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Dr. Maria Minor
Ecology INR
Massey University
P.B.11222
Palmerston North
New Zealand

+64-06-356-9099 ext.84833
M.A.Minor@massey.ac.nz


New Zealand Terrestrial & Freshwater Biodiversity Information System (TFBIS). Find out more...

Isopoda

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Slater (woodlouse), native beech forest, New Zealand.

Common name: slaters, pill bugs, sow bugs, woodlice, Maori papapa.

Scienific name: phylum Arthropoda, class Crustacea, Order Isopoda, Suborder Oniscoidea. From Greek “iso”, equal, and “poda”, legs.

Description

Slaters belong to the order Isopoda, which is a part of the arthropod class Crustacea. Most crustaceans – shrimp, crabs, lobsters, as well as many Isopoda – are marine dwellers, but slaters (suborder Oniscoidea) have become fully terrestrial. Terrestrial Isopoda vary slightly in appearance, but most are conspicuous and easily recognised by their elliptical, flattened segmented bodies, and seven pairs of legs.

Their colour is usually in the shades of grey, from dark to light, often mottled with green and yellow. Often there is a lot of variation in colour and pattern between individuals of the same species. The New Zealand slaters range in size from several millimetres to more than 2 cm, but most are 1.0-1.5 cm in length.


Slater rolled into a ball.

Slater bodies are heavily armoured with calcium carbonate and are quite rigid. The body is divided into three regions - head, thorax, and abdomen, but these are far less distinct than the head, thorax, and abdomen of insects. Overall, the body has a smooth, rounded outline. The head is small and covered with a single shield, sometimes with rounded lobes or processes on the front margin. The head appears to bear one pair of antennae, although there are actually two pairs. The first two antennae are very small, inconspicuous, and are thought to have a chemosensory function (smell and taste). The second antennae are large and prominent, and have a tactile function. On the sides of the head two eyes are visible; in some species the eyes are composed of either a single ocellus (simple eye), in other species the eyes are larger, and have many closely associated ocelli (compound eye). Occasionally (e.g., in cave dwelling species), the eyes are absent. Chewing mouthparts are located ventrally.

The largest part of the body is the thorax. It is comprised of seven segments and covered with seven broad overlapping dorsal plates (tergites), often with projections at the sides. On the underside, each segment bears a pair of walking legs. All legs are of similar size and structure, hence the name Isopoda. The smaller hindmost region, not always distinct, is the abdomen. The abdomen has 5 segments (although only four plates are visible), and ends with a triangular plate called the telson. Each abdominal segment also bears a pair of legs, which are flattened and modified for respiration. In male Isopoda the first pair of abdominal legs is modified into copulatory organs. The last plate, the telson, bears a pair of short stout legs called uropods, which resemble two short tails.


Porcellio scaber, an introduced species now common throughout New Zealand. Porcellio don't roll into a ball.

When disturbed, some terrestrial Isopoda – the "pillbugs" – will roll themselves into a ball, protecting the vulnerable ventral surface. You can predict the ability of a slater to roll into a ball by looking at its posterior body outline. If the prominent uropods are visible, this slater probably does not roll into a ball, while the ones with reduced uropods and smooth body outline do. The New Zealand pillbugs can be confused with pill millipedes (Diplopoda: Sphaerotheriida). Unlike Isopoda, pill millipedes have 2 pairs of legs on each body segment, and are typically shiny brown in colour.

Notes on biology

Slaters are mainly scavengers. They are omnivorous, feeding on a variety of decaying vegetation, tree bark, rotting wood, etc. Slaters may also graze on fungi associated with decay, or consume dead animal matter, such as dead insects or larger animal carcasses. Ant associates feed on ant droppings and fungi. Although slaters usually feed on dead and decaying vegetable matter and do not attack living plants, they can sometimes eat seedlings and root vegetables, or take advantage of fallen fruit previously damaged by birds or insects. Chemical control against isopods is not necessary – frequent removal of decaying plants and other hiding places will keep them away.

Many animals – small mammals, birds, centipedes, ground beetles – will eat terrestrial isopods. The spider Dysdera crocata, often found in New Zealand gardens, is said to prey on slaters almost exclusively. In defence, some slaters secrete a noxious fluid, which makes them distasteful and repels predators.

The isopod abdominal legs (pleopods) are modified to form thin, plate-like gills. To function, these gills need to remain covered with a thin film of water, which is why slaters prefer damp, sheltered environments. The slaters also lose water through the cuticle easily, and are very sensitive to desiccation. Some species have internal breathing structures called pseudotracheae (seen as small white patches on the ventral side), and are more tolerant of dryness.


New Zealand Isopoda, side view.

