Flat-Backed Millipedes: The One Already in Your Garden

Every other millipede I sell is a cylinder. Pick one up and it coils into a flat spiral like a watch spring, and its body in cross-section is a tube. The flat-backed millipedes are not built that way, and the difference is not cosmetic. They belong to a separate order, Polydesmida, and once you know that, three things about keeping them stop being surprising: the chemistry is different, the temperature is different, and the life cycle ends in a way the giants' does not.

They are also, if you garden in Britain, already living outside your back door.

What you are looking at

The common British flat-back is Polydesmus angustus. NatureSpot describes it as up to 25 mm long and 4 mm wide with about 20 segments, a flattened brownish animal with off-white legs. The flattening comes from small lateral keels running down each side, which is what gives the order its shape and its name.

The British Myriapod and Isopod Group lists it as common and widespread across Britain and Ireland, and notes that it is "very catholic in its requirements and is found in almost any habitat where millipedes occur". In practice that means compost heaps, leaf litter, dead wood and the underside of anything left on damp soil. If you have turned over a flowerpot and found a small flat millipede, that was one.

The temperature question, and I had this wrong

Worth dwelling on, because the listing here carried the wrong figure until recently and saying so seems better than quietly editing it out.

The copy said to keep them around 18 to 24°C, which is the band you would give a tropical species. It is the wrong band for an animal that overwinters in a British compost heap. Work by David, Geoffroy and Vannier, published in the European Journal of Entomology in 2003, measured supercooling points of P. angustus egg batches between -14.8 and -30.6°C, and found the eggs unaffected by prolonged exposure to low temperatures above 0°C. Cold hardiness is highest in the egg and decreases as the animal develops, but nothing in that picture describes a species needing to be kept warm.

So: 10 to 22°C, which an unheated British room delivers for most of the year. No heat mat, and certainly nothing underneath the enclosure. The risk with this species is the other direction entirely. Hold them under roughly 26°C, keep the sun off them, and a cool spare room in winter serves them better than a warm lounge.

The cyanide, stated plainly and then put in proportion

Flat-backed millipedes defend themselves with hydrogen cyanide. That is not a hobby rumour: Makarov and colleagues, writing in the Journal of Chemical Ecology in 2010, identified nine compounds in three polydesmid species including Polydesmus complanatus, among them hydrogen cyanide, benzaldehyde and mandelonitrile. The cylindrical millipedes in the rest of the range do something different, leaning on benzoquinones instead, which is why the flat-backs smell faintly of almonds when they are annoyed and the giants stain your fingers brown.

Now the proportion. The quantity an animal 25 mm long can produce is minute, and in open air it does nothing to you. Wash your hands after handling, which you should do with any invertebrate. The one situation worth avoiding is a small sealed container, because it is the only place the gas has anywhere to accumulate. Ventilate the enclosure, do not transport them in an airtight tub for hours, and nothing else here needs thinking about.

The life cycle nobody warns you about

Two facts sit together here, and between them they decide whether you enjoy this animal or not.

The first is that development is slow. The recording group cites work on a related species putting maturity at 30 to 36 months. That is two to three years from egg to breeding adult, in an animal 25 mm long.

The second is the one that catches keepers out. In P. angustus, both sexes die after reproducing, which is why the recording data show adults scarce over the summer. If you buy a group of adults, breed them successfully, and then watch them fade out over the following weeks, nothing has gone wrong. That is the animal completing its life.

In practice you are running a culture, not a collection of individuals. Buy a mixed group instead of a matched pair of adults, leave the bed alone instead of sifting it for eggs, and give the young the two years they need. Anyone hoping for a display animal they will still be watching in five years wants a cylindrical species instead.

Keeping them

Start with depth. Five centimetres is a floor and eight to ten is better, and the recipe matters less than the principle that most of the bed should be rotting wood. Coir and pesticide-free compost make up the bulk, crumbled decayed hardwood does the actual feeding, and a thick cap of oak or beech goes over the top. Softwood is the one thing to keep out, since the resins do not suit millipedes.

Moisture should come from underneath. Keep the bed damp all the way down, aim for seventy to eighty per cent in the air, and ventilate well enough that it never smells sour. Spraying the surface twice a day gives you a wet lid over a dry bed, which is the wrong way round. Keep cuttlebone or crushed eggshell present at all times, because the shell is rebuilt at every moult and that calcium has to come from somewhere.

Height is wasted on them. These are litter-layer animals, so floor area buys you more than a tall box, and an ordinary close-fitting lid does the job. Feeding barely counts as a task once the substrate is right, since the substrate is the food. A slice of cucumber or courgette now and then is welcome, lifted out the next day before it goes mouldy.

Where they fit

They sit alongside isopods and springtails without competing for the same job. Isopods work the substrate, springtails graze the surface film, and the flat-backs process litter at a size that fits a small bioactive tub. For anyone assembling a collection, they are the order-level counterpoint to everything else on the shelf: same class, different architecture, different chemistry, different climate.

If that appeals, the tiny flat-backed millipedes are here, and they are among the least demanding animals I stock, provided you do not try to keep them warm.