Woodlice are fascinating from a classification perspective - they're terrestrial crustaceans that have undergone enormous diversification across roughly 3,700 described species worldwide. This guide covers their taxonomic position, body structure, common UK species, evolutionary history, and ecological roles, with proper attention to where hobby species fit within the broader picture.
Scientific Classification
Worth being precise about woodlouse taxonomy:
- Phylum: Arthropoda (jointed-leg invertebrates)
- Subphylum: Crustacea (alongside crabs, lobsters, shrimp)
- Class: Malacostraca (the largest crustacean class)
- Order: Isopoda (flattened-body crustaceans)
- Suborder: Oniscidea (terrestrial isopods - the woodlice)
Within Oniscidea, woodlice are organised into several families, each containing many genera. Key families include:
- Armadillidiidae - pill bugs (Armadillidium genus), the conglobators that roll into balls
- Porcellionidae - including Porcellio (Dairy Cow, Giant Orange) and Porcellionides (Powder species)
- Oniscidae - Oniscus (Common Shiny Woodlouse)
- Trichoniscidae - small soil-dwelling species like Trichoniscus and Trichorhina (Dwarf Whites)
- Ligiidae - coastal species including Ligia oceanica (Sea Slater)
- Trachelipodidae - including Trachelipus species
For more on the isopod-insect distinction see our article on are isopods insects.
Species Diversity
Globally, around 3,700 described species of terrestrial woodlice (suborder Oniscidea). New species are still being described regularly - the total including undescribed species may approach 4,000-5,000.
Five Common UK Native Species
Britain and Ireland have roughly 35-40 native woodlouse species, but five are most commonly encountered:
- Common Shiny Woodlouse (Oniscus asellus) - smooth shiny grey-brown, 12-16mm, yellow patches on sides
- Common Rough Woodlouse (Porcellio scaber) - grey with bumpy texture, 12-17mm
- Common Striped Woodlouse (Philoscia muscorum) - mottled brown with dark central stripe, 8-11mm
- Common Pygmy Woodlouse (Trichoniscus pusillus) - small pinkish-brown, 3-5mm
- Common Pill Bug (Armadillidium vulgare) - dark grey-brown, 12-18mm, conglobates fully
Aquatic and Semi-Aquatic Relatives
While most woodlice are terrestrial, some related species live in moist coastal environments. The most familiar UK example is the sea slater (Ligia oceanica) - a 25-30mm splash-zone species. Worth being clear: sea slaters are AIR-BREATHING through pleopodal lungs (like other terrestrial woodlice), NOT water-breathing through gills. They survive brief immersion but aren't truly aquatic - they're adapted to splash zone conditions where they get regular wetting from sea spray.
Different from marine isopods (other suborders within Isopoda) which evolved separately and never left the water.
Body Structure
Woodlice exhibit distinctive morphology:
Segmentation
The woodlouse body is divided into 13 distinct segments:
- Pereon (thorax) - 7 segments, each bearing a pair of jointed legs (14 legs total)
- Pleon (abdomen) - 6 smaller segments containing the pleopodal lungs and reproductive structures
The first thoracic segment is often fused with the head, giving woodlice their characteristic "head plus body" appearance.
Size Range
Far wider variation than commonly assumed:
- Smallest UK natives - Trichoniscus pusillus at 3-5mm
- Standard UK natives - Porcellio scaber, Armadillidium vulgare at 12-18mm
- UK natives (largest) - Ligia oceanica at 25-30mm
- Hobby giants - Porcellio expansus, P. hoffmannseggii, P. magnificus at 35-40mm
Colour Range
Woodlice come in a remarkable variety of colours through both natural variation and selective breeding:
- Natural colours - grey, brown, occasional pink-purple in some species
- Selectively-bred morphs - orange, yellow, blue, white, black, multicoloured patterns
Conglobation
The iconic woodlouse defence behaviour - but NOT all species do it. Only conglobators (mainly Armadillidiidae and most Cubaris) can fully roll into balls. Non-conglobators (Porcellio, Porcellionides, Oniscus species) run or flatten when threatened instead.
For more anatomical detail see our woodlouse anatomy article.
Evolutionary History
Worth being clear about the timeline:
- ~300 million years ago (Carboniferous period) - isopods first evolved from marine ancestors
- ~100 million years ago (Cretaceous period) - oldest confirmed terrestrial woodlouse fossils. Possible that woodlice transitioned to land earlier, but fossil evidence is limited
- Today - terrestrial woodlice are the only fully land-adapted crustacean lineage with global distribution
The transition from sea to land required several major adaptations:
- Pleopodal lungs evolved from gills
- Brood pouch developed to protect offspring from desiccation
- Improved water-conservation mechanisms (though woodlice still need humid air)
- Loss of fully aquatic locomotion
One of the few crustacean lineages to fully colonise terrestrial environments.
