A roly poly, also called a pill bug, is not an insect. It’s a crustacean, in the same broad family as shrimp, crabs, and lobsters, and it belongs to the isopods, a huge group with about 5,000 land-living species.12024 IEEE Integrated STEM Education Conference (ISEC). Rolling Across the Continents: Phylogenetic Relationships of the Isopoda Its ancestors lived in the sea, and nearly everything about how a roly poly lives, from its hiding spots to its famous ball, comes down to not drying out on land.
How a Sea Creature Ended Up in Your Garden
Land isopods are the most successful crustaceans ever to colonize land, so scientists use them as a model for how sea animals adapted to life out of water.2Terrestrial Arthropod Reviews. Evolutionary adaptation of oniscidean isopods to terrestrial life: Structure, physiology and behavior The split between the sea and land lines happened in the shallow coastal zone.3Terrestrial Arthropod Reviews. Evolutionary adaptation of oniscidean isopods to terrestrial life: Structure, physiology and behavior That means the move happened directly across sea beaches, and some living species still show in-between stages.4Comparative Biochemistry and Physiology Part A: Physiology. Review Physiology of terrestrial isopods Genetic evidence points to a single move onto land, not several separate ones.5PubMed Central. Phylogenomics supports a single origin of terrestriality in isopods Their family tree is still being revised, though, because genetic results often disagree with groupings based on body shape.62024 IEEE Integrated STEM Education Conference (ISEC). Rolling Across the Continents: Phylogenetic Relationships of the Isopoda
Telling a roly poly from an insect is easy once you count. Insects have six legs and three body parts, while a roly poly has seven pairs of legs and a segmented shell. It also has two pairs of antennae. The first pair is short and often hidden, while the longer second pair senses its surroundings.7Welcome Wildlife. In your yard: pill bugs (roly-poly bugs) Even the word “bug” misleads, since to biologists a true bug is a specific insect group, and roly polies aren’t in it.
Many woodlice look alike, but among them only pill bugs roll into a full, tight ball. Sow bugs are flatter, can’t roll up, and often show two small tail spikes. Millipedes have far more legs, and most coil into a spiral, though pill millipedes also roll into a ball. A quick, gentle touch tells you which one you found.
Why Roly Polies Roll Into a Ball
Pill bugs lose water more slowly when rolled up than when stretched out.8PubMed Central. Conglobation in the Pill Bug, Armadillidium vulgare, as a Water Conservation Mechanism The underside, with its breathing plates, loses water faster than the rest of the shell, so rolling tucks that thirsty underside inward and covers it.9PubMed Central. Conglobation in the Pill Bug, Armadillidium vulgare, as a Water Conservation Mechanism A ball also has less exposed surface for its size, so less moisture escapes.10PubMed Central. Conglobation in the Pill Bug, Armadillidium vulgare, as a Water Conservation Mechanism11PubMed Central. Conglobation in the Pill Bug, Armadillidium vulgare, as a Water Conservation Mechanism
Rolling is also a defense. Male pill bugs that smelled a predator, a toad, stayed rolled up longer than males that smelled harmless rabbit scent or water.12PubMed Central. Keep the ball rolling: sexual differences in conglobation behavior of a terrestrial isopod under different degrees of perceived predation pressure So roly polies seem to judge danger and adjust how long they hide. A rolled-up roly poly stays that way until it senses the danger has passed, then slowly unrolls. Handling it gently and returning it to damp shade causes no harm.
The ball works because the shell is made of overlapping plates across the back. They slide against each other like armor, letting the body curve into a sphere. When the animal curls, the head and the last body segment meet to close the ball, leaving very little gap for a predator to get into.
How Roly Polies Breathe, Drink, and Stay Moist
A roly poly breathes through its underside. Flat plates there handle both breathing and water exchange.13Backyard Nature. What Is a Roly Poly Bug? Its Biology and Habitat In some species these plates fold inward into whitish branching tubes that work as small lungs.14Backyard Nature. What Is a Roly Poly Bug? Its Biology and Habitat Plate lungs evolved in the groups of land isopods that spread most widely.15PubMed. Respiratory physiology of the Oniscidea: aerobic capacity and the significance of pleopodal lungs How much lung surface a species has, along with how thick its back shell is, helps decide how well it resists drying.16PubMed. Eco-morphological studies on pleopodal lungs and cuticle in Armadillidium species (Crustacea, Isopoda, Oniscidea)
Flip one over and you may see pale patches near the rear underside. Those are the lung plates, and they must stay damp to work. If they dry out, the animal can’t breathe properly.
