Woodlice are terrestrialisopods in the suborderOniscidea. Their name is derived from being often found in old wood,[2] and fromlouse, a parasitic insect,[3] although woodlice are neither parasitic nor insects.
Woodlice evolved from marine isopods which are presumed to have colonised land in theCarboniferous, though the oldest known fossils are from theCretaceous period.[4] This makes them quite unique among thecrustaceans, being one of the few lineages to have transitioned into a fully terrestrial environment.
Woodlice have many common names and although often referred to asterrestrial isopods, some species livesemiterrestrially or have recolonised aquatic environments like those of the genusLigia. Woodlice in the familiesArmadillidae,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 basicmorphology of asegmented,dorso-ventrally flattened body with seven pairs ofjointed legs, and specialisedappendages forrespiration. Like otherperacarids, female woodlice carry fertilised eggs in theirmarsupium, through which they provide developingembryos with water, oxygen and nutrients. The immature young hatch asmancae and receive further maternal care in some species. Juveniles then go through a series ofmoults before reaching maturity. Mancae are born with six segments and gain an additional one after their first molt.
Whole woodlouse.
While the broaderphylogeny of the Oniscideans has not been settled, eleveninfraorders/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 speciesextant worldwide.[6] Key adaptations toterrestrial life have led to a highly diverse set of animals; from the marinelittoral zone andsubterranean lakes to arid deserts and desert slopes 4,725 m (15,500 ft) above sea-level, woodlice have established themselves in most terrestrialbiomes and represent the full range of transitional forms and behaviours for living on land.
Recent research has shown that the grouping as traditionally defined may not bemonophyletic, with some taxa likeLigia and possiblyTylidae more closely related to other marine isopod groups, though the majority of woodlice probably do constitute aclade.[7][8]
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]
The woodlouse has a shell-likeexoskeleton, which it must progressively shed as it grows. Themoult 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 keepfertilisedeggs in amarsupium 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 reproducingasexually.[36]
Despite beingcrustaceans like lobsters or crabs, woodlice are said to have an unpleasant taste similar to "strong urine".[36] This is due to their high concentration ofuric acid,[37] which is one of the chemicals in urine. Their flavor has also been compared toshellfish.[38]
Pill bugs (woodlice of the familyArmadillidiidae andArmadillidae) can be confused withpill millipedes of the orderGlomerida.[39] 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 ofconvergent 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 apleotelson, while Glomeris millipedes lack a visually defined thorax and have 12 body segments total. While theuropods of pillbugs are relatively quite small, flipping a pill bug over will reveal the small uropod overlapping the pleotelson.[40] Some woodlouse species, likeArmadillidium maculatum, seem to displayBatesian Mimicry to certain pill millipedes likeGlomeris marginata.
Many members of Oniscidea live in terrestrial, non-aquatic environments, breathing throughtrachea-like lungs in their paddle-shaped hind legs (pleopods), calledpleopodal lungs. Woodlice need moisture because they rapidly lose water byexcretion and through theircuticle, and so are usually found in damp, dark places, such as under rocks and logs, although one species, the desert-dwellingHemilepistus reaumuri, inhabits "the driest habitat conquered by any species of crustacean".[41] They are usuallynocturnal and aredetritivores, feeding mostly on dead plant matter.
Although woodlice, likeearthworms, are generally considered beneficial in gardens for their role in controlling certain pests,[47] producing compost and overturning the soil, some species like those of the genusArmadillidium have also been known to feed on cultivated plants, such as ripeningstrawberries and tender seedlings.[48]
Woodlice can also invade homes in groups searching for moisture, and their presence can indicate dampness problems.[49] 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,[50] or by removing the dampness.
Woodlice have become a popular household pet for children as well as a hobby for invertebrate and insect enthusiasts or collectors.[51]Porcellionidae (sowbugs) andArmadillididae (pillbugs) are seen often as they are the most common terrestrial isopods in Europe and North America.[52]
While some isopod species are kept purely as pets, some can also be used as an addition tobioactive terrariums, due to their ability to break down decaying organic materials.
As isopods are bred in captivity, some hobbyists will discover a newmutation, or they willselectively 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 bypolygenic 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.[53]
Other morphs are the result ofdominant 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 producemelanin, removing all blackpigmentation. However, they are believed to betyrosinase-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.[54][55][56]
Confusion can often arise due to the rate at which unidentified orundescribed isopod species are introduced to the hobby. This has contributed significantly to the genusCubaris being considered awastebasket taxon,[57] 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.
There is general agreement that there are five main lineages in suborder Oniscidea, although the phylogenetic relationships between them are unsettled.[62][63][5][64][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.[62][5] The other places Tylidae in infraorderTylomorpha, with the remaining families placed in three sections in infraorderLigiamorpha.[63] The former scheme is presented below.
^Paoletti, Maurizio G.; Hassall, Mark (1999). "Woodlice (Isopoda: Oniscidea): their potential for assessing sustainability and use as bioindicators".Agriculture, Ecosystems & Environment.74 (1–3):157–165.Bibcode:1999AgEE...74..157P.doi:10.1016/S0167-8809(99)00035-3.
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