15 Fascinating Facts About Pill Bugs

Woodlouse (pill bug, roly-poly)

 Paul Starosta/Getty Images

The pill bug goes by many names—roly-poly, woodlouse, armadillo bug, potato bug. But whatever you call it, it's a fascinating creature...or actually 4,000 species of creature. The nocturnal crustaeans have seven pairs of legs, segmented sections like a lobster's tail, and prefer humid environments. They eat rotting vegetation and help nutrients in it get returned to the soil for plants to feed on, so they're not pests. They don't bother living vegetation. These insights into pill bugs will give you a newfound respect for the tiny tank living beneath your flower pots. 

01
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Pill Bugs Are Crustaceans, Not Insects

Though they're often associated with insects and are referred to as "bugs," pill bugs actually belong to the subphylum Crustacea. They're much more closely related to shrimp and crayfish than to any kind of insect.

02
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Pill Bugs Breathe Through Gills

Like their marine cousins, terrestrial pill bugs use gill-like structures to exchange gases. They require moist environments to breathe but cannot survive being submerged in water.

03
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A Juvenile Pill Bug Molts in Two Sections

Like all arthropods, pillbugs grow by molting a hard exoskeleton. But pillbugs don't shed their cuticle all at once. First, the back half of its exoskeleton splits away and slides off. A few days later, the pillbug sheds the front section. If you find a pillbug that's gray or brown on one end, and pink on the other, it's in the middle of molting.

04
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Pill Bug Mothers Carry Their Eggs in a Pouch

Like crabs and other crustaceans, pill bugs tote their eggs around with them. Overlapping thoracic plates form a special pouch, called a marsupium, on the pill bug's underside. Upon hatching, the tiny juvenile pill bugs remain in the pouch for several days before leaving to explore the world on their own.

05
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Pill Bugs Don't Urinate

Most animals must convert their wastes, which are high in ammonia, into urea before it can be excreted from the body. But pill bugs have an amazing ability to tolerate ammonia gas, which they can pass directly through their exoskeleton, so there's no need for them to urinate.

06
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A Pill Bug Can Drink With Its Anus

Though pill bugs do drink the old-fashioned way—with their mouthparts—they can also take in water through their rear ends. Special tube-shaped structures called uropods can wick water up when needed. 

07
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Some Species of Pill bugs Curl Into a Ball When Threatened

Most kids have poked a pill bug to watch it roll up into a tight ball. In fact, many people call them roly-polies for just this reason. Their ability to curl up distinguishes the pill bug from another close relative, the sowbug.

08
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Pill Bugs Eat Their Own Poop

Yes indeed, pill bugs munch on lots of feces, including their own. Each time a pill bug poops, it loses a little copper, an essential element it needs to live. In order to recycle this precious resource, the pill bug will consume its own poop, a practice known as coprophagy.

09
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Sick Pill Bugs Turn Bright Blue

Like other animals, pill bugs can contract viral infections. If you find a pill bug that looks bright blue or purple, it's a sign of an iridovirus. Reflected light from the virus causes the cyan color.

10
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A Pill Bug's Blood Is Blue

Many crustaceans, pill bugs included, have hemocyanin in their blood. Unlike hemoglobin, which contains iron, hemocyanin contains copper ions. When oxygenated, pill bug blood appears blue.

11
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They "Eat" Metals

Pill bugs are important for ridding the soil of heavy metal ions by taking in copper, zinc, lead, arsenic, and cadmium, which they crystallize in their midgut. Thus, they can survive in contaminated soil where other species can't. 

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They're the Only Land Crustacean

Pill bugs represent the only crustacean that has widely colonized land. They're still a bit "fish out of water," though, as they are at risk of drying out on land; they haven't developed the waterproof waxy coating of arachnids or insects. Pill bugs can survive until they get down to 30 percent dry.

13
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They're Humidity Sponges

If the humidity gets really high in the atmosphere, above 87 percent, pill bugs can actually absorb moisture from the air to stay hydrated or improve their hydration.

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They're European Imports

Pill bugs probably came to North America with the lumber trade. European species may have originated in the Mediterranean region, which would explain why they don't survive winters where it gets below 20 degrees Fahrenheit as ​they aren't underground burrowers.

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Babies Don't Have All Their Legs

When born, pill bug young have only six pairs of legs. They get the seventh pair following their first molt.