Most millipedes people picture are cylindrical: the long, glossy hobby giants that coil into a neat spiral. The Tiny Flat-backed Millipede, Polydesmus sp., is a different animal in a different order. These little flat-backed millipedes belong to the order Polydesmida, and rather than aiming for size and spectacle they earn their keep as micro-cleanup crew, working quietly through leaf litter at a scale that suits a small bioactive setup. Adults reach up to about 25mm, and commonly rather less, which is small by millipede standards, with an orangey-brown to mottled brown body and a flat back edged by little lateral flanges of chitin along each of its twenty segments. The look is understated rather than showy, closer to a temperate woodland floor than a tropical display. These are temperate animals, most likely wild-collected, so I send them out as an establishing culture rather than making any firm captive-bred claim. They travel in careful double-walled packing under live-arrival cover, so you can settle a small starter group and let the culture find its feet. Husbandry is forgiving, and the important thing is that this is a British animal rather than a tropical one. Aim for 10 to 22°C, which an unheated room delivers for most of the year, with no supplementary heating and no bottom heat. Warmth is the risk here, not cold: keep them below about 26°C and out of direct sun, and a cool spare room in winter suits them better than a heated one. Aim for 70 to 80% humidity from damp substrate rather than constant misting, with moderate ventilation to stop the air going stale. Give them depth: at least 5cm of substrate, 8 to 10cm if you can, built from coir, pesticide-free compost and crumbled decaying hardwood, topped with a generous layer of hardwood leaf litter. Never use softwood. A little cuttlebone or crushed eggshell keeps calcium available. They are leaf-litter dwellers rather than climbers, so a wide floor beats a tall enclosure and a snug lid is plenty. When disturbed they tuck into a loose coil rather than rolling into a tight ball the way an isopod conglobates. Feeding is the easy part. They are detritivores, and hardwood leaf litter and decaying hardwood such as oak and beech form the whole foundation of the diet. Beyond that, a little soft veg or fruit now and then, cucumber, courgette or a piece of banana, works as a supplement; lift out anything uneaten after a day or so to keep mould down. They sit alongside springtails and a small isopod colony without treading on the same niche, which is part of why they make such tidy partners in a bioactive tub. The one point worth stating plainly is defence chemistry. Like all flat-backed millipedes, Polydesmus releases a small amount of hydrogen cyanide when stressed, where the cylindrical millipedes rely on benzoquinones instead. The quantity from an animal this size is tiny and poses no danger in open air; wash your hands after handling, and you may catch a faint almond smell as you do. The one thing worth avoiding is a sealed container, since that is the only situation where the gas has anywhere to build up. Keep them away from other sensitive invertebrates in very small, closed containers, and don't mix them with small vertebrates that might try to eat them. Given stable, moist, well-fed conditions they breed steadily. Eggs go down into the substrate, and the young hatch tiny with only three pairs of legs, adding segments and legs through successive moults. Development is slow: the British Myriapod and Isopod Group cites work on a related species putting maturity at 30 to 36 months, so a mixed starter group and a bit of patience serve you far better than any rush. One thing to expect, because it looks like a problem and is not. In Polydesmus angustus, the common British flat-back, both sexes die after reproducing, which is why adults are scarce in the wild over summer. If your adults fade out after a breeding season, that is the animal doing what it does rather than anything you got wrong. What you are keeping going is the culture, not the individuals, so let the young come through and leave the substrate alone. If you are assembling a broader collection, these make a neat order-level counterpoint to the cylindrical species in the millipede collection, and they slot in happily beside the isopod cleanup crews that many keepers already run. Common questions How big do they actually get? Published figures for the common British flat-back put it at up to about 25mm long and 4mm wide, and most animals settle below that, so treat them as small, leaf-litter animals rather than showpiece millipedes. Is the cyanide something to worry about? No. The amount a single small millipede can produce is minute and does nothing to you in a ventilated enclosure; wash your hands afterwards, and keep the tub ventilated rather than sealed, which is the only situation where it could build up at all. Will they get on with my isopods and springtails? Yes. In an invertebrate-only setup they coexist quietly with isopods and springtails and help share the cleanup workload. All in, they are an easygoing, low-key millipede that rewards anyone who values quiet ecological usefulness over drama.
