Armadillidium

Brandt, 1831

pill woodlice, leg pebbles, pill bugs, roly-poly, potato bugs

Species Guides

Armadillidium nasatum(Nosy Pill Woodlouse)
Armadillidium vulgare(common pill-bug)

Armadillidium is a of terrestrial crustaceans commonly known as pill bugs or roly-polies, distinguished by their ability to roll into a ball when disturbed (conglobation). The genus contains approximately 189 recognized , most to Mediterranean regions. These inhabit moist environments and play important roles in decomposition. The most widespread species, A. vulgare, has been introduced globally and serves as a soil bioindicator.

Armadillidium nasatum by (c) Timothy Lindsey, some rights reserved (CC BY), uploaded by Timothy Lindsey. Used under a CC-BY license.

Armadillidium nasatum by (c) Kathy Richardson, some rights reserved (CC BY), uploaded by Kathy Richardson. Used under a CC-BY license.

Armadillidium nasatum by (c) Cole Shoemaker, some rights reserved (CC BY), uploaded by Cole Shoemaker. Used under a CC-BY license.

Pronunciation

How to pronounce Armadillidium: //ˌɑːrmədɪˈlɪdiəm//

These audio files are automatically generated. While they are not always 100% accurate, they are a good starting point.

Identification

Distinguished from other woodlice by ability to roll into complete ball (volvation/conglobation). Key morphological features: convex oblong body shape, multiple black ocelli, epistome passing frontal edge and vertically directed (often triangular), short with slightly wavy margin, present on opercular plates of first two pleopoda pairs. Distinguished from pill millipedes (Glomeris, Sphaerotherium) by having 14 legs (7 pairs) versus millipedes' 30+ legs and two pairs per segment. When rolled, millipedes form perfect sphere with no gaps; Armadillidium leaves small gaps.

Images

Appearance

Small terrestrial crustaceans with dorsoventrally flattened, convex oblong bodies. Size ranges from 5–25 mm. Coloration typically gray, brown, or black; some display distinctive patterns (e.g., A. klugii with red hourglass markings resembling widow spiders). Body divided into distinct segments with multiple black ocelli. First small, second antennae approximately half body length. End segment of triangular or rectangular. Short uropods visible when not conglobated. lacks waxy , making them susceptible to desiccation.

Habitat

Moist terrestrial environments including leaf litter, soil, under rocks, and decaying wood. Require humid conditions due to lack of waxy and dependence on moist pseudotracheae for respiration. Often found in settings such as gardens, greenhouses, and agricultural areas. Seek dark, cramped spaces and aggregate under shelters during daylight hours.

Distribution

Native to Palearctic region, with highest diversity in Mediterranean basin. Most to small regions near Mediterranean Sea. A. vulgare most widespread: native from England to Russia, introduced to North America (including California), South Africa, Australia, Madagascar, Canada, and Japan. A. arcangelii spreading across Europe via plant transport. Records from Slovenia, Tunisia, and global temperate regions.

Seasonality

activity pattern; most active at night. seasonal, typically summer months. Can live 2–5 years depending on and conditions. Activity reduced in winter; travel distances significantly lower than summer. Seek high ground during flooding events. Move underground or deeper into soil during extreme heat and drought.

Diet

Primarily detritophagous omnivores. Feed on decaying organic matter including leaves, decomposed wood fibers, moss, , bark, and lichen. Practice (consumption of fecal pellets), which accounts for approximately 10% of diet and aids nutrient absorption through secondary digestion. Some show feeding preferences: A. vulgare prefers thistle, tarweed, and vetch. Require for hemocyanin-based oxygen transport; copper-deficient individuals select copper-rich foods.

Life Cycle

Direct development without larval stages. Females possess marsupium ( pouch) formed during parturial , 1–200 for 1–2 weeks. Marsupium provides nutrients, calcium, water, and oxygen to developing embryos. Hatchlings called 'mancae'—first instar with 6 pereopods (versus 7 in ). Molt 2–3 times to reach adult form. Sexual maturity at approximately 1 year. : females produce 1–3 broods annually, up to 200 eggs per brood. Larger females produce larger broods. Females can store sperm in seminal receptacle for multiple broods from single mating.

Behavior

Conglobation (rolling into ball) primary defense when disturbed, though seeking shelter in dark spaces is more important avoidance strategy. Photonegative: avoid light, especially UV radiation. Aggregate in groups, attracted by touch and in . Travel up to 13 meters in half-day during summer foraging. Climb to elevated positions (trees) to escape flooding. Move underground during extreme heat. Some produce pungent defensive secretions from glands. determines nocturnality, influenced by moisture and light.

Ecological Role

contributing to decomposition and nutrient cycling. Serve as soil bioindicators, particularly A. vulgare, due to sensitivity to environmental changes including soil pH, temperature, vegetation, and water availability. Prey for various including birds, spiders, lizards, and salamanders. for including Acanthocephalan Plagiorhynchus cylindraceus, which manipulates host to increase risk. Food source for young reptiles and amphibians in bioactive systems.

Human Relevance

Considered minor household and agricultural pests in greenhouses, though beneficial as . Used in pet trade (Spain), with 57% of traded native to that country; A. album, A. arcangelii, and A. assimile commonly traded. Potential risk from introduced . A. vulgare used as global soil bioindicator for monitoring environmental health and effects of climate change. Popular educational organism for teaching crustacean and .

Similar Taxa

Glomeris marginataPill with similar conglobation ; distinguished by 30+ legs (two pairs per body segment versus 7 pairs in Armadillidium) and perfect spherical roll without gaps.
PorcellioRelated in Porcellionidae; cannot conglobate (roll into ball), has flatter body profile, and typically shows more elongate body form.
Perisphaerus (Blaberidae) with superficially similar conglobation ; distinguished by six legs, five-segmented with , and exposed on abdominal segments.

Misconceptions

Frequently confused with pill millipedes despite being crustaceans, not myriapods. 'potato bugs' and 'roly-poly' also applied to unrelated insects. Not insects: have 14 legs (7 pairs) versus insect 6 legs. Despite '' name, not true bugs (Hemiptera). Some ' coloration (A. klugii) resembles venomous widow spiders, but this is harmless , not indication of .

More Details

Physiological specializations

Excrete nitrogenous waste as ammonia gas rather than , without water loss or toxic effects. High and ammonia . Respire through pseudotracheae—white, bean-shaped structures on first two abdominal segments that must remain moist. Hemocyanin-based oxygen transport requires dietary copper. Simple with straight gut and only two pairs of digestive glands.

Reproductive biology

bacterial can feminize males in some , potentially skewing sex ratios. Multiple paternity common; genetic females mate with more males than neo-females (Wolbachia-infected feminized males), maintaining genetic diversity despite infection. triggered by consistent light levels, increased temperature, and male presence (which accelerates female maturation in some ).

Fossil record

Terrestrial isopod fossils rare due to low preservation potential of chitinous skeletons. Armadillidium fossils and traces known from Miocene and Pleistocene, though dating uncertain. Oligocene member described from Hungary in 1985. Suborder Oniscidea (including Armadillidium) represents one of few crustacean lineages to successfully colonize terrestrial environments.