Isopoda - Woodlice, aquatic species, etc.

woodlouse undersidePorcellio scaber (above)

Woodlice overview

On the left and below are woodlice, also known as sow-bugs or slaters, Order Isopoda. In the Crustacea phylum they are the only order that contain species that are wholly terrestrial. Although there are also freshwater (see Ascellus) and marine species.

There are over 4 000 species of isopods in the world. Most are small, although there is a deep sea giant aptly named Bathynomus giganteus, which can reach 42 cm long and 15 cm wide! The 37 species of British and Irish woodlice are listed below, but there are a few introduced species which can breed indoors only. Most range in size from 5 - 15 mm. Most are grey or grey/brown in colour. In the terrestrial species the animal's whole life revolves around the avoidance of desiccation. Therefore it is active mainly during the night. It is thought that isopods colonized the land during the Carboniferous.

woodlouse, Isopoda, Oniscidea

Oniscus asellus, the common shiny woodlouse

Oniscus asellus (above left) is often found under logs and in compost heaps. It is shiny grey with lighter grey patches, and its eyes consist of a number of black ocelli. It can reach 15 mm long and 7.5 mm wide, and is probably the most common British woodlouse, and is found anywhere there are damp conditions, it is also common in north and western Europe, and is present in eastern Europe and north America. It remains motionless when disturbed, but can run quite fast.


Porcello spinicornis, woodlouse

Porcellio spinicornis

Left is Porcellio spinicornis, a particularly attractive woodlouse. Fully grown it reaches about 12 mm long. It has a black head, black eyes, a dark central stripe with a row of yellow blotches down either side. It can run quite fast, but usually stays still for a while when first disturbed. It is found in walls and buildings especially those with lime-rich mortar. It is very common in north east Scotland and in drystone walls in the Cotswolds, also northern France, northern Italy, Russia, Canada, and the USA.

Woodlice in medicine

In olden days woodlice were carried around in a small bag and used to treat stomach aches. As their exoskeleton is mainly calcium carbonate they may have been able to neutralize stomach acids and so treat ulcers, heartburn, and over indulgence in general.

Sub order Oniscidea, species native to Britain and Ireland

Family Species Common name and Notes
Ligiidae Ligia oceanica Common sea slater. Coastal only.
Ligiidae Ligidium hypnorum  
Trichoniscidae Androniscus dentiger Rosy woodlouse
Trichoniscidae Buddelundiella cataractae  
Trichoniscidae Haplophthalmus daniscus  
Trichoniscidae Haplophthalmus mengei  
Trichoniscidae Haplophthalmus montivagus  
Trichoniscidae Metatrichoniscoides celticus  
Trichoniscidae Metatrichoniscoides leydigi  
Trichoniscidae Miktoniscus patiencei Coastal only.
Trichoniscidae Oritoniscus flavus  
Trichoniscidae Trichoniscoides albidus  
Trichoniscidae Trichoniscoides helveticus  
Trichoniscidae Trichoniscoides saeroeensis Coastal only
Trichoniscidae Trichoniscoides sarsi  
Trichoniscidae Trichoniscus pusillus Common pigmy woodlouse
Trichoniscidae Trichoniscus pygmaeus  
Halophilosciidae Halophiloscia couchi Coastal only
Halophilosciidae Stenophiloscia zosterae a.k.a. Halophilosia zosterae
Oniscidae Oniscus asellus Common shiny woodlouse
Philosciidae Philoscia muscorum Common striped woodlouse
Platyarthridae Platyarthrus hoffmannseggi Ant woodlouse
Porcellionidae Porcellio dilatus  
Porcellionidae Porcellio laevis  
Porcellionidae Porcellio scaber Common rough woodlouse
Porcellionidae Porcellio spinicornis  
Porcellionidae Porcellionides cingendus a.k.a. Metoponorthus cingendus
Porcellionidae Porcellionides pruinosus a.k.a. Metoponorthus pruinosus
Armadillidiidae Armadillidium album Coastal only.
Armadillidiidae Armadillidium depressum  
Armadillidiidae Armadillidium nastatum  
Armadillidiidae Armadillidium pictum  
Armadillidiidae Armadillidium pulchellum  
Armadillidiidae Armadillidium vulgare Common pill woodlouse
Armadillidiidae Eluma purpurascens  
Cylisticidae Cylisticus convexus  
Trachelipidae Trachelpus rathkei  

On the right a diagram showing how to tell the difference between male and female woodlice.

