As with the Millipedes and Centipedes, woodlice belong to the Phylum Arthropoda. Everyone can recognise a woodlouse, kids have poked them about for centuries and they are prevalant around the world, apart from the Arctic Circles and arid deserts.
A total of 42 species of woodlice have been recorded from the British Isles, but only about 29 of these can be considered native to these islands. Some of the others have been imported with plants and have become well established as Synanthropes (this is a name that means “with man”, that is, we provide the ideal living conditions in our house, greenhouse, cafe and kitchens etc.), but obviously there will be a restriction with distribution.
Many of our native species are confined to coastal habitats or to woodlands, and only six species can be regarded as garden animals. All woodlice are much more abundant on lime rich soils, than in other regions, as they need the lime for making their shells.
Woodlice are treated harshly by gardener and housewife alike and as soon as they appear they are squashed underfoot. In fact, in many areas it is considered to be unlucky to have a woodlouse in the house. They are in fact harmless creatures however, and certainly don’t deserve to be slaughtered whenever they are found.
They may occasionally nibble young seedlings, but they are generally more interested in dead leaves and decaying material and their presence in the garden is probably more beneficial than harmful. Those species that can roll up into a ball were once thought to have a medicinal value and were swallowed whole, alive, in attempts to cure digestive problems. They were also given to cattle and this is probably why they are called cud-worms in some areas.
Woodlice belong to the class of arthropods known as crustaceans, which is a predominantly aquatic group containing the crabs and lobsters. Although they now live on land the woodlice have not completely shaken off their aquatic habits.
Their skins are not completely waterproof, and the animals very soon desiccate in a dry environment. This is why almost all of them are confined to damp places and why they only come out to feed at night, when the air is cooler and damper. Many of the species can breathe only if their bodies are covered with a thin film of moisture, although they would soon drown if immersed in water.
A woodlouse’s body consists of three main regions, although these are far less distinct than the three regions of an insects body. The head is small and sunk back into the rest of the body to give a smooth, rounded outline. It carries two pairs of antennae or feelers, but only one pair is obvious. As in most other arthropods, the antennae are sensory organs which help the animal to feel and smell its way about. The tough jaws and the rest of the feeding apparatus are concealed under the head. The central and largest part of the body is called the pereion or thorax and it is roofed over with seven broad overlapping plates. It bears seven pairs of legs on the underside.
The hindmost region, which is not always distinct from the perion, is called the pleon or abdomen. It has six segments, but only four of the dorsal plates or shields are very obvious. The last plate, which is often triangular, is called the telson.
The first five pairs of legs on the pleon are quite unlike normal legs. Each leg consists of two leaf like flaps lying on top of each other. The inner flap is very thin and well supplied with blood. It acts as a gill, enabling the woodlouse to absorb oxygen from the air, but it will only work when surrounded by a thin film of water in which the oxygen can dissolve. Woodlice don’t just dry up in dry atomosphere: they suffocate as well. Some species are on the way to solving this problem however, through the development of minute breathing tubes in the outer flaps of some of the abdominal limbs. A tiny pore on the surface allows air to enter the tubes which spread through the limb to a greater or lesser extent. The walls of the tubes are always moist, and so oxygen can easily pass through the walls of the tubes and into the bloodstream. These tubes are called pseudotracheae and they show up as little white patches on the underside of the animal. Woodlice possessing them can certainly tolerate dry conditions far better than those species without them, but the animals still depend to a large extent on their gills.
The last pair of legs on the pleon are much more like legs than the others. They stick out from the hind end and they are called uropods. Each one is forked, with the outer branch usually much stouter than the inner one. The uropods probably act as sensory organs, analogous with the hind legs of some centipedes and they also secrete repellent fluids which protect the animals from some of their enemies. Repellent fluids are also secreted from most segments of the body.
Although protected to some extent by their repellent fluids, the woodlice are eaten by a wide range of other animals. Shrews, toads, ground beetles, centipedes and some spiders are among their most numerous predators.
The animals also suffer from attack by parasites of several kinds. Among the commonest are several kinds of blow-fly. Stimulated by scent, the female blow-fly seeks out the woodlouse haunts and lay their eggs there. The maggots which hatch from the eggs bore their way into the woodlice and remain there, feeding on the tissue of their hosts, (just what the Cluster Fly does to a worm). There is normally only one maggot in each woodlouse..pheww that’s enough isn’t it?..and by the time the maggot is fully grown the woodlouse is nothing more than a shell.
During the breeding season, which lasts for most of the summer, the females develop a brood pouch under the pereion. The pouch is formed by a number of overlapping plates which grow in from the sides of the body, and form a false floor to the body. The space between the body and this false floor is filled with liquid and the female lays her eggs into it. Studies indicate that the eggs of Porcellio scaber take about a month to hatch. The fluid then gradually disappears from the pouch and the little woodlice, about the size of a grain of rice, leave after a few days. They are very pale and only have six thoracic segments, but they soon moult to reveal seven.
It is important to remember that woodlice are not harmful in any way. Certain species will nibble house plants, but they pose no threat to humans. They may wander indoors to protect themselves from inclement weather or if there is a build up of vegetation around the home.
The best way to deal with them is to vacuum them up and spray a residual spray such as Dethlac or Insectrol around entry points to deter them. Blocking any obvious points of entry with a sealant such as decorators caulk is also advisable. Bear in mind though that a cluster of woodlice in one spot may be an indication of a damp problem that should be investigated.
Read more: How to get rid of woodlice