Woodlice are an object of childhood fascination for those fortunate enough to encounter them. For the majority of adults, however, woodlice belong to that great sweep of invertebrate diversity that is simply ignored for having no obvious impact on our daily lives. Even some of my colleagues, with a background in biology, did not realise that forty different woodlice species have been recorded outdoors in Britain, with a further 12 introduced species maintained inside horticultural glasshouses. Four of the species found outdoors are ubiquitous and can be found in very large numbers across Britain. Quite simply, woodlice are the most successful group of crustaceans to have colonised the land.
Woodlice are an essential component in the process of decomposition and nutrient recycling, feeding on a variety of partly decomposed plant material and even the flesh of dead animals. You might think, therefore, that they are rather efficient in this role. In actual fact, they have a rather inefficient digestive system; much of the material that they need passes straight through the gut. Like Rabbits, woodlice eat their own faecal pellets, although in this case they rely on bacterial activity prior to reingestion to get at the nutrients still locked inside.
The successful colonisation of the terrestrial environment has been possible through a number of adaptations. The most notable of these is a fluid-filled brood pouch which protects the young from drying out during the first weeks after hatching from the egg. The problem of water loss shapes other woodlouse features and behaviours. Woodlice have external pleopodal lungs, visible as tiny white patches on the underside of the body, which remain moist and facilitate gaseous exchange between the air and the blood. Nitrogenous waste is excreted as ammonia gas, another water-saving adaptation, and it is this that is behind the smell of urine that can be noted where large numbers of woodlice are found together.
The need to reduce water loss is also the reason why woodlice tend to be found in damp environments, either under dead wood, large stones, within leaf litter or within the soil itself. The choice of these different microhabitats also shapes variation in the basic woodlouse body plan. Those most tolerant of ‘drier’ microhabitats tend to be the larger robust species familiar to any child that has turned over a stone or broken off a piece of dead wood. Some of these species can role into a ball if disturbed. Others, known as ‘clingers’, clamp themselves to the substrate, while others still (known as ‘runners’) have a surprising turn of speed. Soil-dwelling forms are smaller, with shorter legs and one of these lives within ant nests. All in all they are a diverse and fascinating group.