Pill bugs, often recognized as “roly-poly” or “woodlouse,” are common sights in gardens and beneath rocks. Despite their insect-like appearance, these creatures are actually terrestrial crustaceans, making them more closely related to shrimp and lobsters than to true bugs. They are one of the few crustaceans that live entirely on land.
Overall Size and Shape
An adult pill bug typically measures between one-quarter and one-half inch in length. The body silhouette is distinctly oval, appearing longer than it is wide, and is highly arched or convex on top, resembling a miniature tank. The underside of the body is flatter, creating a low-to-the-ground shape. Coloration commonly ranges through shades of slate gray, brown, or brownish-black, providing effective camouflage. Occasionally, individuals may exhibit a blue or purplish tint, often immediately after molting.
The Exoskeleton and Segmentation
The pill bug’s hard, chitinous outer layer is an exoskeleton. This structure is highly segmented, giving the creature its characteristic armored look, and is covered by seven distinct, overlapping plates on the main body (pereon). These plates are connected by flexible membranes, allowing for considerable movement. This segmentation enables the pill bug to perform its signature defensive maneuver. The surface of this armor is generally smooth, though it can sometimes appear slightly granular.
Distinguishing Appendages
The head features two pairs of antennae, though only the larger pair is prominent and easily visible, functioning as primary sensory organs. Small compound eyes, providing basic light detection, are located on the sides of the head. The pill bug has seven pairs of walking legs, totaling fourteen, with one pair attached to each of the seven main body segments. At the posterior end are the uropods, small appendages that do not extend visibly past the end of the abdomen. This short, non-protruding tail end helps distinguish the pill bug from the similar-looking sow bug.
The Defining Behavior (Conglobation)
The most recognizable characteristic of the pill bug is its defensive action known as conglobation, the ability to roll its body into a tight, nearly perfect sphere. The flexible, segmented exoskeleton allows the animal to tuck its legs and soft underside completely inward. When fully conglobated, the pill bug resembles a small, smooth, hard ball. This posture shields its vulnerable belly and legs, exposing only the tough outer plates to a potential predator. This behavior also serves to conserve moisture, an important survival mechanism for this land-dwelling crustacean.