The common garden creatures known as rolly pollies are frequently mistaken for millipedes due to their segmented bodies and shared habit of curling up when disturbed. This widespread confusion stems from a superficial resemblance that hides a profound biological difference. Rolly pollies, also called pill bugs or woodlice, and millipedes are members of entirely separate classes of life within the phylum Arthropoda.
The Rolly Polly: A Terrestrial Crustacean
The rolly polly is a terrestrial crustacean, making it a land-dwelling relative of shrimp and crabs. This classification places it within the subphylum Crustacea, a group overwhelmingly composed of aquatic species. The rolly polly retains certain crustacean features, such as breathing using gill-like structures, or pseudotrachea, which demand a constant source of moisture. Its body is distinctly oval and flattened, covered by a rigid exoskeleton divided into seven segments. This segmented shell allows for its characteristic defense mechanism, where it forms a tight, impenetrable ball, a behavior known as conglobation.
The Millipede: A Segmented Myriapod
Millipedes belong to the class Diplopoda within the subphylum Myriapoda, which also includes centipedes. This classification distinguishes them from crustaceans, placing them in a group defined by their numerous, jointed legs. The millipede body is typically long and cylindrical, giving it a distinct worm-like appearance compared to the rolly polly’s flatter shape. Most species have an elongated body composed of more than 20 segments, often armored with calcareous plates.
Their defining physical feature is the presence of diplosegments, formed by the fusion of two individual body segments. Nearly every segment after the first few has two pairs of legs, a trait that gives rise to the name “millipede.” When disturbed, a millipede’s primary defense is to curl its body into a spiral, sometimes secreting a pungent, toxic chemical or gas to deter predators.
Key Physical Differences
The most immediate distinction between the two creatures lies in their leg count and arrangement. Rolly pollies possess seven pairs of legs, totaling 14, with one pair attached to each of the seven thoracic segments. Millipedes, in contrast, feature two pairs of legs on almost every visible body segment, a signature of their diplosegment anatomy. This difference in limb structure is the most reliable way to tell them apart.
The overall body shape also provides a clear contrast. The rolly polly is broad and dorsoventrally flattened, resembling a small armadillo. Millipedes, on the other hand, are generally round and tube-like, moving with a slow, wave-like motion of their many legs. The antennae of the rolly polly are prominent and easily visible, while millipedes have much shorter, less noticeable antennae.
Their defensive actions also differ in execution, despite both involving curling. The rolly polly (specifically the pill bug species) can completely tuck its head and appendages inside its shell, forming a perfect, sealed sphere. While many millipedes curl, they often leave their head exposed, relying more heavily on the release of defensive chemical secretions.
Why Rolly Pollies Are Important
Rolly pollies play a significant and beneficial role in local ecosystems, mainly through their work as detritivores. They consume decaying plant matter and other dead organic material, which accelerates the process of decomposition. By breaking down this material, they recycle essential nutrients back into the soil, making them available for plants to use.
Their digestive process increases the speed at which organic matter is converted into rich soil, supporting healthy plant growth. A unique feature of the rolly polly is its ability to safely remove heavy metals from the soil. They ingest contaminants like lead, cadmium, and arsenic, crystallizing these toxins in their midguts, which effectively cleans up contaminated areas.