Invertebrates
Woodlouse
Oniscidea · also called pill bug, roly-poly, sowbug, slater
The grey armoured bug under the flowerpot is not an insect at all. A woodlouse is a crustacean — a cousin of crabs and shrimp — and it is the only one of that huge, mostly marine group to have moved fully onto land. It still breathes with modified gills that have to stay damp, which is why it lives under logs and stones and shrivels in the sun.
Overview
Woodlice are the suborder Oniscidea, around 3,700 species, on every continent except Antarctica. They are terrestrial isopods, seven-legged, two-antennae’d crustaceans of damp, dark places — leaf litter, rotting wood, under stones, in soil and compost. They are detritivores, eating dead plant matter, fungi, and their own droppings to recover copper.
A crustacean adapted to land
Woodlice keep gill-derived breathing structures on the underside of the abdomen that must stay moist, and the more terrestrial species add folded pleopodal lungs for breathing air directly — an adaptation that let the desert pill bugs colonise genuinely dry country. They take up water by drinking, through the anus, and along capillary channels on the rear legs.
Rolling up
The pill bugs (“roly-polies”) can conglobate — roll into a tight sphere that hides the legs and breathing organs behind overlapping plates, deterring predators and slowing water loss. The flatter sowbugs cannot roll and flee or flatten instead.
Odd details
Woodlouse blood is faintly blue (copper-based), and one infected with an iridovirus turns bright blue or purple. They moult in two halves, back then front, days apart.
Status
Woodlice as a group are Not Evaluated. Garden and woodland species are abundant and several have spread worldwide with trade, but many cave, coastal, and island specialists have tiny ranges and some are threatened or protected.
Woodlouse: common questions
Is a woodlouse an insect?
No. A woodlouse is a crustacean, specifically a terrestrial isopod, so its closest relatives are crabs, shrimp, lobsters, and the marine isopods, not insects. The differences are clear once you look. A woodlouse has seven pairs of legs, an insect has six. A woodlouse has two pairs of antennae, though one pair is tiny and hard to see, while an insect has one pair. A woodlouse has no wings and never did, and it breathes through gill-derived structures rather than the tracheal tubes of insects. Woodlice are the only crustaceans to have adapted fully to life on land, which is why they seem so out of place among the insects and spiders they live alongside.
What is the difference between a pill bug and a sowbug?
They are both woodlice, and the practical difference is whether the animal can roll into a ball. Pill bugs, also called roly-polies, have a rounded, domed back and can conglobate, curling into a tight armoured sphere when threatened. Sowbugs are flatter, usually have two small tail-like projections at the rear that stick out beyond the body, and cannot roll up, so they flee or flatten themselves against a surface instead. Pill bugs also tend to tolerate drier conditions. Both are harmless detritivores, and in casual use the names woodlouse, pill bug, sowbug, slater, and roly-poly are often applied loosely to any of them.
Are woodlice harmful in the house or garden?
Almost never. Woodlice feed on dead and decaying plant matter and fungi, so in the garden they are useful recyclers that speed up the breakdown of leaf litter and enrich the soil, and they only rarely nibble a soft seedling or a ripe strawberry lying on damp ground. Indoors, woodlice that wander in from outside cannot breed or survive long in the dry air of a heated house, do not bite, sting, spread disease, or damage timber or fabric, and their presence in numbers usually just indicates damp, such as a leak, condensation, or rotting wood, which is worth fixing for other reasons. Simply sweeping them back outside is enough.
Why do woodlice curl into a ball?
In the pill bugs, rolling up is a defence and a survival tactic rolled into one. Curling into a sphere presents a hard, curved, armoured surface to a predator such as a spider, beetle, or shrew, and hides the soft belly, the delicate breathing organs, and the legs where they cannot be bitten or pulled. The same posture greatly reduces the surface area exposed to dry air, so a woodlouse caught in the open can roll up to slow its water loss until it is safe to move again. Not all woodlice can do it, the flatter sowbugs lack the body shape, but where the ability exists it is triggered by touch, vibration, and drying.