The small, oval-shaped creatures often found under garden rocks or decaying leaves are universally known as “roly polies” due to their habit of curling into a tight ball when disturbed. Their segmented bodies and terrestrial nature often lead to the mistaken belief that they are insects. This confusion prompts questions about their correct biological classification and the exact number of limbs they possess. Understanding this creature requires exploring its unique anatomy and surprising place in the animal kingdom.
The Definitive Answer Counting the Legs
A roly poly has 14 walking appendages, which are seven pairs of legs. These appendages are uniformly distributed along the creature’s segmented middle section, known as the pereon or thorax. This number is consistent across all adult pill bugs, providing a reliable count that distinguishes them from many other garden inhabitants. Each of the seven pairs of legs is nearly identical in size and function, allowing the animal to move deliberately across surfaces.
Not an Insect Roly Polies Are Crustaceans
The high number of legs immediately signals that the roly poly is not a member of the Class Insecta, whose defining characteristic is three pairs of walking legs, totaling six. Instead, the roly poly is a terrestrial crustacean, making it a close relative of shrimp, crabs, and lobsters. They belong to the Order Isopoda, which literally translates to “equal feet,” referencing the uniformity of their seven pairs of thoracic legs.
This classification places them in the Subphylum Crustacea, a group predominantly made up of marine and aquatic species. The pill bug species Armadillidium vulgare is biologically remarkable because it is one of the few crustaceans to have successfully adapted to living its entire life on land.
The difference in leg count stems from fundamental developmental blueprints. Insects evolved a body plan with three distinct body segments and six legs. Crustaceans, by contrast, typically exhibit a greater number of body segments and a higher count of paired appendages, often resulting in 10 or more legs. The roly poly’s 14 legs are a direct anatomical inheritance from its aquatic crustacean relatives, highlighting its unique evolutionary path.
Life on Land Anatomy and Habitat
Beyond the leg count, the roly poly possesses anatomical features that facilitate its life away from water. Its segmented outer shell is composed of overlapping, armor-like plates that provide protection and allow for a behavior called conglobation. This ability to roll into a tight ball is a primary defense mechanism against predators and helps the animal conserve body moisture.
A persistent link to their marine ancestry is their method of respiration, as roly polies breathe using modified gill-like structures. These organs require constant moisture to function effectively, meaning the animal is unable to survive in dry conditions. This necessity dictates their ecological existence, forcing them to remain active primarily at night or during damp, cool weather.
The need for a consistently humid microclimate drives roly polies to seek shelter in specific environments. They are commonly found beneath rocks, decaying logs, leaf litter, and compost heaps, where they can escape the sun and retain the moisture required for breathing. As detritivores, their diet consists almost entirely of dead and decaying plant material, making them important recyclers that help break down organic matter and enrich the soil.