The Isopod and Invertebrate Keeper's Glossary

The Isopod and Invertebrate Keeper's Glossary: Every Term Explained

 Every hobby has its jargon, and invertebrate keeping has more than most. Scroll through a care sheet or a keeper's forum and you'll hit a wall of terms - bioactive, isopod dairy, marsupium, morph, dimorphic, springtail, frass - that can leave a newcomer feeling like everyone's speaking a different language. None of it is complicated once explained, but nobody's born knowing it.

This glossary gathers the terms you'll actually encounter as an isopod, millipede or general invertebrate keeper, explained in plain English. Bookmark it, and refer back whenever a care sheet throws a word at you. We've grouped the terms loosely by theme to make browsing easier.

The animals themselves

Invertebrate - Any animal without a backbone. It's a vast group covering isopods, millipedes, insects, spiders, snails, worms and far more - the overwhelming majority of animal life on Earth. "Inverts" is the common shorthand.

Isopod - A type of crustacean, not an insect. The land-living species we keep are the same broad group as woodlice and pill bugs, and they're more closely related to crabs and shrimp than to beetles. Terrestrial isopods are the ones kept in the hobby.

Woodlouse / woodlice - The everyday British name for terrestrial isopods. "Woodlouse" and "isopod" are often used interchangeably, though "isopod" tends to be preferred in the hobby, especially for the exotic species. Some larger relatives are covered in our piece on the giant woodlouse.

Pill bug / roly-poly - Isopods that can roll into a complete ball as a defence (a behaviour called conglobation). Armadillidium species are the classic pill bugs. Not all isopods can do this.

Millipede - A many-legged detritivore (not an insect but a myriapod), with two pairs of legs per body segment. Slow, docile, and a keeper favourite. Despite the name, none actually has a thousand legs.

Springtail (Collembola) - Tiny arthropods, close relatives of insects, used throughout the hobby as a cleanup crew. Named for the furcula, a forked appendage that flicks them into the air.

Detritivore - An animal that eats decaying organic matter - dead leaves, rotting wood, waste. Isopods, millipedes and springtails are all detritivores, which is why their care revolves around a nutritious, decomposing substrate.

Crustacean - The group that includes crabs, lobsters, shrimp - and isopods. This ancestry is why isopods need calcium (for their calcified exoskeleton) and moisture (they breathe through gill-like structures).

Anatomy and biology

Exoskeleton - The hard external skeleton of an arthropod. It doesn't grow, so the animal must moult to get bigger.

Moulting (ecdysis) - Shedding the old exoskeleton to form a new, larger one. The most vulnerable time in an invertebrate's life, as the new exoskeleton is soft until it hardens. Isopods famously moult in two halves - back first, then front a few days later.

Marsupium - The fluid-filled brood pouch on the underside of a female isopod, where she carries her developing eggs and young. It's a crustacean trait, and it's why baby isopods emerge as tiny versions of the adults rather than as larvae.

Mancae (singular: manca) - Newly-released baby isopods. They look like miniature adults and are initially very small and pale.

Pleopodal lungs (pseudotrachea) - The gill-like breathing structures on an isopod's underside, which must stay moist to work - a legacy of the group's aquatic ancestry, and the reason humidity matters.

Furcula - The forked, spring-loaded appendage under a springtail that snaps down to fling it away from danger. The source of the name "springtail."

Gonopods - Modified legs on a male millipede (on the seventh body segment in many species) used in reproduction. Checking for them is the standard way to sex a millipede.

Frass - Invertebrate droppings. In a bioactive setup, frass is part of the nutrient cycle, and in many millipedes the young eat the adults' frass to acquire the gut microbes they need to digest food.

Conglobation - The technical term for an isopod rolling into a ball to protect itself.

Keeping terms

Bioactive - A self-sustaining enclosure where a cleanup crew (isopods, springtails) and microorganisms break down waste and maintain the environment, often supporting live plants, so it largely cleans itself. The foundation of modern invertebrate and reptile keeping.

Cleanup crew (CUC) - The isopods and springtails added to an enclosure to consume mould, waste and debris. The workforce of a bioactive setup.

Substrate - The material lining the bottom of the enclosure. For detritivores it's both home and primary food, so a nutritious, living substrate matters enormously. See our guide to substrate components for isopods.

Flake soil - A fermented hardwood substrate, rich and nutritious, prized for detritivores. Our flake soil article explains why it beats inert substrates.

Leaf litter - Decaying deciduous leaves used as food, cover and humidity buffer. Oak is the gold standard; see our leaf litter guide.

Isopod dairy - A charming hobby term for a small dish of protein food (like fish flakes) offered to isopods - so called because keepers "milk" the colony's growth and breeding from good protein feeding.

Springtail culture - A dedicated pot of springtails kept and bred to seed enclosures or feed other animals. Our culturing guide covers how.

Moisture gradient - A substrate kept damper at one end/depth and drier at the other, letting animals choose their preferred conditions. Central to good enclosure setup.

Ventilation / cross-ventilation - Airflow through the enclosure, ideally moving across it, to prevent stagnant, mouldy conditions. See our guide to ventilation and humidity.

Cork bark - Pieces of cork used as durable hides and surfaces in an enclosure. A hobby staple.

Biotope - An enclosure designed to recreate a specific natural habitat and its conditions. Our piece on what a biotope is explores the idea.

Breeding, genetics and the trade

Morph - A distinct colour or pattern variety within a species, often selectively bred and named. Many of the striking isopods in the hobby are morphs of otherwise ordinary species - the same animal, bred for a particular look.

Isopod culture / colony - A breeding group of isopods kept together. "Culture" and "colony" are used interchangeably.

Founder / starter culture - The initial group of animals you begin a colony from.

Dimorphic / sexual dimorphism - Where males and females look different. Useful for sexing some species.

Captive-bred (CB) - An animal bred in captivity rather than taken from the wild. Generally hardier, better-acclimated and more sustainable - and the basis of responsible keeping.

Wild-caught (WC) - An animal collected from the wild. Sometimes necessary for establishing new species in the hobby, but captive-bred is preferred where available.

Proving out - Establishing a newly-collected or newly-described species reliably in captivity, working out its care and breeding. Our article on proving out a new isopod genus shows what this involves.

Powder / dwarf isopods - Small, fast-breeding isopod species (like Trichorhina tomentosa, dwarf whites) often used purely as cleanup crew rather than display animals.

A living reference

Language in any hobby keeps evolving, and isopod keeping - still young and fast-growing - coins new terms all the time. If you come across something here we haven't covered, our live chat is always happy to translate, and it may well end up added to this list.

If you're just starting out, this glossary pairs well with our complete beginner's guide to keeping isopods in the UK and our first isopod enclosure setup guide - read those with this open in another tab and the jargon will quickly stop being jargon. And when you're ready to start a colony, you'll find everything you need across our collections and accessories.