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Woodlice; Temporal range: Early Cretaceous–present, 113–0 Ma PreꞒ Ꞓ O S D C P T J K Pg N
	Clockwise from top right: Ligia oceanica, Hemilepistus reaumuri, Platyarthrus hoffmannseggii and Schizidium tiberianum
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Malacostraca
Order:	Isopoda
Suborder:	Oniscidea; Latreille 1802
Sections
	Diplocheta; Tylida; Microcheta; Synocheta; Crinocheta;

Woodlice or roly-polies, are terrestrial isopods in the suborder Oniscidea. Their name is derived from being often found in old wood,^[2] and from louse, a parasitic insect,^[3] although woodlice are neither parasitic nor insects.

Woodlice evolved from marine isopods which are presumed to have colonised land in the Carboniferous, though the oldest known fossils are from the Cretaceous period.^[4] This makes them quite unique among the crustaceans, being one of the few lineages to have transitioned into a fully terrestrial environment.

Woodlice have many common names and although often referred to as terrestrial isopods, some species live semiterrestrially or have recolonised aquatic 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 have a basic morphology of a segmented, dorso-ventrally flattened body with seven pairs of jointed legs, and specialised appendages 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 6 segments and gain an additional one after their first molt.

While the broader phylogeny of the Oniscideans has not been settled, eleven infraorders/sections are agreed on with 3,937 species validated in scientific literature in 2004^[5] and 3,710 species in 2014 out of an estimated total of 5,000–7,000 species extant worldwide.^[6] 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 4,725 m (15,500 ft) above sea-level, woodlice have established themselves in most terrestrial biomes and represent the full range of transitional forms and behaviours for living on land.

Woodlice are widely studied in the contexts of evolutionary biology, behavioural ecology and nutrient cycling. They are popular as terrarium pets because of their varied colour and texture forms, conglobating ability and ease of care.

Recent research has shown that the grouping as traditionally defined may not be monophyletic, with some taxa like Ligia and possibly Tylidae more closely related to other marine isopod groups, though the majority of woodlice probably do constitute a clade.^[7]^[8]

Common names

Common names for woodlice vary throughout the English-speaking world. A number of common names make reference to the fact that some species of woodlice can roll up into a ball. Other names compare the woodlouse to a pig. The collective noun is a quabble of woodlice.^[9]

Common names include:

armadillo bug^[10]
boat-builder (Newfoundland, Canada)^[11]
butcher boy or butchy boy (Australia,^[12] mostly around Melbourne)^[13]
carpenter or cafner (Newfoundland and Labrador, Canada)^[14]
cheeselog (Reading, England)^[15]
cheesy bobs (Guildford, England)^[16]
cheesy bug (North West Kent, Gravesend, England)^[17]
chiggy pig (Devon, England)^[18]^[19]
chisel pig^[20]
chucky pig (Devon, Gloucestershire, Herefordshire, England)^[21]
doodlebug (also used for the larva of an antlion^[22] and for the cockchafer)
gramersow (Cornwall, United Kingdom)^[23]
hog-louse^[24]
millipedus
mochyn coed ('tree pig'), pryf lludw ('ash bug'), granny grey in Wales^[25]
pill bug (usually applied only to the genus Armadillidium)^[26]
potato bug^[27]
roll up bug^[28]
roly-poly^[27]
slater (Scotland, Ulster, New Zealand and Australia)^[29]^[30]^[31]
sow bug^[32]
woodbunter
wood bug (British Columbia, Canada)^[33]
wood pig (mochyn coed, Welsh)^[34]

Description and life cycle

The woodlouse has a shell-like exoskeleton, which it must progressively shed as it grows. The moult takes place in two stages;^[35] the back half is lost first, followed two or three days later by the front. This method of moulting is different from that of most arthropods, which shed their cuticle in a single process. It is theorized that this allows woodlice to maintain partial mobility while molting.

A female woodlouse will keep fertilised eggs 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 offspring that look like small white woodlice curled up in balls, although initially without the last pair of legs.^[35] The mother then appears to "give birth" to her offspring. A few species are also capable of reproducing asexually.^[36]

Despite being crustaceans like lobsters or crabs, woodlice are said to have an unpleasant taste similar to "strong urine".^[37] This is due to their high concentration of uric acid,^[38] which is one of the chemicals in urine. Though other sources say that they taste like prawn, shrimp, or crawfish.^[39]

Pillbugs and pill millipedes

Comparison of the pill bug Armadillidium vulgare (left) and the pill millipede Glomeris marginata (right)

Pill bugs (woodlice of the family Armadillidiidae and Armadillidae) can be confused with pill millipedes of the order Glomerida.^[40] 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 defense 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, while 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 total. While the uropods of pillbugs are relatively quite small, flipping a pill bug over will reveal the small uropod overlapping the pleotelson.^[41] Some woodlouse species, like Armadillidium maculatum, seem to display Batesian Mimicry to certain pill millipedes like Glomeris marginata.

Ecology

Environmental extremes

Hemilepistus reaumuri lives in "the driest habitat conquered by any species of crustacean".^[42]

Ligia oceanica is aquatic.

Many members of Oniscidea live in terrestrial, non-aquatic environments, breathing through trachea-like lungs in their paddle-shaped hind legs (pleopods), called pleopodal lungs. Woodlice need moisture because they rapidly lose water by excretion and 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".^[42] They are usually nocturnal and are detritivores, feeding mostly on dead plant matter.