Woodlice body pattern

At the top of the page you can see the underside of a terrestrial woodlouse, they have 7 pairs of walking legs. The first pair of antennae are usually short, and in terrestrial species they are often vestigial. The second pair of antennae are usually well developed, except in the parasitic species. To tell the difference between adult male and female woodlice see the diagram on the right. Woodlice are preyed upon by spiders, toads, and centipedes. Most woodlice have a series of glands running down the sides of their body which release a sticky, foul fluid when the animal is attacked. This helps it repel and escape from predators.

Woodlice life cycle

There is not much in the way of woodlice courtship; just the male tapping the female with his antennae. The female carries her eggs and young in a fluid-filled pouch on her underside, and it takes about two years for the young to reach maturity. On release from the female's brood pouch the young woodlouse has 6 pairs of legs. Within 24 hours it will moult, and the 7th segment which will bear legs appears. After the next moult it will have the full compliment of 7 pairs of legs.

Moulting is a process of two halves; the hind part is cast off a few days before the front part, as can be seen in the photograph of Porcellio scaber.

Very few woodlice live longer than 2 - 3 years, however in California, where the common pill bug Armadillidium vulgare (see below right) was introduced just over 100 years ago, it lives for 4 years, and females can have 3 or 4 broods.

Male and Female woodlouse

Philoscia muscorum, the Common striped woodlouse

On the right is Philoscia muscorum, the Common striped woodlouse, which is common and fairly widespread. It is 11 mm long when fully grown, and a shiny mottled brown with a darker strip running down the middle of its back. Yellow, red and greenish individuals have been reported, though. The eyes consist of numerous black ocelli. It can run fast, and is found in hedgerows, grassland and tussocky grass in woodland and gardens, particularly on raspberry plants.

Woodlice eating habits

UK woodlice (and most others) feed mainly on rotting vegetation, algae, bacteria, fungi and animal remains and so help to return valuable nutrients to the soil. Woodlice rarely eat living plants, so gardeners should not consider them pests. However they do take an occasional nibble of seedlings in greenhouses and cold frames. On the whole they do much more good than harm, and are especially useful in chewing up plant fragments in compost heaps, and their faeces aids decomposition. Although UK woodlice are vegetarian, there are a few others that are not. There is one (Scyphax ornatus) in New Zealand which lives on sandy beeches, and specializes in eating drowned honey bees. And in the USA woodlice are used in museums to clean the flesh off vertebrate skeletons.

Woodlice as pets

Woodlice are fairly easy to keep as pets at home or in the classroom. Plastic containers make the easiest housing. And all that is needed to keep them happy is a layer of soil, a piece or two of bark, and perhaps a stone and some dead leaves.

A small piece of carrot or other vegetable can be added once a week. Spray with water occasionally to keep conditions humid. It is important not to make conditions too wet or too dry. One of the ways to do this is to spray just the one part of the box. In fact if you have a large box and different species of woodlice this can be used to study the differences in species preferences.

Philoscia muscorum, common striped woodlouse

Armadillidium vulgare the pill bug

Armadillidium vulgare, the pill bug (right), can tolerate dry places, also its ability to roll up helps it conserve moisture. It can be found in grassy places, especially chalky and limestone areas, and is often confused with the pill millipede, but can be distinguished by its numerous small rear segments. When fully grown it can be up to 1.8 cm long. It is variable in colour, though most are light grey, but pink, black and yellow individuals have been seen. It is common in S E England and Ireland, throughout Europe, parts of Asia, North America, Australasia, South Africa and some Pacific and Atlantic islands. The pill bug was once used by doctors - that's where it gets its common name - patients had to swallow one whole!

As is the case in European woodlice, the eggs are laid and brooded in a pouch formed by the plates arising from the 2nd - 5th thoracic appendages. On hatching the young have just 6 pairs of legs, and are pale-coloured. They acquire the 7th pair of legs after their first moult, and by this time have left the brood pouch. Moulting occurs throughout their life, and usually takes place in crevices in the soil. They moult one half of the body at a time (see the Porcellio scaber photograph), with the rear half being moulted first. It can be as long as 2 weeks before the front half is moulted.

Armadillidium vulgare, the pill bug
Small logo (C) 1997 - 2016 contact - Cookie info.