Armadillidium vulgare

Distinguished from congeneric species Armadillidium nasatum and A. depressum by forming a complete sphere without gaps when conglobated; the latter two species leave visible gaps. Differs from pill millipedes (Glomeris marginata and relatives) by having only seven leg pairs (14 legs total) versus millipedes' numerous leg pairs, and by the distinct thoracic segmentation visible when unrolled. The unstalked eyes and antennae with dense bristles are additional distinguishing features.

Adults range 8.5–18 mm in length with an oval, highly convex body capable of complete conglobation (rolling into a ball). The exoskeleton consists of protective plates typically dark grey to brown with faint cream or yellow spotting, though albino and other color morphs occur. The 40 μm-thick cuticle contains chitin, hardened proteins, and calcium carbonate in four layers. Two pigment cells control coloration: ommochrome (black) and pteridine (yellow). The head bears unstalked eyes with single photoreceptors and antennae with dense sensory bristles. Seven thoracic segments each bear one pair of legs; females carry eggs in a ventral marsupium on segments 2–5. The abdomen bears white, spongiform pleopods for gas exchange and two tail appendages (uropods) for water transport.

Requires humid microhabitats to prevent desiccation; thrives under rocks, logs, and leaf litter in forests, gardens, and coastal areas. Shows greater drought tolerance than many woodlouse species but restricted to calcareous soils or coastal habitats. In winter, avoids exposed shores for areas near unfrozen water where soil remains workable. Anthropophilic, utilizing cardboard and other human waste materials.

Native to Mediterranean Europe; introduced and naturalized worldwide including North America (where populations may reach 10,000 individuals/m² in California coastal grasslands), Madagascar, Australia, and South Africa. Now among the most abundant invertebrates in California coastal grassland habitats.

Active year-round in suitable microhabitats, with dormancy induced below 0°C. Foraging activity peaks at night when temperatures are lower and humidity higher. Reproduction occurs primarily during warmer months with females capable of up to 3 broods per season.

Primarily detritivorous, feeding on decomposing plant litter; also consumes seeds (granivory), living plant matter, dead animal remains, fungi, and dung. Fungal consumption contributes to carbon cycling by reducing fungal decomposition rates and associated CO₂ release.

Direct development without larval stage; lecithotrophic eggs (yolk-nourished) develop in female's marsupium for 3–4 weeks post-hatching. Juveniles emerge as miniature adults, molting within 24 hours to develop the seventh body segment, then again after two weeks for final leg pair. Adults molt on approximately 29-day cycles, with 2-day vulnerability period while new exoskeleton calcifies. Lifespan up to 3 years. Females produce up to 40 young per brood, with 3 broods possible per season; sperm storage allows up to 12 months between mating and fertilization.

Exhibits conglobation (defensive rolling) in response to tactile stimuli or vibrations, protecting soft ventral surfaces with the thick dorsal cuticle. Death-feigning observed under extreme stress. Negative thermotaxis and positive thigmotaxis (preference for contact/crevices) often conflict, with thigmotaxis sometimes overriding temperature preferences even in dangerous heat. Aggregation behavior reduces desiccation risk and synchronizes female receptivity for mating. Nocturnal foraging pattern with increased sheltering during dry conditions.

Key detritivore in decomposition and nitrogen cycling. Serves as prey for spiders, centipedes, beetles, salamanders, shrews, toads, and carabids. Egg-brooding females experience increased predation risk due to reduced mobility. Mitigates climate feedback effects by suppressing fungal decomposition and associated carbon dioxide release.

Popular in exotic pet trade with selective breeding for color morphs; the "Punta Cana" morph remains taxonomically disputed (A. sordidum versus A. vulgare variant). Laboratory model for mitochondrial genome studies (unusually large 42 kb partially linear mtDNA). Generally considered a benign household presence, non-venomous and non-pathogenic, though sometimes dismissed as a pest.

• Armadillidium nasatumCongeneric species distinguishable by gap left when rolling into ball
• Armadillidium depressumCongeneric species distinguishable by gap left when rolling into ball
• Glomeris marginataPill millipede with superficially similar conglobation behavior; distinguished by numerous leg pairs and different segmentation pattern

• woodlouse
• terrestrial isopod
• conglobation
• detritivore
• model organism
• invasive species
• pet trade
• mitochondrial genome
• desiccation resistance
• calcium carbonate
• marsupium
• direct development
• nocturnal
• aggregation
• thermotaxis
• thigmotaxis
• calcareous soils
• nitrogen cycling
• climate mitigation

• BugGuide
• Wikipedia
• GBIF taxonomy match
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
• Bohart Museum Open House on 'Many Legged Wonders': How Many Legs Does an Isopod Have? | Bug Squad
• ID Challenge #18 | Beetles In The Bush
• Eumastacidae | Beetles In The Bush