What Are Isopods?
Isopods are crustaceans - not insects. They belong to the order Isopoda, which makes them relatives of crabs, lobsters and shrimp rather than beetles or ants. There are around 10,000 species worldwide, living everywhere from deep-sea trenches to freshwater streams to the underside of a damp log in a British garden. The ones most people meet are the terrestrial isopods: woodlice, pill bugs, sow bugs, slaters - all different names for the same broad group of animals. They eat decaying plant matter, they're completely harmless, and they're one of the few groups of crustaceans that managed to leave the water entirely and colonise the land.
They're also, increasingly, kept as pets - and some of them are genuinely spectacular.
Why They're Not Insects
This is the question people ask most, and the answer is clear once you know what to look for.
| Isopods (crustaceans) | Insects |
| Seven pairs of legs (fourteen in total) | Three pairs of legs (six in total) |
| Two pairs of antennae | One pair of antennae |
| No wings, ever | Usually winged |
| Body in many similar segments | Three clear sections: head, thorax, abdomen |
| Carry young in a pouch, give birth to live young | Almost all lay eggs |
The leg count is the giveaway. Count them and you'll never mistake one again.
The evolutionary story is the interesting part, though. Crustaceans are overwhelmingly aquatic - crabs, lobsters, prawns, barnacles. Terrestrial isopods are one of the very few crustacean groups that made the move onto land completely, and they did it without ever fully solving the water problem. They still breathe through structures derived from gills (pleopodal lungs, on the underside of the body), which is why they need damp conditions and die quickly if they dry out. A woodlouse under a plant pot is, in a real sense, a marine animal that never quite finished moving house.
Woodlouse, Pill Bug, Slater - What's the Difference?
Mostly regional dialect, with one genuine distinction:
- Woodlouse - the standard British term for a terrestrial isopod.
- Slater - the same animal, in Scotland and northern England.
- Sow bug - the American term, usually for the flatter species that can't roll up.
- Pill bug / roly-poly - the ones that can roll into a complete ball. This is the real distinction: only some isopods conglobate, and it's a genuine anatomical difference, not just a naming quirk.
- Isopod - the scientific term for the whole order, and the one the hobby uses.
So a pill bug is a woodlouse, but not every woodlouse is a pill bug. If it rolls into a sealed sphere it's almost certainly an Armadillidium; if it just runs away, it's more likely a Porcellio or Oniscus.
Where Isopods Live
On land. The familiar ones - under logs, in leaf litter, beneath plant pots. They need moisture and darkness, and they're everywhere.
In the sea. Most isopod species are marine, and this is where they get strange. Giant isopods (Bathynomus) scavenge the deep-sea floor and reach the size of a small cat - a phenomenon called deep-sea gigantism. They look exactly like an enormous woodlouse, because that's essentially what they are.
In fresh water. Less famous, but they're there too - in rivers, lakes and caves, doing the same decomposition work.
What They Actually Do
Isopods are detritivores. They eat dead and decaying organic matter - leaf litter, rotting wood, fungi - and in doing so they break it down into forms other organisms can use. They're one of the main engines of the process that turns fallen leaves back into soil.
It's unglamorous work and almost entirely invisible, but a woodland floor without them would simply accumulate dead leaves. They don't bite, don't sting, don't carry disease, don't damage buildings and don't eat living plants. A woodlouse in your house is telling you that somewhere is damp - it isn't causing any harm.
Why People Keep Them
Here's the part most people don't expect. Over the last decade, isopods have become one of the fastest-growing exotic pet hobbies in the UK - and selective breeding has produced animals that look nothing like the grey things under your paving slabs.
There are isopods with bold black-and-white stripes, vivid oranges, deep purples, and one - the Rubber Ducky - patterned so that it genuinely resembles a bath toy. They live in a plastic tub on a shelf, cost pennies to feed, need no heating for most species, breed readily, and ask for almost nothing beyond a damp corner and some leaf litter.
They're also the backbone of the "bioactive" approach to keeping reptiles and amphibians, where a colony of isopods lives in the vivarium substrate and processes waste continuously - a self-cleaning enclosure.
If any of that sounds interesting, our guide to isopod species and morphs covers what's actually available, and the beginner's guide to keeping isopods covers how to start. The Dairy Cow is where most people begin.
Frequently Asked Questions
What is an isopod?
A crustacean in the order Isopoda - a relative of crabs and lobsters rather than an insect. Around 10,000 species exist worldwide, in the sea, in fresh water, and on land. The terrestrial ones are what most people know as woodlice or pill bugs.
Are isopods insects?
No. Isopods are crustaceans. They have seven pairs of legs where insects have three, two pairs of antennae where insects have one, no wings at all, and they give birth to live young rather than laying eggs. Count the legs - fourteen means crustacean.
Is a woodlouse an isopod?
Yes. "Woodlouse" is the common British name for a terrestrial isopod. Slater, sow bug, pill bug and roly-poly all refer to the same broad group. Only some of them - the ones that roll into a complete ball - are true pill bugs.
Do isopods bite?
No. Their mouthparts are built for chewing decaying plant matter and cannot meaningfully bite a person. They have no sting, no venom, and no defence beyond running away or rolling into a ball.
Are isopods harmful?
Not at all. They don't bite, sting, carry disease, damage buildings or eat living plants. Finding them indoors means somewhere is damp - they're a symptom of a moisture problem, never the cause of one.
How many legs does an isopod have?
Fourteen - seven pairs. It's the simplest way to tell them from insects, which have six.
Can you keep isopods as pets?
Yes, and thousands of people in the UK now do. They live in a plastic tub, most need no heating, they feed on leaf litter and vegetable scraps, and selective breeding has produced strikingly colourful varieties. They're one of the lowest-maintenance pets available. Browse our isopods for sale to see what's available.
How long do isopods live?
Most common species live two to three years, with some reaching four. A colony, though, is effectively immortal - animals breed and are replaced, so a well-kept culture keeps going indefinitely. Our guide to the life of an isopod covers the full cycle.
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