Mention the word crustacean and most people would think of shrimps, lobsters or crabs — all marine creatures. There are, however, terrestrial crustaceans and they are probably not very far away from you, hidden in some damp, dark corner of the garden. The common woodlouse, a creepy-crawly we are all familiar with, is a crustacean adapted for life on land. It adapted very successfully too, as woodlice first appear in the fossil record 50 million years ago.
Worldwide there are in the region of 3000 known species, whereas in the British Isles there are approximately 35 native species. Often there is little difference between species and an expert eye is needed to distinguish between them. Indeed, new species may be waiting to be discovered, so precise numbers cannot be given. Five distinct species of woodlouse are widespread in Britain and each may be described by the layman as the ‘common woodlouse’. Woodlice also go by many common names, examples being ‘sow bug’, ‘roly-poly’ and ‘slater’. The species that can roll up into a ball, and not all species can, are commonly known as ‘pill bugs’.
In common with other crustaceans, the woodlouse has a shell-like exoskeleton which must be shed, or molted, several times as the animal develops. Unlike the majority of crustaceans which shed their exoskeletons in a single process, the woodlouse first sheds the back-half, followed a couple of days later by the front. Though less distinct than the divisions of an insect body, the body of a woodlouse is split into three main regions. The head is small and the jaws and mouth are concealed underneath. It has two pairs of antennae, used to feel and smell its environment. Analogous to the thorax in insects, the woodlouse body is known as the pereion, and is covered in seven overlapping plates. Seven pairs of legs are attached to the pereion. The final section of the woodlouse is known as the pleon, and is analogous to the abdomen of insects. It is split into six segments although usually only four covering plates are obvious. Because of these covering plates, the head, pereion and pleon are often indistinct.
Woodlice breath through gills, which are to be found on their legs, and must always inhabit damp environments. Because of this and the fact that their skin is not fully waterproof, they can be found hiding away under logs or other dank corners to stop themselves from drying out. Usually only venturing out at night, they feed on dead and decaying matter and play an important role in recycling plant material. In the garden they will only feed on a plant if it has already been damaged by other animals or disease, are more benign than most people think, and are certainly not major pests. They will only enter houses where damp is a problem, modern central heating systems would soon dry out their bodies.
When ready to breed, female woodlice form a pouch under the pereion. This pouch is filled with fluid and it is here that the eggs are laid and hatch after a couple of weeks or so. The fluid drains away and young woodlice emerge from the pouch after a few days, small copies of the parent but with only six segments to their bodies. They soon molt, however, to reveal the normal seven segments of an adult. Like their marine relatives, woodlice need calcium to form their shells and so are more common in areas with lime rich soils. They really are fascinating creatures and an article as short as this cannot really do them justice.