Woodlice are terrestrial isopods in the suborder Oniscidea. Their name is derived from being often found in old wood, and from louse, a parasitic insect, although woodlice are neither parasitic nor insects. Other common names include slater, sow bug, and wood pig, common names varying widely by region.
Woodlice are suggested to have colonised land during the late Paleozoic based on molecular clock analysis, though the oldest known fossils are from the mid-Cretaceous period around 100 million years ago.
Woodlice have many common names and although often referred to as terrestrial isopods, some species live in semiaquatic environments like those of the genus Ligia. Woodlice in the families Armadillidae, Armadillidiidae, Eubelidae, Tylidae and some other genera can roll up into a roughly spherical shape (conglobate) as a defensive mechanism or to conserve moisture; others have partial rolling ability, but most cannot conglobate at all.
Woodlice are primarily detritivorous, and are a major contributor to the humification process by breaking down litter via ingestion. By reingesting their own faeces (coprophagy) they gain more nutrients from poor-quality leaf litter and increase the rate at which plant debris are transformed in humus.
Woodlice have a basic morphology of a segmented, dorso-ventrally flattened body with seven pairs of jointed legs, and have some or all five pairs of their pleopods (swimming legs) adapted for respiration. Like other peracarids, female woodlice carry fertilised eggs in their marsupium, through which they provide developing embryos with water, oxygen and nutrients. The immature young hatch as mancae and receive further maternal care in some species. Juveniles then go through a series of moults before reaching maturity. Mancae are born with six pereon (thoracic) segments and gain an additional one after their first moult.
Key adaptations to terrestrial life have led to a highly diverse set of animals. From the marine littoral zone and subterranean lakes to arid deserts and desert slopes above sea-level, woodlice can be found in every continent other than Antarctica, in almost all kinds of terrestrial and coastal habitats, being absent only from polar regions and very high altitudes. Due to the permeability of their exoskeletons, woodlice are susceptible to water loss and can often be found beneath surfaces such as the underside of logs and rocks to avoid desiccation under sunlight.
Woodlice are widely studied in the contexts of evolutionary biology, behavioural ecology, and nutrient cycling. They are popular as pets often favoured because of their varied colour and texture forms, ease of care, or for the purpose of serving as a terrarium cleanup crew.
Biology
Description Most woodlouse are between 1.2-30 mm in length, their shape taking a generally flattened appearance. Their basic morphology can be divided into the head, pereon (thorax), and pleon (abdomen) sections. The head bears flagellate antennae. The pereon is constituted by seven segments of chitinous cuticle, each segment bears a pair of legs, making fourteen legs in total. The pleon is constitued by five segments, beneath each pleon is a pleopod which may bear pseudo-trachea through which the woodlouse breathes, the number of pseudo-trachea varying by species.
The woodlouse has a shell-like exoskeleton, which it must progressively shed as it grows. The moult takes place in two stages: the back half is lost first, followed a few days later by the front, so that moulting individuals have a curious appearance. This method of moulting is different from that of most arthropods, which shed their cuticle in a single process. It is postulated that maintaining partial mobility make woodlice less vulnerable to predators during ecdysis, and that biphasic moult favours calcium conservation in a terrestrial environment where calcium availability is limited compared to marine habitats.
Woodlice are susceptible to water loss, with evaporation occurring from the cuticle at a high rate, as a result they have developed morphological, physiological, and behavioural traits to avoid desiccation.
Woodlice are attracted to small narrow places such as crevices or the underside of rocks in order to minimise water loss, likewise they are repulsed by bright light to avoid desiccation under sunlight.
Antennae are used to help feel the environment to aid in navigation, and also allow the woodlouse to detect chemical odours which may identify predators; mechanoreceptors, hygroreceptors, and chemoreceptors on the antennae and cuticle enable the woodlouse to detect small changes in the texture of a surface they encounter as well as the presence of chemical odours.
They are usually nocturnal and are detritivores, feeding mostly on dead plant matter, preferring partially decayed or broken down material such as brown leaves. They break the plant material down by ingesting it, then later excrete it as faeces, and may return to eat the faeces in turn (coprophagy), once bacteria and fungi have further broken them down.
Pill bugs and pill millipedes
Pill bugs (woodlice of the families Armadillidiidae and Armadillidae) can be confused with Pill millipedes of the order Glomerida. Both of these groups of terrestrial segmented arthropods are about the same size. They live in very similar habitats, share a similar diet, and conglobate as a defence mechanism. Pill millipedes and pillbugs appear superficially similar to the naked eye. This is an example of convergent evolution.
These two groups can be distinguished in several ways. Glomeris millipedes have 19 (males) or 17 (females) pairs of legs as adults, while adult pill bugs only have 7 pairs of legs. Additionally, pill bugs have a thorax consisting of 7 body segments, 5 abdominal segments, and a pleotelson, while Glomeris millipedes lack a visually defined thorax and have 12 body segments in total. While the uropods of pillbugs are relatively quite small, flipping a pill bug over will reveal the small uropod overlapping the pleotelson. Some woodlouse species, like the harmless zebra isopod Armadillidium maculatum, seem to display Batesian mimicry to certain pill millipedes like Glomeris marginata which secrete chemicals noxious to predators.
