Pill Bug (Armadillidium vulgare): Roly-Poly and Woodlouse Facts

What is a pill bug?

Pill bugs are woodlice, members of a group of about 3,700 described species of land-living isopods. Isopods are crustaceans, so the pill bug’s closest relatives are sea slaters, shrimp, crabs and lobsters rather than beetles or ants. Woodlice are the most successful crustaceans on dry land, yet they still carry many traces of their watery past. The full story is in are pill bugs insects?

The common pill bug is the best-known woodlouse in the world and probably the most studied. Its scientific name, Armadillidium, means “little armadillo”, a nod to the way it curls into an armored ball. Local names vary from place to place: roly-poly, pill bug, potato bug and doodle bug in North America, pill woodlouse in Britain, and slater in Scotland, Australia and New Zealand.

What does a pill bug look like?

An adult pill bug is an oval, strongly domed animal about 8–18 mm (0.3–0.7 in) long and roughly twice as long as it is wide. Its back is covered by a row of smooth, glossy armor plates, usually slate gray to almost black. Many individuals, especially females, are mottled with yellowish or pale patches, and red, orange, brown and pale color forms also occur.

The body has three parts. The small head carries a pair of simple-looking eyes that mostly tell light from dark, one obvious pair of antennae and a second pair so tiny it is easy to miss. Behind it, seven plates cover the thorax, each with a pair of short legs underneath, for 14 legs in all. The abdomen is short, ending in a triangular tail plate with two small, flat appendages that fit neatly into the outline of the shell.

A pill bug that looks bright blue or violet is usually not a separate color form. That shade comes from masses of virus particles in its body, a sign of an iridovirus infection that is usually fatal.

Habitat and range

The common pill bug originated around the Mediterranean and is now found across most of Europe. It is tougher in dry conditions than most woodlice, thanks to a thicker, more waterproof shell, and it often lives in sunnier, more open places such as grassland, scrub, field edges and gardens. In Britain it is most common on chalk and limestone soils and near the coast, where there is plenty of calcium for its armor.

Carried around the world by people, probably in soil, plants and ships’ ballast, it has become one of the most widespread land invertebrates on Earth. It is now common across much of the United States and southern Canada, and is established in South America, southern Africa, Madagascar, Japan, Australia and many other regions. In some California grasslands it is among the most abundant invertebrates.

Wherever it lives, it needs shelter by day: undersides of stones and logs, leaf litter, compost heaps, bark mulch and the gaps under flowerpots. Pill bugs come out mostly at night, when the air is cooler and damper.

How pill bugs breathe and stay moist

Like all woodlice, the pill bug fights a constant battle against drying out. Its breathing organs are on the underside of the abdomen, where some of the small, flat abdominal legs called pleopods contain white, spongy patches of branched air tubes. These pleopodal lungs evolved from the gills of water-living ancestors and only work while they stay moist.

Pill bugs reduce water loss by hiding in damp places by day, moving faster when the air is dry and slowing down when they find humidity. They also huddle together in clusters, drawn by scent signals in each other’s droppings, which cuts the surface area exposed to dry air. Their blood carries oxygen with hemocyanin, a copper-based pigment also found in crabs and lobsters.

What do pill bugs eat?

Pill bugs are detritivores, animals that feed on dead organic matter. Their main foods are fallen leaves, rotting wood and bark, together with the fungi and bacteria growing on them, and they prefer leaves that have already been softened by decay. They also eat algae, dung, dead insects and other carrion, and they regularly eat their own droppings to extract more nutrients.

When they are very numerous or the ground is dry, pill bugs sometimes nibble tender seedlings, low-hanging strawberries and other soft fruit. Most of the time, though, they are recyclers that break leaf litter into small pieces and help turn it into rich soil. Find out more in what do pill bugs eat?

Behavior: rolling up, hiding and huddling

The pill bug’s best-known trick is conglobation, rolling into a ball. When it is touched or feels strong vibrations, it curls its body forward until the armor plates meet and the head is tucked against the tail plate, hiding the soft underside, legs and antennae. The common pill bug closes into an almost perfect sphere, with no gap where an attacker could get in, and rolling up also helps it save water. The details are in why do pill bugs roll into a ball?

Pill bugs move away from bright light and heat and toward dark, damp, tight spaces, which is why they are a favorite subject for classroom experiments with choice chambers. When they walk without landmarks, they tend to alternate left and right turns, a pattern that keeps them moving roughly in a straight line. In stressful situations they may also lie still and play dead.

Reproduction, brood pouch and molting

Pill bugs breed mainly in spring and early summer, and in warm climates they can breed for much of the year. A male mates by climbing onto the female’s back, and she can store sperm for many months and use it for later broods. Females carry their fertilized eggs in a fluid-filled brood pouch called a marsupium, formed by overlapping plates on the underside of the thorax. Their relatives the sand hoppers and opossum shrimps carry their young in the same way.

After several weeks the young hatch inside the pouch as pale miniature adults called mancae. They stay in the pouch for a few days, then crawl out with only six pairs of legs and gain the seventh pair as they molt. A female may raise one to three broods a year, and larger females carry more young.

Pill bugs grow by molting their hard shell throughout life, and they do it in an unusual way: the rear half of the shell is shed first and the front half one to a few days later. For a short time a molting pill bug looks half gray and half pale. Before each molt, it stores calcium from the old shell as white patches on its underside, then uses it to harden the new one. Freshly molted pill bugs are soft and vulnerable, so they stay hidden until the new shell hardens.

In many populations, a bacterium called Wolbachia, passed from mother to young, turns genetic males into functional females, which tilts broods toward daughters. The pill bug has become a standard animal for studying this kind of microbial control of sex.

How long do pill bugs live?

Common pill bugs usually live about 1–3 years. They reach adult size and start breeding at around one year old, and each female may breed over more than one season. Many die young: the first days after each molt and the early life outside the brood pouch are the riskiest times.

Their predators include spiders, centipedes, ground beetles, toads, frogs, salamanders, shrews, hedgehogs and many birds. The woodlouse spider (Dysdera crocata) has unusually long fangs that can pierce a pill bug’s armor. Woodlouse flies, a family of small parasitic flies, have larvae that grow inside living woodlice and eventually kill them.

Conservation status

The common pill bug has not been assessed for the IUCN Red List. It is one of the most abundant and widespread small animals in its range and faces no known threat as a species. Because pill bugs depend on damp shelter, prolonged drought can reduce local numbers, and intensive farming that removes leaf litter and hedges can make fields less suitable.

Pill bugs and people

For gardeners, pill bugs are mostly helpers. Along with earthworms, millipedes and springtails, they shred dead leaves and other plant debris, speeding up the return of nutrients to the soil. They cannot bite or sting people, are not venomous and do not damage sound wood or buildings. Pill bugs that wander indoors are usually seeking moisture and soon dry out.

Scientists value them too. Woodlice store heavy metals such as copper, zinc, cadmium and lead in their digestive gland, so they are used as living indicators of soil pollution. Their behavior, molting, virus infections and Wolbachia bacteria have made the common pill bug a model animal in many laboratories.

A common mix-up is with the pill millipede (Glomeris marginata) of Europe, which also rolls into a ball. A pill millipede has 17 or 19 pairs of legs instead of seven, and is a myriapod, not a crustacean. Like the armadillos that roll into a ball, the two animals evolved the same defense independently.