The creature known as the “roly poly” or “pill bug” is often confused with many similar-looking organisms. This animal, which curls into a tight sphere, is not an insect but a terrestrial crustacean belonging to the order Isopoda. It is a relative of shrimp and crabs that has adapted to life on land. Other species share its general shape, but a closer look reveals distinct differences in anatomy and behavior.
Defining the True Roly Poly
The true roly poly, Armadillidium vulgare, is a species of woodlouse known for its defensive posture. Its oval body is covered in a segmented, armor-like exoskeleton, typically gray or brown, measuring 8.5 to 18 millimeters long. This segmented shell is composed of seven overlapping dorsal plates, or tergites.
The defining characteristic is conglobation, the scientific term for rolling completely into a perfect, tight ball when threatened. This behavior protects the softer underside from predators and helps conserve moisture. Like all terrestrial isopods, it possesses seven pairs of legs, with one pair attached to each of the seven thoracic segments.
The Closest Look-Alike: The Sow Bug
The creature most frequently mistaken for a roly poly is the sow bug, Porcellio scaber. Both are terrestrial isopods, sharing the same segmented body structure and preference for dark, moist environments. Sow bugs are generally flatter and have a more elongated, oval shape.
The most reliable way to differentiate them is by observing the sow bug’s posterior end. Two tail-like appendages, called uropods, protrude visibly from the rear of the body. Unlike the roly poly, the sow bug cannot roll completely into a ball when disturbed; it remains flattened or attempts to flee.
Other Imposters and How to Spot Them
Beyond the sow bug, other arthropods cause confusion due to their segmented appearance. Millipedes, particularly small, segmented species, can be mistaken for roly polies, but they belong to a different class entirely, Myriapoda. Millipedes are noticeably longer and more cylindrical than the broad, flattened isopods.
The most obvious distinction is the leg count. Roly polies have seven pairs of legs (14 total), while millipedes possess many more, with two pairs of legs on almost every body segment. Some millipede species curl into a tight coil when disturbed, but their bodies are long and worm-like, not forming a smooth sphere. Certain small beetles also share a resemblance, but true insects have three distinct body sections (head, thorax, abdomen) and only three pairs of legs.
Are They Pests: Their Ecological Role
Roly polies are primarily detritivores, meaning they play a beneficial role in the ecosystem by feeding on dead or decaying organic matter. Their diet consists mainly of rotting leaves, mulch, and wood, which helps accelerate the decomposition process and cycle nutrients back into the soil. By breaking down this material, they improve soil quality, which is beneficial for plant life and garden health.
They are generally considered harmless to humans, as they do not bite, sting, or spread disease. While they can occasionally feed on tender plant seedlings or soft fruit lying on the ground, this usually happens only when preferred decaying matter is scarce. They are a sign of a healthy environment, and they will only venture indoors seeking moisture, where they quickly perish in dry conditions.