FaunaHub

Woodlouse (suborder Oniscidea)

InvertebrateCrustaceanDecomposer

Woodlouse (Armadillidium vulgare), a grey land crustacean with segmented armour.

Common pill woodlouse (Armadillidium vulgare).

Image: Michel Langeveld, CC BY-SA 4.0, via Wikimedia Commons.

Overview

The woodlouse (suborder Oniscidea) is a small, oval, armoured little animal found under logs, stones, leaf litter, and plant pots almost everywhere — and despite living on land among insects, it is not an insect at all. Woodlice are crustaceans, relatives of crabs, shrimps, and lobsters, and they are the only major group of crustaceans to have fully conquered life on land. Their segmented, overlapping plates of armour and seven pairs of legs give them their distinctive look.

Woodlice go by a wonderful array of local names — pill bugs, sow bugs, roly-polies, slaters, and many more — and some species (the “pill” woodlice) can roll up into a tight ball for protection.

Note: “woodlouse” covers many species; details here describe the group broadly. Treat general statements as approximate and verify against authoritative sources.

Habitat & Range

Woodlice live in damp, sheltered places on land worldwide — under logs, bark, stones, and leaf litter, in soil and compost, and in gardens, cellars, and greenhouses. Because they breathe through gill-like structures that must stay moist, they need humid microhabitats and avoid drying out, hiding by day and becoming active in cool, damp conditions.

Diet

Woodlice are detritivores, feeding mainly on dead and decaying plant material — fallen leaves, rotting wood, and other organic debris — along with some fungi and occasionally their own moulted skins or droppings to recover nutrients. By breaking down dead plant matter, woodlice are valuable recyclers that help return nutrients to the soil and keep decomposition cycles turning.

Behavior

As crustaceans on land, woodlice keep some watery habits: they breathe through modified gills that need moisture, so they cluster in damp hideaways and are most active at night. Pill woodlice can roll into a protective ball (conglobation) when threatened, shielding their soft undersides behind their armour — a trick that also helps reduce water loss. They moult in two stages, shedding the back half of their armour first and then the front, and females carry their eggs and young in a fluid-filled brood pouch on the underside of the body, a relic of their aquatic ancestry. Woodlice are completely harmless to people and do not bite or sting.

Human Interaction & Conservation

Woodlice are harmless and helpful garden allies, speeding the decay of dead plant matter and enriching the soil; they only very rarely nibble living plants and are not pests in any meaningful sense. They are common and widespread, of no conservation concern as a group, and are a great, safe introduction to the wonders of invertebrate life for curious children and naturalists. Consult authoritative sources for details on specific species.

More photos of the woodlouse

A woodlouse on soil, showing its overlapping armoured plates.

Woodlouse (Armadillidium vulgare).

Image: Daniel Das, CC0, via Wikimedia Commons.

Frequently Asked Questions — Woodlouse

Is a woodlouse an insect?
No — it's a crustacean, more closely related to crabs, shrimps, and lobsters than to insects. Woodlice are the only major group of crustaceans fully adapted to life on land. You can tell they're not insects by their seven pairs of legs (insects have three pairs) and their segmented, overlapping armour.
Why do some woodlice roll into a ball?
The 'pill' woodlice can roll up into a tight ball, a behaviour called conglobation, to protect their soft undersides behind their hard armour when threatened. It also helps reduce water loss. Not all woodlice can do this — the flatter 'sow bugs' generally can't roll up as completely as the rounder 'pill bugs.'
What do woodlice eat, and are they harmful?
Woodlice are decomposers that eat dead and decaying plant material, rotting wood, and fungi, helping recycle nutrients into the soil. They're harmless — they don't bite, sting, or carry disease, and only very rarely nibble living plants. In the garden they're beneficial recyclers, not pests.
How can a land animal be a crustacean?
Woodlice descend from aquatic crustaceans and still carry traces of that heritage: they breathe through gill-like structures that must stay moist, so they live in damp places, and females brood their young in a fluid-filled pouch. By keeping their gills wet and hiding from dry conditions, they manage a fully terrestrial life — a rare feat for a crustacean.

Sources and further reading

Authoritative wildlife references used for general educational context. Conservation status should always be verified against current IUCN Red List data. External links open in a new tab.