Unlike insects, which have a waxy coat and sealable breathing pores, a roly poly exchanges gas through thin, moist surfaces that also let water escape. That trade-off is behind its need for damp places.
Tail-like tips at its rear wick up water, so a roly poly can drink from both ends of its body.17Colorado State University Extension. Pillbug (“Roly-poly”) Water drawn up at the tail is thought to travel along grooves on its sides toward the mouth.18Backyard Nature. What Is a Roly Poly Bug? Its Biology and Habitat
Waste is handled just as thriftily. Most animals flush ammonia waste out in liquid, which costs water.19Welcome Wildlife. In your yard: pill bugs (roly-poly bugs) Pill bugs instead release ammonia as a gas straight through their shell.20Welcome Wildlife. In your yard: pill bugs (roly-poly bugs) That way they avoid losing precious moisture to urine.
If you keep roly polies, give them a corner of the container that’s damp but not flooded. Too much standing water can drown them, since they breathe air. A faint ammonia smell in a closed container comes from their gas waste.
Where Roly Polies Live and Why
Pill bugs live in leaf litter, soil, compost, mulch, and under logs or rocks.21Field of Mars EEC. Pill bug fact sheet They choose dark, damp spots to stay moist and avoid dry air and temperature extremes.22Extension. Pillbugs: An interactive investigation into animal behaviors [STEM Activity] In hot or dry weather they hide in cool spaces and become more active at night.23Field of Mars EEC. Pill bug fact sheet
A good spot is dark, holds moisture, and has food nearby, since decaying leaves and wood serve as both shelter and meals. Places that stay shaded and covered cool down in summer and stay above freezing longer in winter than open ground does. Bare, sunny, or sandy ground makes a poor home, which is why you rarely find roly polies on an open lawn in the afternoon.
To find them, lift a flowerpot, board, or stone after rain, where clusters often gather. Check under mulch and in the compost pile too. They huddle in groups, which helps each one lose less water. Put the cover back afterward so they don’t dry out.
Land isopods live everywhere from the seashore to treetops in tropical forests.242024 IEEE Integrated STEM Education Conference (ISEC). Rolling Across the Continents: Phylogenetic Relationships of the Isopoda The common pill bug has spread with people to gardens on many continents.
Are Roly Polies Good or Bad for Your Garden
Pill bugs are recyclers that feed on dead plant matter.25Green Pest Management. Pill Bug Fun Facts: What Roly Polies Are, Why They Roll Up & Roly Poly Pest Control They help break down leaf litter and compost and return nutrients to soil.26Field of Mars EEC. Pill bug fact sheet On farms they speed up decomposition of decaying organic matter.27Vegetable Crops Hotline. Are Pillbugs Pests? How Can They be Managed?
They can also feed on fruit, vegetables, young shoots, roots, or low leaves touching soil.28Vegetable Crops Hotline. Are Pillbugs Pests? How Can They be Managed? Trouble usually starts when populations grow large.29Green Pest Management. Pill Bug Fun Facts: What Roly Polies Are, Why They Roll Up & Roly Poly Pest Control Heavy feeding on seedlings can stunt growth, especially in small gardens.30Green Pest Management. Pill Bug Fun Facts: What Roly Polies Are, Why They Roll Up & Roly Poly Pest Control In southern Australia they are increasingly reported damaging emerging crop seedlings, including cereals, legumes, and oilseeds.31PubMed. Crop Seedling Susceptibility to Armadillidium vulgare (Isopoda: Armadillidiidae) and Ommatoiulus moreletii (Diplopoda: Iulidae)
Before acting, check whether the damage is on seedlings or on soft fruit resting on the soil, since healthy established plants rarely suffer meaningfully. A pattern of repeated loss of new seedlings, not a single nibbled leaf, is the sign that numbers are too high. A garden with plenty of roly polies but undamaged plants needs no action at all.
