Woodlouse Anatomy Explained

Woodlice are land-living crustaceans, and their anatomy is a record of how they made the move from sea to land. Beneath that familiar armoured shell sits a segmented body with seven pairs of legs, antennae for sensing the world, gill-derived lungs for breathing air, and a brood pouch for raising young in a pocket of moisture. This guide walks through woodlouse anatomy from the outside in - exoskeleton, segments, senses, breathing, reproduction and digestion - and the adaptations that let these former sea creatures thrive on land.

It's worth remembering throughout that woodlice are crustaceans, not insects - more closely related to crabs and shrimp - which is exactly why so much of their anatomy looks the way it does.

The Exoskeleton and External Body

A woodlouse's most obvious feature is its tough, segmented exoskeleton. The overlapping plates act as armour against physical damage and, just as importantly, help slow water loss - a constant challenge for a land animal descended from the sea. The body is flattened, which lets a woodlouse squeeze into narrow gaps under bark and stones where it stays damp and hidden.

At the head are two pairs of antennae. The longer, obvious pair are the main sensory tools, feeling out the surroundings and detecting movement and vibration; the second pair are much smaller and far less conspicuous. The antennae also carry chemoreceptors, so a woodlouse effectively "tastes" and "smells" its environment to find food, detect moisture and sense danger. The mouthparts sit below, with an upper and lower lip enclosing the jaws used to scrape and chew decaying matter.

How Many Legs Do Woodlice Have?

Woodlice have fourteen legs, arranged in seven pairs - one pair on each segment of the mid-body. The legs are uniramous, meaning each is a single unbranched limb, unlike the branched (biramous) legs of many aquatic crustaceans. This simpler design suits walking on land, giving woodlice steady, stable movement across the uneven world of leaf litter, soil and rotting wood.

The legs do more than walk: woodlice also use them to handle and manipulate food as they feed. A young woodlouse hatches with only six pairs of legs and gains its seventh pair, along with the final body segment, after its first moult - a small detail that often surprises people watching a colony grow.

Body Segments: Pereon and Pleon

A woodlouse's body divides into two main regions. The front, larger section is the pereon (the thorax), made up of seven segments, each bearing a pair of legs. Behind it is the pleon (the abdomen), a smaller rear section that houses the breathing structures. This clear division - walking up front, breathing behind - is typical of isopods and is easiest to see when a woodlouse is viewed from below.

Sensory Organs: Eyes and Antennae

Woodlice have compound eyes, though simple ones compared with an insect's - made up of relatively few units. They're enough to detect light, shadow and movement, which is what a nocturnal animal needs to avoid predators and stay out of drying daylight. Woodlice show strong negative phototaxis: they actively move away from bright light and toward dark, damp shelter.

For everything else, the antennae do the heavy lifting. Between touch, vibration and chemical sensing, a woodlouse navigates, locates food and decaying matter, finds mates and judges humidity largely through its antennae rather than its modest eyes.

How Do Woodlice Breathe?

This is one of the most remarkable parts of woodlouse anatomy. They breathe using pleopodal lungs - sometimes called pseudotracheae - on the underside of the pleon, evolved directly from the gills of their marine ancestors. These structures provide a large, folded surface area for absorbing oxygen from air while helping to limit water loss.

Because these lungs are gill-derived, they work best kept moist, which is a big reason woodlice need humid conditions and cluster in damp microhabitats. In some species you can even see the breathing structures as pale patches ("white bodies") on the underside. It's a neat example of an old marine organ repurposed for life on land.

Reproductive Anatomy and the Brood Pouch

Woodlice reproduce sexually, with the male transferring sperm to the female to fertilise her eggs. The female then carries those eggs in a marsupium - a brood pouch on her underside, formed by overlapping plates between her legs and filled with fluid. This pouch is the key reproductive adaptation: it lets the eggs and developing young grow in a protected, moist pocket, sidestepping the need for standing water that their aquatic relatives rely on.

The young develop inside the marsupium and emerge as mancae - tiny, pale versions of the adults - ready to live independently from the start. Carrying the brood in this internal pouch is a major reason woodlice succeed on land, shielding the most vulnerable stage from drying out.

Internal Anatomy: The Digestive System

Internally, the woodlouse gut is adapted for a tough, low-nutrient diet of decaying plant matter. The functional midgut is reduced; in its place, paired digestive glands known as the hepatopancreas (the midgut glands or caeca) do the work a midgut would in other animals - producing digestive enzymes and absorbing nutrients. A long hindgut then reabsorbs water and ions before waste is compacted and expelled.

That water-saving hindgut is itself a land adaptation, helping woodlice hold on to precious moisture. As decomposers, woodlice also rely heavily on gut microbes to help break down cellulose from the leaves and wood they eat - which is why a varied diet of well-rotted material suits them so well.

Defences and Terrestrial Adaptations

Woodlice defend themselves in several ways. Pill species (such as Armadillidium vulgare) can conglobate - rolling into a tight ball that shields the soft underside and legs and also reduces water loss. Some species produce mildly distasteful chemical secretions to deter predators. Others simply rely on staying hidden, moving at night, and using their antennae to sense and avoid threats - though specialist hunters like the woodlouse spider have evolved to get past the armour anyway.

Underpinning all of this, woodlice lack the waxy waterproof layer that insects use to seal in moisture, which is why they're tied to damp, sheltered places. Their pleopodal lungs, water-recycling hindgut, brood pouch, group-huddling behaviour and unusual two-stage moult (shedding the back half first, then the front a few days later) all point to the same theme: a crustacean body steadily reworked for a life on land.

Frequently Asked Questions

How many legs does a woodlouse have?

Fourteen, in seven pairs - one pair per body segment of the pereon. The legs are uniramous (single, unbranched), suited to walking on land. Young hatch with six pairs and gain the seventh after their first moult.

How do woodlice breathe?

Through pleopodal lungs (pseudotracheae) on the underside of the rear body, evolved from the gills of their marine ancestors. These need to stay moist to work, which is why woodlice require humid conditions.

Are woodlice insects?

No - woodlice are crustaceans, more closely related to crabs, lobsters and shrimp than to insects. Their gill-derived lungs, two pairs of antennae and brood pouch all reflect that crustacean ancestry.

How do woodlice carry their young?

The female carries fertilised eggs in a fluid-filled brood pouch (marsupium) on her underside. The young develop there and emerge as mancae - miniature versions of the adults - ready to live independently.

Do woodlice have a midgut?

The functional midgut is reduced. Its role is taken over by the hepatopancreas (paired digestive glands/caeca), which handles enzyme production and nutrient absorption, while a long hindgut reabsorbs water - an adaptation to their decomposer diet and to conserving moisture on land.

Why do woodlice need damp conditions?

They lack the waxy waterproof layer insects have, and their gill-derived lungs must stay moist to function. Both leave woodlice prone to drying out, so they shelter in dark, humid places and are active mainly at night.