The question of whether woodlice are the same as roly-polies is common, and the simple answer is that the relationship is one of parent group to subset. The terms “woodlouse,” “pill bug,” and “roly-poly” all refer to terrestrial crustaceans belonging to the suborder Oniscidea. These small, segmented organisms are among the few types of crustaceans that have successfully adapted to living entirely on land.
Found in backyards across the globe, they are a familiar sight, often scurrying away when disturbed under a damp log or rock. Their presence is a quiet indication of a healthy, functioning ecosystem where they play a crucial, if often overlooked, role.
Decoding the Common Names
The term “woodlouse” functions as the general, umbrella name for all members of the suborder Oniscidea, encompassing over 3,500 species worldwide. These creatures are also widely known as terrestrial isopods, distinguishing them from their marine relatives like shrimp and crabs. The specific names “roly-poly” and “pill bug” are common nicknames reserved for a select group of species, most notably Armadillidium vulgare. These names specifically describe the defensive behavior of conglobation, which is the ability to roll into a tight, nearly perfect ball when they feel threatened.
This ability to curl up is not universal among all woodlice species. Other woodlice, often called “sow bugs” (from genera like Porcellio and Oniscus), are unable to perform this maneuver because their body segments do not interlock properly. Sow bugs will instead flatten themselves or attempt to quickly run away when disturbed. Therefore, while every roly-poly is a woodlouse, not every woodlouse is a roly-poly.
The Scientific Identity
Woodlice are classified as crustaceans, placing them in the same phylum, Arthropoda, as insects. Their segmented body is protected by a tough, shell-like exoskeleton, or cuticle, which does not have the waxy layer characteristic of insects. This lack of a waterproof layer means they are highly susceptible to desiccation, requiring them to seek out perpetually damp environments.
Their evolutionary history as aquatic creatures is evident in their respiratory system, as they do not possess the spiracles and tracheae of insects. Woodlice breathe using modified abdominal structures called pleopodal lungs, or pseudotracheae, which are adaptations of their ancestral gills. These structures must remain moist to facilitate the exchange of oxygen. The woodlouse body plan is distinct, featuring seven pairs of legs, one pair attached to each of the seven segments of their thorax.
Why They Roll and Where They Live
The remarkable ability of the roly-poly to roll into a sphere is a specific defense mechanism triggered by physical pressure or strong vibrations. This action is possible due to flexible joints connecting their overlapping, armored body plates, which are pulled inward by specialized muscles. The resulting ball is a hard, continuous surface that protects their softer underside and appendages from most small predators.
Conglobation also serves as a mechanism for water conservation, which is vital given their vulnerability to drying out. By reducing their exposed surface area, they can significantly lower the rate of water loss from their pleopodal lungs. These organisms thrive in dark, humid microhabitats, such as beneath stones, leaf litter, and decaying wood. They are detritivores, meaning they feed primarily on dead and decaying plant matter, performing the ecological service of nutrient recycling by returning essential compounds back into the soil.