Pillbugs and Sowbugs (Land Isopods)

Land-dwelling members of the crustacean order Isopoda

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Photo of a sowbug (left) and pillbug (right).
Sowbug (left) and pillbug (right).
MDC staff
Family

There are several families that include land species.

Description

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: 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.

Size

Adult length: about ¼ to ½ inch.

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Photo of a sowbug, a type of terrestrial isopod.
Sowbug
Unlike pillbugs, sowbugs cannot roll into a ball, and they have two short, pointy “tails” at the hind end.

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Photo of a juvenile sowbug walking on a leaf.
Sowbug
Sowbugs are crustaceans that breathe via gills, so they require a moist environment.

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Photo of a pillbug walking on soil.
Pillbug
The common woodlouse, pillbug, or roly-poly was introduced from Eurasia long ago.

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Photo of a pillbug curled into a ball for protection.
Pillbug Rolled In A Ball
The terrestrial isopods called pillbugs have the ability to curl their bodies into a ball.

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Photo of a sowbug (front) and pillbug (rear).
Sowbug and Pillbug
Sowbugs and pillbugs eat decaying plant material, contributing to decomposition and soil fertility.

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Photo of a pillbug walking on soil.
Pillbug
Pillbugs are harmless and do no damage when they sometimes enter buildings.

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A pillbug, or roly-poly, crawling on the ground
Pillbug
Habitat and conservation

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.

Foods

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.

image of Pillbugs and Sowbugs Land Isopods Distribution Map
Distribution in Missouri

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.

Status

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.

Human connections

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.

Ecosystem connections

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.