Woodlice 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 enviroment.
Woodlice have many common names and although often referred to as terrestrial isopods, some species live
semiterrestrially or have recolonised aquatic environments. 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.
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]
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.
A female woodlouse will keep
fertilisedeggs 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.
Pill bugs (woodlice of the family
Armadillidiidae and
Armadillidae) can be confused with
pill millipedes of the order
Glomerida.[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 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.[40]
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".[41] They are usually
nocturnal and are
detritivores, feeding mostly on dead plant matter.
Although woodlice, like
earthworms, 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 genus
Armadillidium have also been known to feed on cultivated plants, such as ripening
strawberries and tender seedlings.[48]
Woodlice can also invade homes en masse in search of moisture and their presence can indicate dampness problems.[49] They are not generally regarded as a serious household pest as they do not spread malady 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 damp.
As pets
Woodlice have become a popular household pet for children as well as a hobby for invertebrate and insect enthusiasts or collectors.[51]Porcellionidae (sowbugs) and
Armadillididae(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 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.[53]
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.[54][55][56]
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,[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 infraorder
Tylomorpha, with the remaining families placed in three sections in infraorder
Ligiamorpha.[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.