Woodlice and roly-polies belong to the same group of land-dwelling crustaceans, but the two names don’t map onto each other perfectly. “Woodlouse” is a broad term used mainly in Europe for any terrestrial isopod, while “roly-poly” (or “pillbug”) typically refers to one particular species, Armadillidium vulgare, famous for curling into a tight ball when disturbed. Every roly-poly is a woodlouse, but not every woodlouse is a roly-poly. That distinction matters more than it sounds, because the woodlouse family tree includes creatures with noticeably different body shapes, defensive strategies, and habits.
Why So Many Names for the Same Creature
Few animals collect regional nicknames the way terrestrial isopods do. Depending on where you live, you might call them woodlice, pillbugs, roly-polies, slaters, cheese logs, chucky pigs, or potato bugs. In Europe, “woodlouse” covers the whole group because these animals turn up under rotting logs and in damp cellar corners. In much of North America, “roly-poly” zeroes in on the species that rolls into a ball, and “sowbug” is reserved for the flatter relatives that cannot roll up at all.1EDIS. Pillbug, Roly-Poly, Woodlouse Armadillidium vulgare (Latreille) (Malacostraca: Isopoda: Armadillidiidae) The confusion is understandable: all of these animals are small, grey, segmented, and found in the same damp habitats. But they belong to different families within the order Isopoda, and some of them are about as closely related to each other as dogs are to cats within the broader mammal group.
Pillbugs Versus Sowbugs
The most useful everyday distinction is between pillbugs and sowbugs. Pillbugs, the classic roly-polies, belong to the family Armadillidiidae. Their body segments fit together like interlocking armor plates, letting them curl into a near-perfect sphere. Sowbugs, by contrast, belong to families like Oniscidae and Porcellionidae. They are flatter, their rear segments stick out in two small tail-like projections (called uropods), and they simply cannot roll up. If you flip one over and it curls into a ball, it’s a pillbug. If it scrambles away with its legs waving, it’s a sowbug.
Both types share the same general lifestyle: nocturnal, moisture-dependent, and found under rocks, leaf litter, and decaying wood. But the rolling ability of pillbugs is not just a party trick. Laboratory experiments with Armadillidium vulgare showed that rolling into a ball reduced water loss by about 35% and cut carbon dioxide release by a similar amount, at least at moderate humidity levels.2PubMed Central. Conglobation in the pill bug, Armadillidium vulgare, as a water conservation mechanism So conglobation, as biologists call the rolling behavior, serves double duty: it protects against predators and helps the animal hang on to precious moisture.
Crustaceans Living on Land
One of the most surprising facts about woodlice is that they are not insects at all. They are crustaceans, more closely related to crabs, lobsters, and shrimp than to beetles or ants. Of all the crustacean groups that have tried to colonize land, terrestrial isopods have been by far the most successful. A 2024 phylogenomic study found strong evidence that this transition happened just once, around 298 million years ago near the boundary between the Carboniferous and Permian periods. That makes the move to land considerably later than for other terrestrial arthropods like insects and arachnids.3PubMed Central. Phylogenomics supports a single origin of terrestriality in isopods
Earlier genetic studies had suggested that isopods might have crawled onto land multiple separate times, which would have meant that different woodlouse lineages weren’t as closely related as they looked. The 2024 analysis, drawing on far more genetic data, overturned that idea and found the group to be genuinely monophyletic, meaning all terrestrial isopods share a single land-colonizing ancestor.3PubMed Central. Phylogenomics supports a single origin of terrestriality in isopods That single origin helps explain why all woodlice, whether they roll up or not, share the same basic toolkit for surviving out of water.
How They Breathe Without Gills
The crustacean heritage of woodlice shows up most clearly in how they breathe. Their marine relatives use gills, and woodlice haven’t entirely abandoned that plan. Instead, they’ve modified the flat, plate-like appendages on their undersides (called pleopods) into organs that function somewhere between gills and lungs. In the more advanced lineages, these pleopods contain branching, air-filled tubes called pseudotracheae that pull oxygen directly from the air, somewhat like the tracheal systems of insects but evolved independently.4PubMed Central. Terrestrial crustaceans (Arthropoda, Crustacea): taxonomic diversity, terrestrial adaptations, and ecological functions These structures originally derived from the pleopodal gills of aquatic ancestors.5PubMed. Pleopodal lung development in a terrestrial isopod, Porcellio scaber (Oniscidea)
This breathing system works well enough on land, but it comes with a catch: the respiratory surfaces still need to stay moist. That’s why woodlice are so strongly tied to damp environments and why you almost never see them out during the heat of the day. If their pleopods dry out, they can’t exchange gases. This is also part of why they huddle together in groups under logs and stones. Gregarious behavior, the tendency to cluster with others of their kind, significantly reduces water loss while also boosting growth and reproductive success.6PubMed Central. Avoidance behaviour toxicity tests should account for animal gregariousness: a case study on the terrestrial isopod Porcellio scaber
Blue Blood and Copper-Based Oxygen
Unlike most animals you encounter in a garden, woodlice don’t use iron-based hemoglobin to carry oxygen through their bodies. Instead, they rely on hemocyanin, a copper-containing protein dissolved freely in their hemolymph (the invertebrate equivalent of blood). When hemocyanin binds oxygen, the copper gives it a blue tint, so woodlouse blood is quite literally blue.7PubMed. Multifunctional Roles of Hemocyanins Hemocyanin also plays roles beyond oxygen transport, including immune defense. It’s a system inherited from their marine crustacean ancestors and shared with spiders, scorpions, and many mollusks.
