Quick answer: a woodlouse is a small crustacean that lives on land, a relative of crabs and shrimps rather than an insect. An adult has 14 legs and two pairs of antennae, breathes through flat limbs under its tail, moults in two halves and carries its young in a pouch of fluid under its body. Mostly nocturnal, woodlice feed on dead leaves, rotting wood and fungi, and most live about two years. Britain and Ireland have 43 species living wild, and the ones most often found in gardens are the common shiny, rough, pill, striped and pygmy woodlice.
What Is a Woodlouse?
Woodlice are isopods (order Isopoda), in the same class of crustaceans as crabs, lobsters and shrimps. They make up the suborder Oniscidea, the largest group of isopods, with more than 3,700 described species worldwide and perhaps 5,000 to 7,000 in all, and they are the only crustaceans that have spread into almost every kind of land habitat. One animal is a woodlouse and two or more are woodlice. The name comes from their habit of living in old wood, and they are not lice, which are insects. For the wider family, see our guide to what an isopod is.
The Common Woodlice in Britain
Britain and Ireland have 43 woodlouse species living wild, native or naturalised, and about 20 more that have turned up only in glasshouses. The ones most often found in gardens are these five:
- Common shiny woodlouse (Oniscus asellus) - smooth and glossy, grey to brown with pale patches, up to about 18 mm. It keeps to damper spots than the rough woodlouse and does not roll up.
- Common rough woodlouse (Porcellio scaber) - bumpy and usually slate grey, up to 17 mm. It copes with drier places than most, climbs walls and trees, and is the woodlouse most often found indoors.
- Common pill woodlouse (Armadillidium vulgare) - domed and usually slate grey, up to 18 mm, and rolls into a tight ball. It is commonest in south-east England; further north it keeps mostly to chalk and limestone and to the coast.
- Common striped woodlouse (Philoscia muscorum) - usually mottled brown with a dark stripe down the back and a darker head, up to 11 mm, and quick to run when disturbed. It favours grassy places.
- Common pygmy woodlouse (Trichoniscus pusillus) - purplish red and tiny, about 4.5 mm at most, in damp leaf litter and moss. Males are rarely found, because this species breeds without them.
The biggest British woodlouse is the sea slater (Ligia oceanica) of rocky shores, at up to about 30 mm, and the smallest species are only a few millimetres long. The pill woodlouse in the photos on this page is the southern pill woodlouse, Armadillidium depressum, which reaches about 20 mm and is commonest in south-west England and south Wales, often on walls around buildings.
How Many Legs Does a Woodlouse Have?
An adult woodlouse has 14 legs, one pair on each of the seven body segments behind its head, and all 14 are much the same shape, which is what "isopod" (equal foot) refers to. The young leave the pouch with 12 legs and grow the seventh pair over their first three moults; in most of the species that have been timed, it is in use after the third moult, about three to four weeks after leaving the pouch at around 20°C. Woodlice also have two pairs of antennae, though the first pair is so small you need a lens to see it, and the long second pair does most of the sensing. Apart from the sea slater, whose eyes are far bigger, their small compound eyes, one each side of the head, have 30 or fewer facets each.
How Do Woodlice Breathe?
Behind the legs is a short tail section, the pleon, and its flat paired limbs (pleopods) do the breathing. In many species, the rough and pill woodlice among them, some of these limbs carry pleopodal lungs: tiny branched air tubes that show as white patches under the tail, two pairs of them in Porcellio and Armadillidium. The shiny, striped and pygmy woodlice have no lungs you can see and breathe through the surface of the limbs. All of them need damp air, because their shell is far less waterproof than an insect's, which is why woodlice hide by day under stones, logs and bark.
How Do Woodlice Moult?
Woodlice moult in two halves, which few other animals do. The back half of the shell comes off first and the front half follows, usually one to three days later, so a woodlouse that is half grey and half white is mid-moult rather than ill. In the days before a moult, white patches of stored calcium show under the front of the body, and that calcium goes into the new shell. Woodlice often eat the old skin too, and they keep moulting all their lives.
How Do Woodlice Have Babies?
A female carries her eggs in a brood pouch (the marsupium) under the front of her body, formed from overlapping plates at the bases of her legs. Isopods in the sea have a pouch too, but in most woodlice it became a closed chamber filled with fluid, which the mother supplies along with oxygen, so the young develop in water she carries for them. In the common pill woodlouse the eggs hatch inside the pouch after three to four weeks, and the young stay there for another week or two; broods develop faster in warmth. They come out as mancae, live young that look like pale miniatures of the adults but are still short of their last pair of legs. Our woodlouse life cycle guide follows them from egg to adult.
How Long Do Woodlice Live?
Most woodlice live about two years, and some three or four; the common pill woodlouse, Armadillidium vulgare, can live up to about five. In the wild most never reach those ages: a study of pill woodlice on Breckland heath found high death rates among the newborn young. The pill woodlouse takes about a year to reach breeding age in laboratory studies, and a female produces one to three broods a season, larger females producing more.
Where Do Woodlice Live?
Almost anywhere damp and sheltered: under stones, logs, flowerpots and loose bark, in leaf litter and compost heaps, in the gaps of old walls, and in cellars and greenhouses. The sea slater lives on rocky shores, boulder beaches and harbour walls all round the coast. Woodlice live on every continent except Antarctica, and the common rough woodlouse alone has spread with people to all of them.
What Do Woodlice Eat?
