Sowbugs and pillbugs are both small, gray, multi-legged creatures you find under rocks and flowerpots, and they are far more closely related to shrimp and crabs than to any insect in your garden. The easiest way to tell them apart is to poke one gently: a pillbug curls into a tight ball, while a sowbug cannot. That rolling ability is the single most reliable difference between the two, but it reflects deeper distinctions in body structure, taxonomy, and even how they manage water loss. Despite being lumped together in casual conversation, these two animals have been on separate evolutionary paths for a long time.
Crustaceans Living on Land
The first thing worth knowing about both sowbugs and pillbugs is that neither one is an insect. They are isopods, a type of terrestrial crustacean that belongs to the suborder Oniscidea.1EDIS. Pillbug, Roly-Poly, Woodlouse Armadillidium vulgare (Latreille) (Malacostraca: Isopoda: Armadillidiidae) Count the legs and you will notice seven pairs, not the three pairs an insect has. They breathe through gill-like structures on the underside of their bodies rather than through the tracheal tubes insects use. This is why you always find them in damp spots: those modified gills need moisture to function, and a sowbug or pillbug left on a sunny sidewalk will dry out and die within hours.
Their crustacean heritage also means they molt their exoskeleton in a distinctive two-stage process. Unlike insects that shed their entire skin at once, isopods molt the back half first, then the front half a few days later. During moulting they are especially vulnerable, so they tend to hide in crevices and stay very still. Females carry their developing eggs and young in a fluid-filled pouch called a marsupium on their underside, another feature that ties them to their aquatic relatives rather than to anything you would call a bug.
The Rolling Test
The defining behavioral difference is conglobation, which is the scientific name for rolling into a ball. Pillbugs, most famously the species Armadillidium vulgare, can tuck their head and tail together and form a nearly perfect sphere. This is why they are called roly-polies in many parts of the United States, and the pill-like shape is exactly where the name “pillbug” comes from.2EDIS. Pillbug, Roly-Poly, Woodlouse Armadillidium vulgare (Latreille) (Malacostraca: Isopoda: Armadillidiidae): EENY630/IN1099 The hard plates of their exoskeleton fit together snugly, enclosing the soft underbelly and legs entirely.
Sowbugs cannot do this. Their body shape is flatter and wider, and they typically have two small tail-like appendages called uropods that stick out from the rear end. Those uropods physically prevent the back end from sealing against the head. Common sowbug genera include Porcellio and Oniscus. When threatened, a sowbug’s main option is to run or press flat against a surface, relying on speed and camouflage instead of armor. So if you flip a rock and the little gray creatures scatter rather than curl up, you are looking at sowbugs.
Why Rolling Is More Than Just Defense
Conglobation clearly helps a pillbug avoid being eaten, but research shows it also serves a second, less obvious purpose: conserving water. In laboratory experiments, pillbugs in their rolled-up position lost water roughly 35% more slowly than when they were uncurled, and their carbon dioxide output dropped by a similar amount.3Europe PMC. Conglobation in the pill bug, Armadillidium vulgare, as a water conservation mechanism The sealed ball reduces the surface area exposed to dry air, slowing evaporation from those gill-like breathing structures underneath.
This water-saving benefit was measurable at low and moderate humidity levels but disappeared once humidity climbed above about 53% relative humidity, at which point evaporation was already slow enough that posture did not matter much.3Europe PMC. Conglobation in the pill bug, Armadillidium vulgare, as a water conservation mechanism The finding helps explain why pillbugs can sometimes survive in slightly drier microhabitats than sowbugs. Sowbugs, lacking the ability to roll up, are generally more tightly dependent on staying in very moist conditions. If you notice sowbugs clustering in a wetter corner of a basement while pillbugs are scattered more broadly, that difference in desiccation tolerance is probably the reason.
Sorting Out the Common Names
The naming situation is genuinely confusing, and it is not your fault if you have been using the wrong term. In Europe, both sowbugs and pillbugs are called woodlice, and the distinction between species usually does not come up in casual speech.2EDIS. Pillbug, Roly-Poly, Woodlouse Armadillidium vulgare (Latreille) (Malacostraca: Isopoda: Armadillidiidae): EENY630/IN1099 In North America, “roly-poly” almost always means a pillbug, while “sowbug” tends to refer to the flatter, non-rolling species. But in some regions people call any small gray isopod a “potato bug,” “doodlebug,” or “cheesy bug,” regardless of whether it rolls.
