Pill bugs are common, harmless invertebrates found in gardens and damp environments. Often mistaken for insects, they are terrestrial crustaceans, related to crabs and lobsters. This article explores their appearance and how they differ from similar sow bugs.
Key Physical Characteristics
Pill bugs typically measure between 0.25 and 0.75 inches (about 6 to 19 mm) in length, with an oval-shaped body that is somewhat flattened and convex on top. Their coloration usually ranges from slate gray to brownish-gray, though some individuals may exhibit a blue or purplish hue, especially after molting.
The body of a pill bug is covered by a hard, segmented exoskeleton. This exoskeleton is divided into a head, a seven-segmented thorax, and an abdomen. They possess seven pairs of short, jointed legs, with one pair attached to each thoracic segment. Pill bugs have two pairs of antennae; one pair is shorter and less visible than the longer, prominent pair used for sensing. Their simple eyes, known as ocelli, detect light and dark but lack detailed vision.
The “Pill” Feature
Pill bugs can roll into a tight ball, a behavior known as conglobation. This action is a primary defense mechanism, protecting their softer undersides from predators with their tough, armored plates. Their segmented body structure, with overlapping plates called tergites connected by flexible joints, enables this curling ability.
Beyond predator defense, conglobation also aids water conservation. Pill bugs require high humidity to breathe through their gill-like structures, and rolling into a ball helps reduce moisture loss. This adaptation is important for their survival in terrestrial environments, as they are susceptible to desiccation.
Pill Bugs Versus Sow Bugs
Pill bugs are often confused with sow bugs due to similar appearances. A main difference lies in their defensive behavior and anatomy. Sow bugs cannot roll into a tight ball when disturbed; instead, they flatten their bodies and scurry away.
Another visual distinction is the presence of two prominent, tail-like appendages, called uropods, at the rear of a sow bug’s body. Pill bugs lack these visible appendages or have them less noticeably recessed. While both share a segmented, oval body, sow bugs may appear slightly flatter and less dome-shaped than pill bugs.