The small, segmented creature known as the rolly polly, pill bug, or woodlouse is a familiar sight under rocks and logs in backyards across the world. Despite its common name often including the word “bug,” and its appearance resembling many common garden pests, the rolly polly is definitively not an insect. It belongs to a completely separate group of arthropods. This animal represents the only group of crustaceans to successfully adapt to a life lived entirely on land.
Defining the Rolly Polly
The rolly polly belongs to the order Isopoda, which places it in the class Malacostraca, making it a terrestrial crustacean. This classification means that the creature is more closely related to marine animals like shrimp, lobsters, and crabs than it is to any true insect. The most common species, Armadillidium vulgare, is often called a pill bug due to its unique defense mechanism of rolling into a tight, armored ball. The name woodlouse is also used, referring to its habit of living under decaying wood. These small arthropods are the descendants of ancient marine organisms that successfully transitioned to a life on dry land millions of years ago.
Key Differences Between Pill Bugs and True Insects
The physical anatomy of the pill bug distinguishes it from true insects. Insects possess six legs, arranged in three pairs, while the pill bug has seven pairs of legs, totaling 14 walking limbs. Furthermore, an insect’s body is clearly divided into three distinct sections—the head, thorax, and abdomen—but the isopod body appears to have only two or three sections that are largely fused together. The pill bug’s head is fused with the first segment of its thorax, and its remaining body segments are less clearly defined than in an insect.
Their respiratory systems reveal their marine ancestry. Insects breathe through a network of internal tubes and external openings called spiracles. Pill bugs, however, breathe using modified gills called pleopods, which must remain moist to function. Some species have evolved lung-like structures on these pleopods, known as pseudotrachea, to aid in oxygen absorption. They still rely on a high level of environmental humidity to survive, which is a direct link to their aquatic relatives and a major reason they are found in damp environments.
Life as a Terrestrial Crustacean
Its most famous behavior is conglobation, the action of rolling into a perfect sphere when disturbed. This serves as a defense against predators and helps to conserve precious body moisture. They must excrete ammonia, a nitrogenous waste product, as a gas directly through their exoskeletons. This is unlike most animals that excrete waste in a liquid urine form.
Their feeding habits position them as important decomposers, or detritivores, in the ecosystem. Pill bugs consume decaying plant matter and other dead organic material, helping to recycle nutrients back into the soil and improving its overall quality. Females brood their young in a specialized, fluid-filled pouch located under their abdomen called a marsupium. The young, called mancae, emerge from this pouch looking like miniature adults, but initially possess only six pairs of legs, gaining the seventh pair after their first molt.