Woodlice are common in gardens and damp basements. They are terrestrial crustaceans (suborder Oniscidea), making them relatives of crabs and shrimp, not insects. They are one of the few crustacean lineages that successfully adapted to life on land, evolving specialized structures to breathe air. Woodlice are known by common names including pill bug, roly-poly, and sow bug.
General Physical Characteristics
Woodlice have a compact, oval, and slightly flattened appearance, usually ranging from 1/3 to 3/4 of an inch long. Their coloration is typically slate gray or brown, though some species are mottled or appear blue or purplish after molting. The body is protected by a tough, shell-like exoskeleton, giving them an armored look. This armor is heavily segmented into overlapping plates called tergites. The body is divided into a small head and a larger, segmented thorax, which provides flexibility for movement and defensive coiling in some species.
Unique Identification Features
Woodlice possess two pairs of antennae, though one pair is often small and difficult to see. The longer, visible pair has sensory structures that help the woodlouse navigate and detect moisture. A primary feature distinguishing them from insects is their leg count: woodlice have seven pairs of jointed legs, attached to the seven segments of the thorax. At the rear of the body is a pair of small, tail-like appendages known as uropods, which are sometimes used to deter predators or aid in navigation.
Distinguishing Between Pill Bugs and Sow Bugs
The most frequent point of confusion for the general public is differentiating between the common pill bug and the sow bug, two distinct types of woodlice. The easiest way to tell them apart is by observing their defense mechanism when disturbed. Pill bugs, such as Armadillidium vulgare, have the unique ability to roll completely into a tight, nearly spherical ball, a behavior known as conglobation.
Sow bugs, like Porcellio scaber, lack the necessary body structure to fully roll up and instead rely on fleeing when threatened. The second key difference is visible on the rear of the animal. Sow bugs have two prominent, flattened, tail-like uropods that clearly project out from the end of the body. In contrast, the pill bug’s uropods are recessed and barely visible, giving their rear end a smooth, rounded appearance when not rolled up.
Contextual Identification: Where Woodlice Live
Woodlice have retained a need for moisture from their aquatic ancestors, making their habitat a strong clue for identification. They are highly susceptible to drying out because they lack a waxy cuticle and breathe using specialized, gill-like structures. As a result, they are almost exclusively found in dark, damp, and humid microhabitats.
These crustaceans commonly hide under rocks, logs, flowerpots, and leaf litter in gardens. They are frequently seen in compost piles, where they feed on decaying plant matter. Finding them indoors, often in basements, crawl spaces, or laundry rooms, is a sign that the area has the high humidity and darkness they require.