Pill Millipede

Glomeris marginata — the European pill millipede — is one of about 280 species in order Glomerida (the pill millipedes) and one of the most-cited examples of CONVERGENT EVOLUTION in arthropod biology. The species is widespread across temperate Europe in moist deciduous forest leaf litter. Adults are 7-20 mm long, dark brown-to-black with bright yellow-cream margins on each body segment. Like other Glomerida, the species' defining defensive behavior is the ability to ROLL INTO A TIGHT ARMORED BALL — the body segments interlock so that the hardened dorsal sclerites form a continuous protective shell, and the soft ventral surface, head, and legs are completely enclosed inside. The rolled ball is approximately the size of a small pea (4-7 mm diameter) and is essentially impervious to most predators (bird beaks, ant mandibles, small mammal jaws cannot pry open the locked sclerites). The species' major scientific significance is the parallel between pill millipedes and PILLBUGS (Armadillidium and related Isopoda crustaceans). Pillbugs and pill millipedes look almost identical when rolled into balls — same size, same general dorsal sclerite architecture, same rolled-up posture, same response to threats. But the two groups are completely unrelated: pillbugs are CRUSTACEANS (class Crustacea, with one pair of legs per body segment, breathing through gills), while pill millipedes are DIPLOPODS (class Diplopoda, with two pairs of legs per body segment, breathing through tracheae). The rolled-up-ball morphology is one of the most-cited examples of convergent evolution in introductory biology curricula. Field-identification tip: unrolled pill millipedes have many more body segments (and twice as many legs per segment) than pillbugs; pill millipede heads are also visible only at the front (vs. pillbugs, where the head is more clearly demarcated). The species is detritivorous, feeding on decaying leaf litter, and is an important participant in temperate forest decomposition cycles. Pill millipedes are completely harmless to humans (no venom, no bite, no sting).