THE ANIMAL FILES

Common pill woodlouse

Armadillidium vulgare

Rolling into a ball protects a body that also needs moisture.

The common pill woodlouse is a terrestrial crustacean, not an insect. Its segmented armour lets it roll into a ball, while the risk of water loss helps explain its use of shaded, humid places beneath stones and wood.[1][2]

Follow the evidence ↓
Common pill woodlouse
Image: Franco Folini · Commons · CC BY 2.5
Animal group
A terrestrial isopod crustacean[2]
Defensive movement
Conglobation: curling into a ball[1]
Frequent food
Decomposing plant litter[1]

A ball made from segmented plates

The exoskeleton consists of protective plates containing chitin, proteins and calcium carbonate. Body segmentation permits the characteristic inward curl. Rolling brings the tougher outer covering around the softer underside, and can occur after touch or nearby vibration.[1]

Moisture changes the feeding opportunity

Pill woodlice are vulnerable to drying out and often shelter in humid, shaded microhabitats. Feeding increases under suitable moist conditions, while drier air can keep them under cover for longer. Time spent hiding can therefore reflect water balance as well as the presence of predators.[1]

Litter is part of a mixed detritivore diet

Decomposing plant material is a principal food, but fungi, seeds and other organic matter are also eaten. By consuming material on the forest floor, woodlice participate in the movement of organic resources through a litter community. This role depends on suitable moisture for the animal to stay active.[1]

A COMMON MIX-UP

Is a pill woodlouse an insect because it lives on land?

No. NCBI places it among isopod crustaceans. Habitat alone does not determine animal classification, and crustacean lineages include terrestrial forms as well as aquatic ones.[2]

TRY A LITTLE REASONING

Why might woodlice stay hidden on a dry day without a predator nearby?

Reveal the explanation

Exposure can increase water loss. Sheltering preserves a more humid setting, so the decision to forage reflects a balance between obtaining food and maintaining the conditions needed for survival.[1]

Sources & editorial notes

Adapted and translated from the cited Wikipedia revisions, with source-linked reasoning. Image credit and licence are listed separately above.

Adapted and translated from Wikipedia contributors; exact revisions are linked below. CC BY-SA 4.0

  1. Wikipedia contributors — Common pill woodlouse (fixed revision; adapted and translated)
  2. NCBI Taxonomy — Armadillidium vulgare (identity and classification)

Last fact check:

One discovery leads to another

Giant tiger prawnA striped prawn whose address changes as it grows.Penaeus monodon ↗European lobsterTwo claws, two different kinds of work.Homarus gammarus ↗Noble crayfishA night forager with a sheltered daytime address.Astacus astacus ↗