolerates drier conditions than the tropical species.
Feeding
The substrate and leaf litter are the staple diet. On top of that, offer fresh vegetables every few days: cucumber, courgette, sweet potato, and mushrooms go down well. Remove anything uneaten before it goes mouldy, usually within 48 hours.
Don't overdo the fresh food. The bulk of their nutrition comes from the decomposing wood and leaves in the substrate. The vegetables are supplementary. Overfeeding fresh food, especially fruit, can attract grain mites and create mould problems.
Protein is occasionally useful. Some keepers offer a small amount of fish food flakes or dried shrimp once a week or so. It's not strictly necessary for all species, but millipedes do opportunistically consume small amounts of animal protein in the wild.
Handling
Most large millipede species tolerate handling well. Let them walk from your hand into the other hand, rather than gripping them. Support the full body. They'll curl up if startled, which is fine. Just set them back down and try again later.
Always wash your hands after handling. Many species produce defensive secretions from pores along the sides of their body. In the order Spirostreptida (which includes A. gigas), these are typically benzoquinones, which can stain your skin brown and irritate eyes and mucous membranes. Some Polydesmida species produce hydrogen cyanide, which is more serious. It's unlikely to harm you in the tiny quantities a single millipede produces, but it's another good reason to wash your hands and avoid touching your face during handling.
Breeding
Most large millipede species breed very slowly in captivity. Females lay eggs in small chambers in the substrate. The eggs can take weeks to months to hatch. The young are tiny, pale, and have far fewer segments and legs than the adults. They grow by moulting, adding new segments each time, which is called anamorphic development.
Growth is slow. It can take two to four years for some species to reach adult size. If you're hoping to breed millipedes, patience is the main requirement. Make sure the substrate is deep enough for egg-laying, the calcium supply is consistent, and the conditions are stable. Beyond that, it's mostly waiting.
Lifespan
Large tropical millipedes can live 7-10 years in captivity with good care. That's a reasonable commitment, though nothing compared to a tortoise. Smaller species tend to have shorter lifespans, often 3-5 years.