Reproduction and Life Cycle
The Marsupium
Distinctive feature of isopod reproduction. Female woodlice carry fertilised eggs in a marsupium (brood pouch) on the underside of the body, formed by overlapping plates attached to the bases of the first five pairs of legs.
Typical egg counts vary by species:
- A. vulgare - typically 50-150 eggs per brood
- Smaller species - 20-60 eggs
- Larger species - can produce more
Mancae Development
When eggs hatch within the marsupium, the offspring are called mancae. Distinctive features:
- Small, white, curled forms initially
- Six leg-bearing pereon segments (one less than adults)
- Lack the seventh pair of legs at first
- Develop the missing leg pair after their first moult
- Gradually mature through successive moults
Important distinction: woodlice don't lay eggs that hatch externally. They give birth to live young that emerge from the marsupium as miniature versions of adults. For more on this see our woodlouse life cycle article.
Common Names and Regional Variation
Woodlice have a remarkable variety of common names reflecting local culture and linguistic diversity:
UK Regional Names
- Cheeselog/Cheesy bug - used in parts of southern England (especially Berkshire/Reading area)
- Chucky pig - used in parts of England
- Granny grey - used in some regions
- Pill bug - for conglobating species (Armadillidium)
International Names
- Slater/Slater bug - Australia and New Zealand
- Sow bug - North America
- Roly-poly/Pill bug - North America
- Doodlebug - some regional US use
The linguistic diversity reflects how widely distributed woodlice are across temperate and tropical regions worldwide.
Habitat Preferences
Most woodlice prefer damp, dark environments. Common habitats:
- Garden settings - under stones, in compost heaps, beneath bark
- Woodland - leaf litter, decaying logs, under loose bark
- Coastal - splash zones (Ligia oceanica)
- Caves - specialist species adapted to dark stable conditions (some Cubaris)
- Urban environments - basements, around foundations
- Soil ecosystems - throughout temperate and tropical regions
The common requirement: humidity. Without humid air, woodlice's pleopodal lungs can't exchange gas.
Diet and Ecological Role
Woodlice are detritivores - animals that feed on decaying organic matter:
- Decaying leaves - the dietary foundation
- Decaying wood - particularly hardwoods
- Fungi - both as food and source of moisture
- Decaying plant matter generally
- Occasional protein - dead invertebrates, occasionally other organic matter
Worth being clear: woodlice generally don't damage healthy plants. They process decaying material that would otherwise sit unbroken-down.
Ecological Contributions
- Decomposition acceleration - processing leaf litter and dead wood
- Nutrient cycling - returning organic matter to soil as frass
- Soil structure - burrowing species contribute to aeration
- Food web links - prey for spiders, ground beetles, frogs, birds, hedgehogs
- Microbial distribution - spreading beneficial fungi and bacteria
Predators
Despite their defences, woodlice have several specialised predators:
- Woodlouse spider (Dysdera crocata) - evolved specifically to prey on woodlice
- Ground beetles - process small woodlice
- Centipedes - larger species take small woodlice
- Amphibians - frogs and toads opportunistically feed
- Birds - particularly insectivorous species
- Hedgehogs and other small mammals
Why Classification Matters for Keepers
Understanding woodlouse classification helps keepers make better decisions:
- Family informs husbandry - Armadillidiidae (conglobators) generally need humidity gradient; Porcellionidae often need more ventilation
- Avoiding species confusion - one mix-up to avoid is filing the Giant Orange under Porcellio expansus; the Giant Orange is an orange form of P. laevis, the Dairy Cow's species, while P. expansus is a larger Spanish animal
- Choosing compatible groups - within-family species often have similar needs
- Understanding hobby names - "Magic Potion", "Rubber Ducky" are hobby names for specific morphs of specific species
Browse our Armadillidium collection for the pill bug family, or our full isopods collection for the broader range across families.
The Honest Summary
Woodlice (suborder Oniscidea) are the only fully terrestrial crustaceans:
- ~3,700 described species worldwide - with new descriptions still being added
- 13 body segments - 7 thoracic with leg pairs, 6 abdominal with respiratory and reproductive structures
- 14 legs total - 7 pairs on the thoracic segments
- Pleopodal lungs - air-breathing structures evolved from gills
- Live-bearing reproduction - via marsupium
- Ancient lineage - terrestrial isopods established for at least 100 million years
- Ecologically essential - detritivores supporting soil ecosystems globally
For UK keepers, the practical implication is that the species in our hobby (Dairy Cow, Powder Orange, Zebra, Rubber Ducky) all fit within this same Oniscidea suborder despite their dramatic visual differences. Understanding the classification framework helps navigate the species diversity. Browse our isopods collection for the UK hobby range.
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