8 Fascinating Facts About the Playful-Looking Roly-Poly

A brown pill bug walking on a flat rock

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The roly-poly, or pill bug, is a terrestrial crustacean that looks just like an insect. Oval shaped, with seven sets of legs and a hard outer shell, these creatures are best known for their ability to roll themselves into a perfectly shaped ball when threatened. Native to the Mediterranean, roly-polies can be found in nearly all temperate terrestrial ecosystems worldwide.

From superb composting skills to unusual bodily functions, discover the most fascinating facts about the roly-poly.

1. Roly-Poly Is Just One of Their Names

For one small bug, they go by a lot of different names. Their scientific name is Armadillidium vulgare, and they are officially called pill bugs, but they’re also known as doodle bugs, wood shrimp, and woodlice. People in the United Kingdom refer to them as chiggypigs, penny sows, and cheesybugs. Whatever name you use, keep in mind that while these gentle critters may eat a few of your plants, they are not harmful to humans.

2. They're Not Really Bugs

Even though their name is pill bug and they have a buglike appearance, they’re not insects at all; they are actually terrestrial crustaceans. They're more closely related to lobsters, crabs, and shrimp than to beetles or butterflies. Roly-polies are the only crustaceans that have adapted to living completely on land. These creatures range from one quarter-inch to one half-inch in length, and have segmented bodies and seven sets of legs.

3. They Have Gills

Pill bugs breathe with gills, like their ancestors. While gills are great in the water, they are not ideal on land because they can dry out. To preserve moisture and avoid desiccation, pill bugs are active at night and spend the daylight hours in wet, damp areas under things like logs, mulch, and stones, where they can roll into a ball to protect any moisture they have on their gills.

4. They Roll Into a Ball When Disturbed

A pill bug rolled into a tight ball laying on light orange sand
Konrad Wothe / Getty Images

The reason they are called roly-polies is also one of their most visually delightful qualities. When these critters are disturbed or frightened, they roll into a tight little ball, a process known as conglobation. It’s a defense mechanism that is hypothesized to have evolved to protect the pill bugs' soft underparts from predators and to allow them to retain moisture on their gills.

5. They Have Unusual Bodily Functions

Pill bugs have a high tolerance for ammonia gas, so they don’t urinate. Instead, they excrete waste fluids through their shells. As for solid waste, their diet includes self-caprophagy (eating their own feces), which allows them to obtain nutrients they may have missed in the first digestive cycle. When it comes to drinking, roly-polies have two choices: they can drink from their mouths like most creatures, or they can use tube-shaped structures that jut out of their rear ends.

6. They Compost Soil

The head and front legs of a pill bug working in the soil
Joao Paulo Burini / Getty Images

Pill bugs’ preferred food is dead organic plant matter, so if you’re looking for a great composting partner, look no further. By chewing up rotting vegetation and returning it back to the soil, they help speed up decomposition and provide an incredible free service to gardeners. Thanks to bacteria in their guts, they can process dead fruits, leaves, and other vegetation and return it to the earth or compost heap to be digested by more fungi and bacteria.

7. They Eat Metals

Roly-polies play important roles in the environment. They are able to take in heavy metals such as copper, zinc, and lead, and then crystallize them in their bodies. This has made them an ideal test subject in studies of pollution and related environmental research. The pill bugs’ unique ability to remove heavy metal ions from contaminated soil allows them to thrive in polluted locations where other species cannot.

8. They Carry Their Eggs in a Pouch

Just like other crustaceans, female pill bugs have a brood pouch — called a marsupium — on their underside. The females carry their eggs in the pouch for two to three months until they hatch. Even after hatching, young pill bugs may return to the pouch and continue to grow and be nourished through their mother’s marsupial fluid before heading out into the world.