Pillbugs and sowbugs are some of the small creatures you’ll commonly find on the surface of the soil. You’ll often encounter them under rotting wood, in leaf litter, under rocks, and in other dark, moist places. While they are common, they have several truly odd characteristics and they are important recyclers in the ecosystem.

These creatures are terrestrial isopods. Although they are called bugs, they are not insects. They’re a type of crustacean like shrimp and crabs. Isopods evolved in a shallow marine environment during the Paleozoic era roughly 500 to 200 million years ago (this is also known as the Cambrian period).

Upside down Oniscus sowbug showing seven pairs of legs. photo by John F. Williams

Weird fact: terrestrial isopods breathe with their legs, sort of. They have two to five pairs of pleopods (small leg-like appendages) on the abdomen. Aquatic isopods use their pleopods for swimming and breathing. Terrestrial isopods have pleopodal gills covered by an operculum (a type of flap) that allow gas exchange on land. That’s why pillbugs and sowbugs need to live in a damp environment so that these gills can stay moist.

Did you know that pillbugs and sowbugs share a feature with kangaroos and opossums? They also have a pouch, called a marsupium, where they raise their young. Female pillbugs and sowbugs can lay up to 200 eggs in their pouch. The eggs are carried in the marsupium for three to seven weeks. When they hatch, the babies are barely visible and stay in the pouch for another several weeks. Once they leave the pouch they never return.

Woodlouse or sowbug. drawing by Thomas Noland

Like crabs and insects, pillbugs and sowbugs have a hard exoskeleton that they shed and replace as they grow, a process called molting. Depending on the species, they can molt up to a dozen times during their lifetime, growing bigger each time. These isopods generally live to around two years, but some have been known to live as long as five years.

Front end of a sowbug. photo by Thomas Noland

Pillbugs and sowbugs do not urinate, but instead release ammonia waste as a gas through their bodies. They also eat their own feces (known as coprophagy). This behavior is not really understood but scientists think it may be a way for the isopods to retain copper, a mineral needed for survival that is often not easily obtained. Also, these critters drink from both ends of their bodies: using both their mouths and straw-like structures called uropods attached to their abdomens.

Aquatic sowbug. photo by Thomas Noland

So when you’re out getting firewood from your woodpile, digging in the garden, or looking under rocks or logs, watch out for the roly-polies scurrying away. Be careful and place that rotten wood, stone, or leaf litter back over them carefully as these little backyard cleaning crews have important jobs to do.

Thomas Noland is a naturalist and photographer living in Everett. In addition to his interests in paleobiology, he is a dedicated entomologist and caretaker of numerous rescued cats.

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