Woodlice: Armoured Invertebrates
Since you are most likely to find woodlice in forests with chalky soils, it may come as a surprise to realise that these little animals are crustaceans – related to the crabs and lobsters inhabiting the sea. They are usually out and about at night when conditions are damp and cool.
Woodlice are recognisable by their seven pairs of legs, which are attached to the main part of the body. The head has eyes, a pair of antennae, and mouthparts for biting and chewing. At the rear end of the body is a short section with special limbs used in respiration and (in the male) for sperm transfer during mating.
Woodlice are isopods, which are a major group of crustaceans; in basic structure they resemble marine isopods such as the gribble, which bores into wood, and Idotea, which lives among seaweed. Marine isopods are found well back in the fossil record, but terrestrial species appear to be relatively recent (50 million years old!). It is thought that when they moved on to the land, they lived at first on the seashore, where a number of species still remain, including the sea slater.
Eggs in a pouch
Woodlice mate when the male mounts the female and transfers sperm using his modified rear limbs. By twisting his body first to one side and then the other. He places sperm in the genital openings on her underside. Once fertilised, the eggs are laid into a brood pouch filled with liquid. Forming a false floor under the main part of the body.
In this protected environment the embryos develop and then hatch, but the young remain in the pouch until they are well formed, resembling miniature adults. In this way woodlice protect the early developing stages of their young from the worst rigours of the terrestrial environment. The number of eggs produced varies between species, and is also related to adult size, ranging from 4-5 in the small species to 300 or more in large adults of the largest species. It takes between three months and two years for the young to reach maturity – depending on species and locality. Growth takes place during the warmer months of the year only, although woodlice remain active all year round. A few species have only one brood a year, but most manage a second brood in the autumn. Maximum life span is four years (in the common pill bug, Armadillidium vulgare) but usually it is no more than two years.
Woodlice eat decaying matter of all kinds, with leaf litter being the staple diet. This is eaten in large quantities, and woodlice in many habitats are key members of the community that breaks down dead plant material and return nutrients to the. Fungi, green algae growing on tree trunks, and carrion are consumed as well. Woodlice are unable to absorb all the essential nutrients from their food the first time it passes through their gut, so they eat their own faeces. The bacteria in these breaks down the food sufficiently during its second journey through the digestive system to release the nutrients.
Woodlice even eat each other. Such cannibalism is rare in the wild, but can be seen if woodlice are kept in captivity in large numbers. It is usually moulting or moribund animals which suffer. Woodlice which get intocan be a nuisance because they eat succulent young seedlings, particularly and .
A meal for others
Predators of woodlice were once thought to be few, because of the distasteful and sticky fluids that woodlice can discharge from glands along the sides and rear of the body. However, recent research has shown that they are taken in large numbers by centipedes, spiders and beetles; while little owls, toads and shrews also eat them readily. Any that die from other causes are rapidly scavenged by ants.
Woodlice, unlike insects, do not have a waterproof cuticle and their chief problem on land is water loss. They have largely overcome this by developing a set of behavioural responses that make them instinctively come to rest in damp, dark and confined places. For instance, they move away from light, and they move more slowly the greater the humidity. The intensity of these responses varies with the degree of water stress experienced, and is reversed if an excess of water builds up in the body. Furthermore, response varies on a 24-hour basis, to allow escape from shelter sites at night to find food and mates (conditions in the open at night are generally much more humid than in the day).
Where to find them
Woodlice occur almost everywhere in the British Isles and are usually more abundant in forests and ungrazed grassland where there is plenty of leaf litter and shelter. Densities of 500 per square metre are quite common. However, many woodlice require large amounts of calcium to make up the calcium carbonate which strengthens their hard outer surface (exoskeleton). So they are most abundant on calcareous soils. Most species are a good deal less common in the north than elsewhere because of the prevalence of acid soils which they do not like.