Woodlice are detritivores. They eat dead and rotting leaves, decaying wood, fungi, dead plants and animals, and their own droppings, and the rough woodlouse grazes the green algae on bark and walls. They prefer leaves that have started to rot, which are softer and easier to digest. They rarely eat established living plants while they have enough to eat, though young seedlings are at more risk. Eating droppings is part of how they feed: microbes grow on the droppings once they are out, and a 1982 study found that woodlice kept from their droppings on a diet of leaf litter grew more slowly. The same study tested the idea that they do it mainly to recover copper and did not support it. Our guide to what woodlice eat covers their diet in more detail.
Which Woodlice Roll Into a Ball?
Only some can. Rolling into a ball, called conglobation, is what the pill woodlice (Armadillidium) do best, closing into a sphere with the legs and antennae tucked inside; the shiny, rough and striped woodlice cannot, and clamp down or run instead. Rolling up protects the softer underside and also cuts water loss. Around the world the ability has evolved in at least eight families of woodlice, among them the Armadillidae, the family of the hobby's Cubaris.
Copper-Based Blood
Woodlice carry oxygen in their blood on haemocyanin, a protein built around copper, as crabs and lobsters do, rather than on the iron-based haemoglobin in our blood. A 2009 study of the rough woodlouse suggests its haemocyanin may also play a part in fighting infection. Woodlice take their copper from food and keep a store of it: in one experiment rough woodlice fed a copper-free diet for four weeks lost copper from that store.
How Do Woodlice Drink?
Woodlice drink through their mouths from damp surfaces, and when their tail end is in contact with water they can take it in through the anus as well. A 1954 study found both routes in all four species it tested, the shiny, rough and pill woodlice and the sea slater, and in the sea slater most of the water went in at the rear. Many species can also absorb water vapour straight out of very humid air, once it is above a threshold of roughly 86 to 93% relative humidity, depending on the species.
More Woodlouse Facts
- They give off ammonia. Most of a woodlouse's nitrogen waste leaves its body as ammonia gas, which is why a crowd of woodlice can smell faintly of ammonia.
- They have a chemical defence. Glands in the skin release a sticky secretion against predators such as spiders, and in 2025 researchers identified its active compounds in the rough woodlouse, the first defensive chemicals identified from any crustacean. We found no record of it harming a person.
- Some have no eyes. The ant woodlouse (Platyarthrus hoffmannseggii), which lives in ants' nests, is blind.
- A bright blue woodlouse is ill. A woodlouse that has turned bright blue or violet is usually infected with an iridovirus; our article on whether isopods are poisonous explains why a woodlouse turns blue.
Woodlouse Names
Researchers have recorded at least 250 local names for woodlice across the UK. Slater is the usual word in Scotland and Northern Ireland, chucky pig belongs to south-west England around Bristol and Bath, cheeselog survives near Reading, granny grey is a South Wales valleys name, and in Welsh a woodlouse can be mochyn coed, "wood pig". In North America the rolling kinds are pill bugs or roly-polies and the others are sow bugs. Our guide to woodlouse names maps the rest.
Where Did Woodlice Come From?
Woodlice came from the sea. Isopods as a group appear in the fossil record about 300 million years ago, as marine animals, and DNA dating puts the start of the woodlice's own move to land at roughly the same time, around the end of the Carboniferous (a 2024 study gives about 298 million years, with a wide margin either side). The oldest woodlouse fossils are much younger, about 105 million years old, preserved in amber from northern Spain. That 2024 study points to a single move onto land, and the lung-like breathing organs came later, probably evolving more than once.
Woodlice in the Hobby
Several garden woodlice have colour forms in the hobby. The rough woodlouse comes as Dalmatian, Lava, Whiteout and Yin Yang, among others, in our Porcellio collection, and the pill woodlouse in lines such as Magic Potion and Jelly Bean in our Armadillidium collection. The wider isopods collection runs from those to tropical species, and the wild ones under a log in your garden are worth a closer look too.
Frequently Asked Questions
Is it woodlouse or woodlice?
Both are right: woodlouse is one animal and woodlice is two or more, just as one louse becomes two lice.
Are woodlice nocturnal?
Mostly, yes. They feed at night and spend the day hidden under stones, logs and bark, where the air stays damp, and they move away from light, so a torch sends them back under cover.
Do woodlice drink water?
Yes. They drink through their mouths and can take water in through the anus, and in very humid air many can absorb water vapour as well. They still need a damp hiding place, because they lose water quickly in dry air.
Do woodlice have teeth?
No. They have a pair of small, blunt jaws (mandibles) for scraping and grinding soft, rotting plant matter, and they cannot bite a person. Our article on whether isopods bite covers handling them.
What does woodlouse poo look like?
Small, dark, dry pellets, usually in heaps where woodlice gather under a log or pot. Woodlice eat some of their own droppings again once microbes have grown on them.
Why are woodlice called granny greys or cheeselogs?
They are local names. Granny grey belongs to the South Wales valleys and probably comes from the grey colour, and cheeselog is a southern English name, still used near Reading.
Is a woodlouse an insect?
No. A woodlouse is a crustacean. An insect has six legs, three body sections and one pair of antennae, while a woodlouse has 14 legs, two pairs of antennae and breathes through limbs under its tail. It is not a beetle either, though a rolled-up pill woodlouse is sometimes taken for one.
How big do woodlice get?
The common garden species reach about 11 to 18 mm, the pygmy woodlouse under 5 mm, and the sea slater, the largest British woodlouse, about 30 mm.
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