The word “bug” in both names is itself misleading. True bugs belong to the insect order Hemiptera and have piercing-sucking mouthparts, which is not what these animals are. Calling them woodlice is at least taxonomically neutral, but even “lice” conjures the wrong image. The one name that actually gets the biology right is “land shrimp,” which you occasionally hear from people who have just learned these creatures are crustaceans. It is whimsical, but it is accurate.
Where They Live and What They Eat
Both sowbugs and pillbugs thrive in the same general habitat: cool, dark, moist spots with access to decaying organic matter. Under logs, beneath mulch, inside compost piles, and in the leaf litter along foundation walls are classic locations. They are detritivores, meaning they feed primarily on dead plant material, breaking it down into smaller particles that soil microbes can then process further. In this way, they play a useful role in nutrient cycling comparable to earthworms.
They can become a nuisance when they wander indoors, usually through ground-level door gaps or cracks in a foundation during especially wet or dry spells. Indoors, they tend to die quickly because the air is too dry, so finding a handful of dead ones in a basement corner is more common than finding a thriving population. Occasionally, very large numbers will congregate in greenhouses or crawl spaces where moisture stays high. In gardens, they sometimes nibble on seedlings or strawberries resting on damp ground, but the damage is minor compared to what slugs or caterpillars do, and most gardeners consider them beneficial decomposers.
If you want fewer of them near your home, the strategy is moisture management. Move mulch a few inches away from the foundation, fix leaky hose bibs, improve drainage around basement walls, and reduce ground-contact wood. Eliminating their preferred microhabitat is more effective and less toxic than pesticides, and given that they are harmless to humans and pets, most pest-control professionals recommend tolerating them outdoors.
How They Navigate When Threatened
When sowbugs and pillbugs flee rather than curl up, they do not run randomly. Both use a strategy called turn alternation: after making a left turn, they tend to turn right next, and vice versa. Over a series of turns, this zigzag pattern keeps them moving roughly in a straight line away from danger, which is a simple but effective escape tactic for an animal without great eyesight.
Experiments with both the sowbug Porcellio laevis and the pillbug Armadillidium vulgare found that sustained exposure to predator cues, specifically the chemical presence of ants kept nearby for a week, caused both species to increase the rate of alternating turns. Brief exposure did not trigger the change, suggesting the behavior requires time to ramp up, possibly as the isopods detect that the threat is persistent rather than a passing encounter.4PubMed Central. Do predator cues influence turn alternation behavior in terrestrial isopods Porcellio laevis Latreille and Armadillidium vulgare Latreille? For a creature with a relatively simple nervous system, adjusting navigation strategy based on long-term threat levels is a surprisingly sophisticated response.
Spiders That Evolved to Hunt Them
One group of predators has taken isopod-hunting to an extreme. Spiders in the genus Dysdera, sometimes called woodlouse hunters, have evolved specialized mouthparts specifically for piercing isopod armor. Research on several Dysdera species in the Canary Islands documented four distinct jaw shapes, from standard unmodified fangs to elongated, concave, or flattened versions. The more specialized the jaw shape, the more strongly the spider preferred isopod prey and the less able it was to handle other types of food.5PubMed Central. Prey acceptance and metabolic specialisations in some Canarian Dysdera spiders
If you have ever seen a reddish-brown spider with disproportionately large, forward-pointing fangs lurking in the same damp spots where you find isopods, it was probably a Dysdera. These spiders are nocturnal and not dangerous to people, though they can give a mild bite if handled roughly. Their existence is a testament to how abundant terrestrial isopods are in soil ecosystems: there is enough isopod biomass to sustain an entire lineage of specialist predators. A pillbug’s ability to roll into an armored ball makes sense as a counter-adaptation when you realize there are spiders with jaws literally shaped to crack that ball open.