Chemical Warfare Against Predators
Rolling into a ball is the pillbug’s signature move, but woodlice that can’t roll up need other ways to stay alive. Research published in 2025 revealed that the rough woodlouse (Porcellio scaber), a common sowbug, secretes chemical compounds from glands on its body that are remarkably effective at repelling spiders. In controlled experiments, false widow spiders (Steatoda grossa) preyed on 15 out of 20 mealworm beetles offered to them but killed only 6 out of 20 woodlice. When the researchers extracted the woodlouse secretions and applied them to beetles, spider predation on those beetles dropped from 16 out of 20 down to just 6 out of 20.8PubMed Central. Glandular quinoline-derivates protect crustacean woodlice from spider predation The active compounds turned out to be quinoline derivatives, nitrogen-containing molecules with a foul taste that spiders clearly want no part of.
So pillbugs and sowbugs have adopted different defensive strategies. Pillbugs armor up and roll into an impenetrable ball. Sowbugs stay flat, run, and rely on chemical deterrents. Both strategies work well enough that these animals have thrived on every continent except Antarctica.
Their Senses Are Not What You’d Expect
Woodlice have simple eyes that can distinguish light from dark but can’t form detailed images. They navigate the world primarily through two pairs of antennae loaded with chemical sensors, using these to locate food, find suitable moist microhabitats, detect predators, and communicate with other woodlice.6PubMed Central. Avoidance behaviour toxicity tests should account for animal gregariousness: a case study on the terrestrial isopod Porcellio scaber If you’ve ever watched a woodlouse moving across the ground, you’ll notice those antennae constantly tapping and sweeping ahead, reading the chemical landscape much the way a dog reads scent trails.
Pillbugs also display a curious navigational behavior called turn alternation. When forced to turn one direction by an obstacle, they tend to compensate by turning the opposite direction at the next opportunity, keeping their overall path roughly straight.9PubMed. Self-corrective behavior for turn alternation in pill bugs (Armadillidium vulgare) This self-correcting strategy is surprisingly sophisticated for an animal with such minimal visual ability, and it’s robust enough that it has been studied in psychology as a model of simple spatial decision-making.
Quiet Engines of Decomposition
Woodlice play a role in soil ecology that far outweighs their small size. As saprophagous macroarthropods, they are key regulators of leaf litter decomposition in both temperate and tropical ecosystems.10PubMed. The ecology of saprophagous macroarthropods (millipedes, woodlice) in the context of global change They chew up dead leaves, drag fragments from the surface deeper into the soil, and pass the material through their guts. What comes out the other end is dramatically different from what went in. Classic laboratory work showed that fungal spore densities were over three times higher in woodlouse feces than in intact leaf litter, and viable bacteria counts were over a hundred times higher in feces derived from fresh leaves than in the uneaten litter.11PubMed. Effects of terrestrial isopods on the decomposition of woodland leaf litter
The physical act of shredding, combined with the chemical changes during digestion, creates a substrate that microbes can colonize far more effectively. In the field, the simple act of moving litter from the dry surface to damper layers below may be the single biggest contribution woodlice make to the decomposition chain.11PubMed. Effects of terrestrial isopods on the decomposition of woodland leaf litter For gardeners, that means woodlice in your compost are a feature, not a problem.
A Gut Full of Helpers
Woodlice eat things that should, by all rights, be nearly indigestible. Dead leaves and rotting wood are made largely of cellulose, hemicellulose, and lignin, compounds that resist breakdown. Woodlice manage this with help from their gut microbiome. Researchers studying Armadillidium vulgare found that the animal produces its own enzymes for breaking down plant cell walls, but its gut bacteria and archaea contribute a complementary and overlapping set of enzymes that fill in the gaps.12PubMed Central. Lignocellulose degradation at the holobiont level: teamwork in a keystone soil invertebrate It’s a genuine partnership: neither the animal nor its microbes could fully break down tough plant material alone.
