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 below) and marine species. There are over 7 500 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! 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 colonised the land during the Carboniferous.

On the left 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.

The female carries her eggs and young in a fluid-filled pouch beneath her (see Ascellus below left), and it takes about two years for the young to reach maturity.

On the left and below is the freshwater louse or hog louse, Ascellus sp. The drawing on the left shows the underside of a female with eggs (just one set of legs are drawn). it can grow to 2.5 cm long. It is grey-brown, but the colour varies. The female lays eggs in April or May, and carries them around in her marsupium (pouch). Even after hatching the young remain in the pouch for a while.