Roly poly damage looks like ragged holes low on plants, near the soil. Slugs leave a shiny slime trail, and roly polies leave none. A flashlight check at night shows who is feeding.
If numbers do get high, pull mulch a few inches back from young seedlings to reduce damp hiding spots. Water in the morning so the soil surface dries by night. Lift ripening strawberries and squash off the soil.
Pesticides are usually unnecessary, since removing excess damp debris usually solves problems. Killing them also removes free compost helpers.
How Roly Polies Grow and Raise Their Young
Adults shed the back half of their shell first, then the front half a few days later.32Welcome Wildlife. In your yard: pill bugs (roly-poly bugs) A roly poly with different-colored front and back halves is probably mid-molt.33The Evolving Naturalist. roly-poly Molting in halves likely lowers predator risk and reduces water loss.34The Evolving Naturalist. roly-poly The new half often looks paler or whitish. Molting animals are soft and best left undisturbed, and they may eat their shed shell to recycle calcium. The hard shell can’t stretch, so the animal must shed it periodically to grow larger, and adults keep molting through life.
Females carry eggs in a fluid-filled brood pouch on their underside. Development runs from the first cell division to a hatched baby that eventually leaves the pouch.35PubMed. The embryonic development of the malacostracan crustacean Porcellio scaber (Isopoda, Oniscidea) Depending on species, eggs and young sit in sacs hung from the pouch roof or are attached singly by small stalks.36Invertebrate Reproduction & Development. Brood-pouch structures in terrestrial isopods Those stalks can be seen alongside developing babies inside the pouch.37PubMed Central. Histological studies on the marsupium of two terrestrial isopods (Crustacea, Isopoda, Oniscidea)
A female with a pale bulge on her underside is carrying young. Tiny white babies may emerge and cluster near her. Newborns look like miniature adults with one fewer leg pair.
Why Some Roly Polies Turn Blue
A virus called invertebrate iridescent virus 31 makes land isopods turn iridescent and kills many.38PubMed. Sex-specific survival and absence of Wolbachia-mediated protection during invertebrate iridescent virus 31 infection in Armadillidium vulgare In lab infections, the blue sheen appeared about 6 days after infection and every infected animal died by 80 days.39PubMed. Sex-specific survival and absence of Wolbachia-mediated protection during invertebrate iridescent virus 31 infection in Armadillidium vulgare Blue to violet woodlice have now been reported from Africa, South America, and Oceania, and the virus was confirmed in Australia.40PubMed. Citizen science data opens multiple avenues for iridovirus research and prompts first detection of Invertebrate iridescent virus 31 in Australia Photos of blue roly polies shared by the public are helping scientists track it.
Infected animals take on a blue, violet, or purple shimmer that looks different from the normal gray or brown. A roly poly that is simply damp, freshly molted, or covered in a bit of dirt may look odd but isn’t the same as the distinct blue sheen.
If you find a blue one, leave it or remove it from any pet colony to protect the others. The virus affects these small creatures, not people or pets. Snapping a photo for a citizen science app helps map the virus.
Hidden Life Inside Roly Polies
Many common pill bugs carry Wolbachia, bacteria passed from mother to young.41PubMed. Evidence for a new feminizing Wolbachia strain in the isopod Armadillidium vulgare: evolutionary implications The bacteria turn genetic males into functional females.42PubMed. Evidence for a new feminizing Wolbachia strain in the isopod Armadillidium vulgare: evolutionary implications More females means more mothers passing the bacteria on through eggs.43PubMed. Effects of parasitic sex-ratio distorters on host genetic structure in the Armadillidium vulgare-Wolbachia association
Roly polies also record their surroundings. Pill bugs eating food tainted with cadmium and lead build up the metals in step with the dose.44PubMed. Bioaccumulation of cadmium and lead and its effects on hepatopancreas morphology in three terrestrial isopod crustacean species High doses damage their digestive gland, so their cells can serve as useful signals of soil pollution.45PubMed Central. Flow cytometric analysis of hepatopancreatic cells from Armadillidium vulgare highlights terrestrial isopods as efficient environmental bioindicators in ex vivo settings
Lots of healthy roly polies don’t prove the soil is clean. A soil test is the reliable way to check before growing vegetables.