This has attracted interest from researchers looking for new ways to break down agricultural waste and other plant-based biomass. The gut of another woodlouse species, Porcellio dilatatus, has been mined for bacteria capable of degrading lignocellulosic material, with an eye toward industrial applications.13PubMed Central. Isolation of lignocellulosic biomass-degrading bacteria from Porcellio dilatatus gut-enriched cultures The humble woodlouse gut turns out to be a surprisingly rich library of enzymes that biotechnologists would like to borrow.
Living Pollution Detectors
Because woodlice spend their lives in direct contact with soil and leaf litter, absorbing metals through their food and their permeable undersides, they accumulate heavy metals in their tissues in ways that reflect what’s actually biologically available in the environment. This makes them potentially valuable as bioindicators of soil contamination. Isopods tolerate metals like cadmium and copper by sequestering them in specialized storage structures within their hepatopancreas, the organ that functions as both their liver and pancreas.14Agriculture, Ecosystems & Environment. Woodlice (Isopoda: Oniscidea): their potential for assessing sustainability and use as bioindicators
Different woodlouse species handle individual metals in distinctly different ways. A study of urban soils found that the two common European species Oniscus asellus and Porcellio scaber bioconcentrated metals in different rank orders, with copper being the most concentrated in both but the handling of zinc, cadmium, and other metals varying considerably between them.15PubMed. Metal uptake by woodlice in urban soils The widespread distribution of Porcellio scaber, which has been introduced to most countries worldwide, has led researchers to propose a global “woodlouse watch” program, analogous to the “mussel watch” schemes used to monitor marine pollution. The idea is that measuring metal levels in this single cosmopolitan species could give a standardized picture of soil contamination across regions.16Science of The Total Environment. The isopod Porcellio scaber as a monitor of the bioavailability of metals in terrestrial ecosystems: towards a global ‘woodlouse watch’ scheme
The Taxonomy Is Still Being Sorted Out
You might assume that a group of common backyard animals would have a settled family tree by now, but woodlouse taxonomy remains surprisingly unsettled. Molecular studies have revealed that some traditional family groupings don’t hold up under genetic scrutiny. The family Porcellionidae, for example, clusters with Trachelipodidae rather than with Armadillidiidae, even though Armadillidiidae was long considered its closest relative. Meanwhile, the genus Ligia, a shore-dwelling woodlouse often treated as the most “primitive” living terrestrial isopod, may actually sit outside the true terrestrial isopod group entirely, closer to aquatic isopod families.17Scientific Reports. Genetic evidence against monophyly of Oniscidea implies a need to revise scenarios for the origin of terrestrial isopods
These aren’t just academic reshufflings. If Ligia really does fall outside the terrestrial isopod clade, it changes the story of how woodlice colonized land. Were shore-dwelling species the stepping stone between sea and soil, or did they arrive at the shoreline from a different evolutionary direction? The broader phylogenomic analysis recovered Oniscidea as monophyletic only after including far more genetic loci, and earlier studies based on fewer genes kept getting contradictory results.3PubMed Central. Phylogenomics supports a single origin of terrestriality in isopods Expect the classification to keep shifting as sequencing gets cheaper and more obscure species get sampled. For now, the rough shape is clear: all the woodlice you find under flowerpots and compost bins share a common land-dwelling ancestor, but the precise branching pattern is still being drawn.
Keeping Them or Getting Rid of Them
Woodlice have become unexpectedly popular as pets in recent years, especially among people who keep bioactive terrariums for reptiles and amphibians. Isopod species in the genera Armadillidium, Porcellio, and Cubaris are bred in a range of color morphs and sold as “cleanup crews” that consume waste in vivariums. The roly-poly’s ability to curl into a ball makes it a perennial favorite with children, too, and because these animals don’t bite, sting, or carry diseases meaningful to humans, they’re about as harmless as a garden invertebrate gets.
On the flip side, large populations of woodlice in a greenhouse or around young seedlings can occasionally cause minor feeding damage to tender stems and roots, though this is far less common than their reputation suggests. They strongly prefer dead and decaying matter over living plant tissue. If you find them nibbling strawberries or lettuce, it’s almost always fruit that was already damaged or sitting on damp soil. Reducing ground-level moisture and clearing decaying organic material is usually enough to bring numbers down without chemicals. Diatomaceous earth around plant bases works as a physical barrier because it damages their moisture-retaining cuticle, but given the ecological benefits woodlice provide through decomposition and soil turnover, most gardeners are better off leaving them alone.