Order Isopoda – Woodlice

There are 37 species of native or naturalised woodlice in Britain and Ireland, and they are one of the most common and widespread of animals. Although familiar to gardens and households alike, they are mostly ignored. Belonging to the order Isopoda, they are quite a diverse group, and are members of the subphylum Crustacea (crustaceans), which also includes crabs, shrimps and lobsters.There are around 10,000 species of Isopoda worldwide, most of which are marine.

Woodlice are often considered as pests in gardens and greenhouses, and when they enter houses as they try to escape oversaturation in wet weather, or when they simply lose their way. They are completely harmless, and although they may nibble on the odd tender seedling, they play a very important role in recycling and the enrichment of soil in ecosystems, especially woodland. All British woodlice are predominantly vegetarian, and feed mainly on dead plant matter like leaf litter and rotting wood, which in digestion is turned into nutrients, and with the help of fungi helps speed up decomposition and fertilises the earth. Woodlice are also a good food source for birds, frogs, toads, shrews, spiders, centipedes and other predaceous invertebrates. 

All  juvenile and adult woodlice have seven pairs of legs which aid in distinguishing them from other arthropods, along with their characteristic flattened bodies. During spring, woodlice produce eggs which  are retained inside the female’s body until they hatch. The newly emergent woodlice, called mancas, are kept in a brood pouch on the underside of the female for a few days before they disperse to fend for themselves. As they develop, woodlice go through various stages of moulting up until and after they reach sexual maturity, usually shedding the exoskeleton in two parts. It usually takes a year for them to reach adulthood. They can live up to two or three years. 

Woodlice occur in a wide range of habitats, and can be found by searching through leaf litter, where rotting wood is present, under loose bark, flower pots and paving slabs, and amongst moss, rocks and stones. They tend to stay hidden in dark and damp places during the daytime, but will emerge after dark to forage for food.

Common Pygmy Woodlouse Trichoniscus pusillus agg.
Common Striped Woodlouse Philoscia muscorum
Common Striped Woodlouse Philoscia muscorum
Common Shiny Woodlouse Oniscus asellus ssp. asellus
Common Pill Woodlouse Armadillidium vulgare
Least Pygmy Woodlouse Trichoniscus pygmaeus
Least Pygmy Woodlouse Trichoniscus pygmaeus
Ant Woodlouse Platyarthrus hoffmannseggii
Common Rough Woodlouse Porcellio scaber
Common Rough Woodlouse Porcellio scaber

Many thanks to Warren Maguire and Steve Gregory for their expert assistance in identifying and reconfirming species.

For further reference see the links and literature below:

British Myriapod and Isopod Group (BMIG)
Facebook Isopods and Myriapods of Britain and Ireland Group
NBN Atlas

An AIDGAP publication A Key To The Woodlice of Britain And Ireland by Stephen Hopkin is also a very good reference guide.