The common pill bug, Armadillidium vulgare, is a small, terrestrial crustacean often encountered in gardens and damp environments, earning it the familiar nickname “roly-poly.” These creatures are land-dwelling relatives of shrimp and crabs, not insects.
Physical Characteristics for Identification
The pill bug is easily recognized by its distinct, armored appearance, which gives it the look of a miniature, segmented tank. Adults typically measure between one-third and three-quarters of an inch in length, with a body that is oval-shaped and highly convex, or domed, on top. Their coloration ranges from a uniform slate gray to a darker brown or black, sometimes featuring lighter spots.
The body is divided into three main sections, but the thorax is the most prominent feature, covered by seven distinct, overlapping plates. This segmented outer shell, or exoskeleton, provides protection. The pill bug has seven pairs of legs, totaling fourteen, with one pair extending from beneath each thoracic segment. They also have two pairs of antennae, though only one pair is long and noticeable, used to sense their surroundings.
How Pill Bugs Differ From Sow Bugs
Pill bugs are frequently confused with the closely related sow bug, Porcellio scaber. The most reliable distinguishing feature is the presence of two small, tail-like appendages, called uropods, which visibly extend from the back of the sow bug. The pill bug’s uropods are either hidden or very short and smoothly tucked into the body, giving it a fully rounded, smooth rear end.
Sow bugs also tend to have a flatter, more oval body shape compared to the pill bug’s distinctly higher, domed arch. This difference in body structure is directly related to their defense mechanisms. When disturbed, a sow bug will quickly scurry away, as its body shape prevents it from fully curling up.
Habitat, Behavior, and Classification
Pill bugs require moisture to survive because they breathe using gill-like structures, which necessitates a constant supply of humidity. They are most commonly found in dark, damp locations, such as beneath rocks, logs, leaf litter, and mulch. They are nocturnal, seeking cover during the day to avoid desiccation.
When threatened, the pill bug exhibits a defensive behavior called conglobation, where it tightly rolls its body into a protective sphere. This action shields its softer underside and legs behind its hard, segmented plates. This ability to roll into a ball is not only for defense against predators but also helps the pill bug conserve moisture.