A few woodlice have returned to water. Evolutionary ancient species are amphibious, such as the marine-intertidal sea slater (Ligia oceanica), which belongs to family Ligiidae. Other examples include some Haloniscus species from Australia (family Scyphacidae), and in the northern hemisphere several species of Trichoniscidae and Thailandoniscus annae (family Styloniscidae). Species for which aquatic life is assumed include Typhlotricholigoides aquaticus (Mexico) and Cantabroniscus primitivus (Spain).^[43]

Woodlice are eaten by a wide range of insectivores, including spiders of the genus Dysdera, such as the woodlouse spider Dysdera crocata,^[32] and land planarians of the genus Luteostriata, such as Luteostriata abundans.^[44]

Woodlice are sensitive to agricultural pesticides, but can tolerate some toxic heavy metals, which they accumulate in the hepatopancreas. Thus they can be used as bioindicators of heavy metal pollution.^[45]

Evolutionary history

The oldest fossils of woodlice are known from the mid-Cretaceous around 100 million years ago, from amber deposits found in Spain, France and Myanmar, These include a specimen of living genus Ligia from the Charentese amber of France, the genus Myanmariscus from the Burmese amber of Myanmar, which belongs to the Synocheta and likely the Styloniscidae,^[46] Eoligiiscus tarraconensis which belongs to the family Ligiidae, Autrigoniscus resinicola which belongs to the family Trichoniscidae, and Heraclitus helenae which possibly belongs to Detonidae all from Spanish amber,^[47] and indeterminate specimens Charentese amber.^[4]^[46] The widespread distribution and diversification apparent of woodlice in the mid-Cretaceous implies that the origin of woodlice predates the breakup of Pangaea, likely during the Carboniferous.^[4]

As pests

Although woodlice, like earthworms, are generally considered beneficial in gardens for their role in controlling certain pests,^[48] producing compost and overturning the soil, some species like those of the genus Armadillidium have also been known to feed on cultivated plants, such as ripening strawberries and tender seedlings.^[49]

Woodlice can also invade homes in groups searching for moisture, and their presence can indicate dampness problems.^[50] They are not generally regarded as a serious household pest as they do not spread disease and do not damage sound wood or structures. They can be easily removed with the help of vacuum cleaners, chemical sprays, insect repellents, and insect killers,^[51] or by removing the dampness.

As pets

Woodlice have become a popular household pet for children as well as a hobby for invertebrate and insect enthusiasts or collectors.^[52] Porcellionidae (sowbugs) and Armadillididae (pillbugs) are seen often as they are the most common terrestrial isopods in Europe and North America.^[53]

While some isopod species are kept purely as pets, some can also be used as an addition to bioactive terrariums, due to their ability to break down decaying organic materials.

Morphs and species in the hobby

As isopods are bred in captivity, some hobbyists will discover a new mutation, or they will selectively breed isopods for a specific color/pattern expression. These populations with unique appearances are referred to as 'morphs'. Morphs are given nicknames, usually by the breeder who discovered/created the morph. The standard appearance of an isopod species is often referred to as 'Wild Type'.

Some isopod morphs are characterized by polygenic traits, such as 'Orange Vigor' (Armadillidium vulgare) and 'Pink Rubber Ducky' (Cubaris sp. "Rubber Ducky"), the result of selectively breeding isopods that best match the desired appearance. These genes can vary in their expression greatly, as they are not the result of a specific genetic mutation.^[54]

Other morphs are the result of dominant or recessive mutations, as seen with 'T+/T− Albino' and 'Whiteout' (Several spp.). As an example, T+ albino isopods are the result of an isopod being born without the ability to produce melanin, removing all black pigmentation. However, they are believed to be tyrosinase-positive (hence the T+), and therefore can still create some darker pigments such as brown and purple. T− albino isopods are thought to lack both melanin and tyrosinase, and therefore only express light yellows, oranges, and white.^[55]^[56]^[57]

Confusion can often arise due to the rate at which unidentified or undescribed isopod species are introduced to the hobby. This has contributed significantly to the genus Cubaris being considered a wastebasket taxon,^[58] as many of the unidentified or undescribed isopod species are incorrectly labelled as "Cubaris sp." even when they do not fit the formal description of the genus.

In the British Isles

Woodlice are the most species-rich group of terrestrial crustaceans.^[59] Of the 4,000 described species found worldwide,^[60] 35 species in 10 families are native to the British Isles. One of these species, Acaeroplastes melanurus, had been considered extinct in the British Isles but was rediscovered in 2002 at its only site (Howth, County Dublin, Ireland), and a further ten species have become naturalised in greenhouses, presumably transported with exotic plants.^[61] Five species are especially common throughout the British Isles, and are known as the "famous five species".^[62] They are Oniscus asellus (the common shiny woodlouse), Porcellio scaber (the common rough woodlouse), Philoscia muscorum (the common striped woodlouse), Trichoniscus pusillus (the common pygmy woodlouse) and Armadillidium vulgare (the common pill bug). One species, Metatrichoniscoides celticus, is endemic to Glamorgan, and is listed as a vulnerable species in the IUCN Red List.

Classification

There is general agreement that there are five main lineages in suborder Oniscidea, although the phylogenetic relationships between them are unsettled.^[63]^[64]^[5]^[65]^[6] Two main schemes for the classification that differ in which group is considered sister to the remaining oniscideans. One places Ligiidae in section Diplocheta, with the remaining families divided between four sections in infraorder Holoverticata.^[63]^[5] The other places Tylidae in infraorder Tylomorpha, with the remaining families placed in three sections in infraorder Ligiamorpha.^[64] The former scheme is presented below.