|Clockwise from top right: Ligia oceanica, Hemilepistus reaumuri, Platyarthrus hoffmannseggii and Schizidium tiberianum|
The first woodlice were marine isopods which are presumed to have colonised land in the Carboniferous, though the oldest known fossils are from the Cretaceous period. They 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 as a defensive mechanism; 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, specialised appendages for respiration and like other peracarids, females 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.
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 and 3,710 species in 2014 out of an estimated total of 5,000–7,000 species extant worldwide. 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.
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.
Common names include:
- "Jomits" (Cloneganna)
- "armadillo bug"
- "billy baker" (South Somerset)
- Billy Button (Dorset)
- Granny grey (Wales)
- "boat-builder" (Newfoundland, Canada)
- "butcher boy" or "butchy boy" (Australia, mostly around Melbourne)
- Bunty Nathans (Western Australia)
- "carpenter" or "cafner" (Newfoundland and Labrador, Canada)
- "carpet shrimp" (Ryedale)
- "charlie pig" (Norfolk , England)
- "cheeselog" (Reading, England)
- "cheesy bobs" (Guildford, England)
- "cheesy bug" (North West Kent, Gravesend, England)
- "cheesy lou" (Suffolk)
- "cheesy papa" (Essex)
- "cheesey wig"
- "chiggy pig" (Devon, England)
- "chucky pig" (Devon, Gloucestershire, Herefordshire, England)
- "chuggy peg" (North Devon)
- "crawley baker" (Dorset)
- "daddy grampher" (North Somerset)
- "damp beetle" (North East England)
- "dandy postman" (Essex and East London)
- "doodlebug" (also used for the larva of an antlion)
- "gramersow" (Cornwall, England)
- "Mochyn Coed" (meaning "Tree Pig"), "Pryf lludw" (meaning "Ash fly"), "granny grey" in Wales
- "granny grunter" (Isle of Man)
- "hardback" (Humberside, England)
- "horton bug" (Deal, Kent, England)
- "humidity bug" (Ontario, Canada)
- "menace" (Plymouth, Devon)
- "monkey-peas" (Kent, England)
- "monk's louse" (transl. "munkelus", Norway)
- "parson's pig" (Isle of Man)
- "pea bug" or "peasie-bug" (Kent, England)
- "pennysow" (Pembrokeshire, Wales)
- "piggy wig"
- "pill bug" (usually applied only to the genus Armadillidium)
- "potato bug"
- "roll up bug"
- "rosary bug" (Turkey)
- "slater" (Scotland, Ulster, New Zealand and Australia)
- "saw bug" (Dingwall, Nova Scotia)
- "sour bug" (Cambridgeshire)
- "sow bug"
- "wood bug" (British Columbia, Canada)
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; 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 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. The mother then appears to "give birth" to her offspring. Females are also capable of reproducing asexually.
Pillbugs and pill millipedes
Pillbugs (woodlice of the family Armadillidiidae, also known as pill woodlice) 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, and they can both roll up into a ball. Pill millipedes and pillbugs appear superficially similar to the naked eye. This is an example of convergent evolution.
Pill millipedes can be distinguished from woodlice on the basis of having two pairs of legs per body segment instead of one pair like all isopods. Pill millipedes have 12 to 13 body segments and about 18 pairs of legs, whereas woodlice have 11 segments and only seven pairs of legs. In addition, pill millipedes are smoother, and resemble normal millipedes in overall colouring and the shape of the segments.
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". 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).
Woodlice are eaten by a wide range of insectivores, including spiders of the genus Dysdera, such as the woodlouse spider Dysdera crocata, and land planarians of the genus Luteostriata, such as Luteostriata abundans.
The oldest fossils of woodlice are known from the mid-Cretaceous around 100 million years ago, including 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, and unidentified specimens in Spanish and Charentese amber. The widespread distribution of woodlice in the mid-Cretaceous implies that the origin of woodlice predates the breakup of Pangaea, likely during the Carboniferous.
Although woodlice, like earthworms, are generally considered beneficial in gardens for their role in controlling certain pests, producing compost and overturning the soil, they have also been known to feed on cultivated plants, such as ripening strawberries and tender seedlings.
Woodlice can also invade homes en masse in search of moisture and their presence can indicate dampness problems. 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, or by removing the damp.
- Infraorder/Section Diplocheta
- Infraorder Holoverticata
- Section: Tylida
- Section: Microcheta
- Section: Synocheta
- Section: Crinocheta
- Invertebrate iridescent virus 31 – a species of virus hosted by woodlice
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