Glomeris marginata is found throughout central and north-western Europe, from Poland and Scandinavia to Spain and Italy.^[3] In the British Isles, it is found in all areas south of the Central Belt of Scotland.^[4]

Glomeris marginata grows up to 20 millimetres (0.8 in) long and 8 mm (0.3 in) wide, and is covered by twelve black dorsal plates with white rims.^[5][6] Each segment except those at the front and back bears two pairs of legs, with around 18 pairs in total.^[5] This distinguishes pill millipedes from pill woodlice, both of which are called "pillbugs" — woodlice have 7 pairs of walking legs, one per body segment, while millipedes have more pairs, and with two pairs to each apparent body segment.

When rolled into a ball, G. marginata can be distinguished from a rolled–up pill woodlouse by the asymmetrical ball it rolls into; pill woodlice roll into much more perfect spheres.^[7] The cuticle is also darker and shinier, and the antennae are shorter.^[8] The head and tail of a woodlouse have a greater number of small plates, while the head of Glomeris is protected by a single large shield, and it lacks the uropods of woodlice at the read end.^[5] Although usually black, red, yellow and brown individuals do occasionally occur.^[2]

Glomeris marginata lives in leaf litter as well as in grass and under stones, with a preference for calcareous soils.^[2] In domestic gardens, they are most frequent along hedgerows and at the bases of old walls, where the mortar has started to crumble, leaching lime into the soil.^[5][8] It is less prone to desiccation than other millipedes and can be found in the open, even in sunny weather,^[6] although they are more active at night and prefer more humid areas.^[2] G. marginata feeds on old, rotting leaves, despite the higher nutrient content of freshly–fallen leaves, and G. marginata can be responsible for recycling a significant proportion of the nutrients in the leaf litter.^[9]

Predators of Glomeris marginata are reported to include the starling, the common toad, the woodlouse spider,^[8] and hedgehogs.^[6] As well as rolling up into a ball for protection, G. marginata produces noxious chemicals to ward off potential predators, as many millipedes do. One to eight drops of a viscid fluid are secreted, containing the quinazolinone alkaloids glomerin and homoglomerin, dissolved in a watery protein matrix.^[10] These chemicals act as antifeedants and toxins to spiders, insects and vertebrates, and the fluid is sticky enough to entrap the legs of ants.^[10] Having completely discharged these chemical defences, it can take up to four months for the millipede to replenish their supplies.^[10]

Breeding takes place cyclically in spring and summer. Male G. marginata are capable of producing a pheromone which attracts females, although this is only believed to be effective over short distances.^[11] They then transfer sperm to the female using their gonopods, specially modified legs. After fertilisation, the females lay 70–80 eggs, each about 1 mm long, and each wrapped singly in a capsule of digested earth.^[2] The eggs hatch after a period of about two months, with the duration depending on factors including temperature.^[6] Development lasts for up to three years, and involves nine moults, following which moults occur periodically as adults.^[2] Females are fertile for several years, and can produce six broods over their lifetime,^[6] which can be up to eleven years.^[2]

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