When Isopods Turn Blue
Sowbugs and pillbugs are normally gray or brownish gray, so a bright blue or purple individual immediately stands out. That unusual color is not a rare genetic variant. It is a sign of disease. An iridovirus that infects terrestrial isopods causes a characteristic blue discoloration of the normally gray exoskeleton.6Journal of Invertebrate Pathology. Isolation of an iridovirus from two terrestrial isopods, the pill bug, Armadillidium vulgare, and the sow bug, Porcellio dilatatus The virus has been isolated from both pillbugs (Armadillidium vulgare) and sowbugs (Porcellio dilatatus), confirming that it crosses the taxonomic boundary between the two groups.
Iridoviruses produce crystalline protein arrays inside infected cells, and those arrays refract light in a way that creates the striking iridescent blue sheen. The infected isopods are not necessarily dead yet when you see the color change, but the infection is typically fatal. If you spot a blue pillbug or sowbug in your garden, it is not a lucky find or a special morph. It is a sick animal. The virus can spread through populations in moist habitats, but outbreaks tend to be localized and do not pose any risk to humans, pets, or plants.
Bacteria That Change Their Sex
One of the stranger stories in isopod biology involves a bacterium called Wolbachia, an endosymbiont that lives inside the cells of a huge fraction of arthropod species. Wolbachia is transmitted from mother to offspring, so the bacterium has an evolutionary incentive to skew host reproduction toward producing more females, which will carry it forward. In pillbugs, Wolbachia does exactly that: it feminizes genetic males, causing individuals that are chromosomally male to develop as functional females.7PubMed Central. Feminizing Wolbachia: a transcriptomics approach with insights on the immune response genes in Armadillidium vulgare
In some wild populations of Armadillidium vulgare, the infection is so widespread that all individuals are genetically male. The only thing determining which ones develop as females is whether they carry Wolbachia. The bacterium overrides the male-determining signal on the Z chromosome, effectively replacing the host’s own sex-determination system with one that benefits the parasite.8Genetics. Conflict between feminizing sex ratio distorters and an autosomal masculinizing gene in the terrestrial isopod Armadillidium vulgare Latr.
The story gets even more dramatic. Researchers discovered that in some pillbug lineages, a large chunk of Wolbachia DNA, roughly three million base pairs, has been transferred directly into the pillbug’s own nuclear genome. This inserted bacterial DNA now acts as a female sex-determining region, essentially functioning as a brand-new W sex chromosome. The chromosome carrying the Wolbachia insert is hemizygous in females and absent in males, behaving exactly like a conventional sex chromosome even though it originated from a bacterial parasite.9PubMed Central. Birth of a W sex chromosome by horizontal transfer of Wolbachia bacterial symbiont genome The pillbug essentially domesticated its own parasite’s genome and repurposed it. This is one of the most striking examples known of a bacterium fundamentally rewriting how an animal determines sex.
Physiology That Bridges Sea and Soil
Because sowbugs and pillbugs are crustaceans that colonized land, their physiology is a patchwork of marine adaptations jury-rigged for a terrestrial existence. Reviews of their physiology describe a suite of features that make them unusual among land-dwelling arthropods: they exchange gases through pleopodal lungs (modified abdominal appendages that evolved from the gill-bearing limbs of aquatic ancestors), they excrete nitrogen primarily as ammonia gas rather than as the uric acid that insects use, and some species can actively absorb water vapor from humid air.10ScienceDirect / Elsevier. Physiology of terrestrial isopods
The ammonia excretion is a particularly telling holdover. Aquatic crustaceans can afford to release ammonia directly into the surrounding water, where it is diluted and carried away. On land, ammonia is toxic at even moderate concentrations, so most terrestrial animals convert it to something less harmful before excreting it. Isopods have stuck with ammonia, releasing it as a gas through their exoskeleton. It works, but only because they stay in humid, ventilated microhabitats where the gas disperses quickly. This is yet another reason they are locked into damp environments and why you rarely find them far from moisture. Every aspect of their biology points back to an ancestry in the ocean, and the compromises they have made to live on land are what shape the moist, hidden, under-the-rock world they inhabit.