Pill bugs and millipedes are often confused due to their segmented appearance and shared preference for damp, dark habitats under rocks and logs. However, they belong to entirely different biological classes within the phylum Arthropoda, meaning their relationship is distant. Understanding the difference requires examining their unique classifications and anatomical features.
The True Identity of the Pill Bug
The pill bug, often called a roly-poly, is a terrestrial crustacean belonging to the Order Isopoda, not an insect. This means the pill bug is more closely related to marine animals like shrimp and crabs. This classification explains its need for moisture, as it has not fully adapted to life away from water.
Pill bugs breathe using specialized gill-like structures on the underside of their abdomen, sometimes called pseudotracheae. These respiratory organs must remain damp to function, which is why these animals hide in moist soil or under leaf litter. Their body has a hard, segmented exoskeleton, consisting of seven large thoracic segments, each bearing a pair of legs. The most famous trait of the pill bug is its ability to roll its body into a perfect, protective sphere, a defense mechanism known as conglobation.
Understanding the Millipede
Millipedes belong to the Class Diplopoda, which is part of the Subphylum Myriapoda, meaning “many feet.” These segmented arthropods typically have a long, cylindrical, or slightly flattened body composed of many segments. Unlike the pill bug, the millipede’s body is adapted for pushing through soil and decaying plant matter, which is its primary food source.
Their respiratory system relies on a network of internal tubes called tracheae. Air enters this system through small external openings called spiracles, located along the body segments near the legs. Millipedes use a calcified exoskeleton for protection, and many species curl into a tight coil when threatened.
Distinguishing Features
The primary distinction lies in the number and arrangement of their legs. A pill bug has seven distinct pairs of jointed legs, totaling 14, with one pair attached to each of the seven thoracic segments. Millipedes, however, possess a remarkable feature called diplosegments, where two body segments are fused together. This fusion results in the millipede having two pairs of legs, or four legs total, on almost every visible body segment, giving them the appearance of having many more legs than the pill bug.
Although both animals can roll up for defense, the pill bug (specifically the family Armadillidiidae) typically forms a near-perfect, closed ball. In contrast, most millipedes simply coil into a tight spiral.