Life cycle A female woodlouse will keep fertilised eggs (around 40 eggs, 400 to 600 µm in diameter, for Porcellio scaber) in a marsupium on the underside of her body, which covers the under surface of the thorax and is formed by overlapping plates attached to the bases of the first five pairs of legs. They hatch into mancae that look like small white woodlice curled up in balls, although initially without the last pair of legs. The mother then appears to "give birth" to her offspring. A few species are also capable of reproducing asexually by parthenogenesis, e.g. Trichoniscus pusillus.
Eggs, such as those of Porcellio scaber, typically develop for 1-2 months before hatching, with post embryonic woodlouse taking between 2-3 months to fully mature. The total lifespan of a woodlouse is often between 1-4 years.
Distribution
Woodlice can be found in every continent other than Antarctica, in almost all kinds of terrestrial and coastal habitats, being absent only from polar regions and very high altitudes. A poleward expansion of woodlice populations has been observed and attributed to global climate warming in Britain, like many other plant and animal taxa, but this postulate is still awaiting confirmation from dispersal studies, because several other mechanisms can interfere. Some species, like Hyloniscus riparius, are moving out of their original western European range, and are now invasive in a number of Russian ecosystems.
Woodlice are commonly found under stones or under the bark of decaying logs, in the moss at the base of trees, and they have been observed to climb on trees at night. Some species are commonly found on concrete and limestone walls and even on rooftops. By staying under rocks and logs during the day, they avoid desiccation in the sun, favouring to come out at night and during the morning or evening when the humidity is high enough.
Ancestral Oniscidea species are littoral, such as the marine-intertidal sea slater Ligia oceanica. They rely on gills (pleopodal endopods) for respiration and are thus bound to wetter environments. Other examples include some Haloniscus species from Australia (family Scyphacidae), and in the northern hemisphere several species of Trichoniscidae, e.g. Brackenridgia heroldi, and Thailandoniscus annae (family Styloniscidae). Species for which aquatic life is assumed include Typhlotricholigoides aquaticus (Mexico) and Cantabroniscus primitivus (Spain).
Many members of Oniscidea live in terrestrial, non-aquatic environments, breathing through pleopodal lungs (paddle-shaped posterior appendages) with pseudotrachea (branched air tubules). The pseudotrachea structure is well described in Porcellio scaber. Some Crinocheta members evolved to have spiracles that control airflow into their pleopodal lungs. Many woodlice still retain the gill-like endopod structures that continue to serve osmoregulatory functions. Woodlice need moisture because they rapidly lose water by excretion and evaporation through their cuticle, and so are usually found in damp, dark places, such as under rocks and logs, although one species, the desert-dwelling Hemilepistus reaumuri, inhabits "the driest habitat conquered by any species of crustacean". Pericyphis is another xerophillic isopod with unique pseudotrachea structure, extending from the pleopods into their body cavity.
Behaviour Kinesis and taxis
Much study has been done on the locomotion and stimulus response in woodlouse since Robert Hughes' seminal studies on the adaptive function of response alternation in Porcellio scaber. Woodlice move towards preferred (dark and humid) environments by a combination of directional movements (taxis) and random turning movements (kinesis). Simple experiments for classrooms demonstrate that woodlice are able to choose within ten minutes the dark side (against the light side) of a petri dish, negative photokinetic response to light being reinforced by thigmotaxis when individuals stop moving when close to each other, resulting in rapid aggregation in the favourable environment. Learning capacity was demonstrated in behavioural processes involved in spatial navigation. In an experiment, woodlouse exposed to 'rotational stress', by being rotated rapidly, exhibited stress behaviour such as a notable reduction in exploratory locomotion, however if the woodlouse had previously been exposed to the experiment they would exhibit these behaviours far less than they had after the initial exposure, a case of non-associative learning (habituation).
Aggregation Aggregation mediated by pheromones and environmental heterogeneity is commonplace in woodlice, much is still unknown about the nature of the pheromones used. The behaviour is thought to be the result of several adaptive functions. Individuals huddle closely together thereby minimising the gaps through which moisture can escape, reducing the chance of desiccation in low humidity. In winter the same behaviour may reduce heat loss from the body of individuals (kleptothermy). The close proximity of individuals of the same species also increases the rate of successful mating. Groups preferentially aggregate in locations with suitable conditions, and with food sources nearby. The information exchanged between individuals during aggregation is still awaiting to be documented.
Conglobation Some species of Woodlouse are capable of either partial or complete conglobation, which is the ability to roll up into a ball often referred to as volvation in animals. Families with members capable of either partial or complete conglobation include Armadillidae, Armadillidiidae, Eubelidae, and Tylidae.Western Australian Museum. 1991.15(1) 109·116 Fiona Lewis