Pillbugs and Sowbugs (Land Isopods)

Photo of a sowbug (left) and pillbug (right).
Scientific Name
Land-dwelling members of the crustacean order Isopoda
Several families include land species

Land isopods have the following characteristics: bodies flattened top-to-bottom, obviously segmented, usually oval so that head, thorax, and abdomen aren’t immediately distinct; eyes compound and not on stalks; 2 pairs of antennae (one pair large, the other pair tiny); mouthparts for chewing; 7 pairs of walking legs that are all pretty much the same (iso- means “same” or “equal,” and pod means “foot”); 5 pairs of 2-parted pleopods (gills), each protected by a platelike operculum; the rear end of the body (last abdominal segment) with uropods and a telson (analogous to the “tail fan” of a crayfish).

Some common land isopods in Missouri include:

  • The common woodlouse, pillbug, or roly-poly (Armadillidium vulgare), was introduced from Eurasia long ago. It can roll itself into a ball (thus "pill bug").
  • Isopods in the genera Oniscus and Porcellio are called sowbugs. They are also from the Old World; they cannot roll into a ball, but they have two short, pointy “tails” at the end of the body.

Adult length: about ¼ to ½ inch.

Where To Find
image of Pillbugs and Sowbugs Land Isopods Distribution Map

Statewide. There are more than 10,000 species of isopods globally; most live in the sea, and some live in fresh water. Worldwide, about 5,000 species live on land.

Land isopods have special adaptations allowing them to live on land. They will drown if submerged in water too long. They have gills, however, which must be kept moist. This is why they live in damp, humid places such as under rocks and logs, have nocturnal habits, and some can roll up in a ball (as pillbugs do). The underside of the body is especially vulnerable to drying out. Some species can push their tail-like appendages into dewdrops and channel water to the gills under their bodies.

Land isopods are herbivores, scavengers, and omnivores. Mouthparts are for chewing. Common foods include decaying plant material, such as rotting wood, and fungi. They sometimes chew on living plants, if they are tender enough.

Fossil isopods date to 300 million years ago. As crustaceans, how are they related to crabs and crayfish? Crustaceans are a subgroup of the phylum Arthropoda (a huge group including insects, spiders, centipedes, and all other jointed-legged invertebrates). Crustaceans are divided into classes, and one of these is Malacostraca, which includes several orders. These orders include the isopods, the amphipods (scuds), the decapods (crayfish, crabs, lobsters, prawns, and shrimp), and others.

Life Cycle

Like most other crustaceans, young isopods hatch from eggs and look like tiny versions of the adults. As they grow, they molt (shed their skins) in two phases: first the back half, later the front half. Mature females have special appendages under their bodies that form a water-filled brood pouch, the marsupium (the same term is used for opossum and kangaroo pouches). The female lays her eggs directly into the marsupium; as the young develop, they undergo their initial molts within its safety.

It’s hard to consider isopods pests, since they are harmless and do no damage when they sometimes enter buildings. It’s our choice whether to view them with revulsion, or to see them as interesting, humble little creatures that look something like tiny VW microbuses or Airstream trailers.

By chewing and eating organic detritus, such as rotting wood and other decaying plant material, and fungi, isopods contribute to decomposition and soil fertility. Isopods are eaten by a variety of predators, including centipedes, spiders, beetles, and small mammals.

Media Gallery
Similar Species
About Land Invertebrates in Missouri
Invertebrates are animals without backbones, including earthworms, slugs, snails, and arthropods. Arthropods—invertebrates with “jointed legs” — are a group of invertebrates that includes crayfish, shrimp, millipedes, centipedes, mites, spiders, and insects. There may be as many as 10 million species of insects alive on earth today, and they probably constitute more than 90 percent all animal species.