Most species reproduce sexually. Male and female slaters mate during warm times of the year; the males use modified abdominal legs to transfer sperm. The females carry the eggs in a fluid-filled brood pouch between the bases of the legs, which protects the eggs and the newborns from drying out. The eggs remain in the brood pouch for a few weeks, the young slaters hatch within the pouch and are released a few days later. The newly hatched slaters are soft, white, and have only 6 pairs of legs. They gain the 7th pair of legs after their first moult. Like all arthropods, slaters grow through a series of moults, expanding their body while the new cuticle is still soft. Juvenile slaters resemble adults, and moult at regular intervals until they reach sexual maturity, usually within 1-2 years. The adult slaters continue to moult, although less frequently. Slaters can live for 2-4 years, although most die as juveniles. The development time and frequency of breeding differ between species.

Terrestrial Isopoda play an important role in soil ecosystems. They shred dead and decaying organic matter, mix the litter, disperse fungal spores, and produce numerous faecal pellets. These activities enhance the decomposition process, and speed up the recycling of soil nutrients. Most slaters in New Zealand remain active throughout the year, but in cold climates they may burrow down into the leaf litter, soil or rotting wood to protect themselves from the frosts.

Where to find them?


Slaters (woodlice) in a native beech forest, New Zealand.

Terrestrial Isopoda are among the easiest soil animals to find and collect. They occur in a wide range of habitats, and are particularly abundant in lime-rich soils, as they need calcium to build their hard cuticle. Slaters can be found in dark, damp, sheltered places in almost any kind of environment. They spend the daylight hours congregated under shelters, sometimes in large numbers; at night they wander around in search of food, and even climb tree trunks. Look for them under logs, in leaf litter, in rotting wood, under loose tree bark, under garden debris, in compost heaps, under animal carcasses, under rocks and bricks. Once uncovered, slaters will try to escape the light, but they are not very fast runners. Slaters do not bite.

If dried, slaters become very brittle and will fall apart. If necessary, slaters can be preserved in 75% alcohol, although this will discolour them. Most species will survive for many days in a sealed container with soil, leaf litter, pieces of tree bark, or moss, placed in a cool dark area (for example, the veggie section of the refrigerator).

Distribution and conservation

About 3,500 species of terrestrial Isopoda have been described worldwide; many more species are probably still undiscovered. Slaters have colonised some of the most extreme environments, including Antarctica and Australian deserts. In New Zealand, about 36 species of terrestrial Isopoda are known. Slaters in New Zealand are common inhabitants of nearly all environments. Native species can be found in forested areas, grasslands, coastal dunes; several introduced species are inhabitants of disturbed areas and are very common in parks, gardens, and on farmlands. A pale coloured Trichoniscus commensalis lives in ant nests.

Slaters are easily transported with plants, soil, and garden materials. Several common European species are synanthropic (prefer human-modified habitats), and have become well established in gardens and farmlands of New Zealand. Two introduced species, Armadillidium vulgare and Porcellio scaber, are particularly common around New Zealand. European Oniscus asellus has been placed on the New Zealand list of invasive species as a regulated pest. Many of the native species are probably confined to protected native forests. Currently, there is no conservation program directed on terrestrial Isopoda in New Zealand.

Included images:

Family Porcellionidae
Porcellio scaber (Latreille, 1804)* - Rabbit Island, NN, South Island
Porcellio scaber (Latreille, 1804)* - Kaiteriteri Road, NN, South Island
Other
Unidentified species - Kaikawa scenic Reserve, ??, North Island (2 images)
Unidentified species - Ohinetonga Scenic Reserve, TO, North Island
Unidentified species - Rimu Valley Walk, SD, South Island (2 images)
Unidentified species - Nelson, NN, South Island (3 images)
Unidentified species - Blue Duck Reserve, KA/NC, South Island
Unidentified species - Kawhatau Base, RI, North Island
Unidentified species - Peel Forest, Dennistoun Bush, SC, South Island (3 images)
Unidentified species - Temple Valley, Lake Ohau, MK, South Island
Unidentified species - Tautuku Estuary, Caitlins, Southland, SL, South Island (3 images)
Unidentified species - Trounson Kauri Park Northland, ND, North Island (2 images)
Unidentified species - Palmerston North, WI, North Island (2 images)
Unidentified species - Palmerston North, WI, North Island (2 images)
* - species exotic in NZ

Further information on New Zealand terrestrial Isopoda:

Chilton, C. 1901. The terrestrial Isopoda of New Zealand. Transactions of the Linnean Society of London, 2 (Zoology) 8, p. 99-152.

Hopkin, S.P. 1991. A Key to the Woodlice of Britain and Ireland. 56 p.

Hurley, D.E. 1961. A Checklist and Key to the Crustacea Isopoda of New Zealand and the Subantarctic Islands. 34 pp.

Lewis, F. 1998. New genera and species of terrestrial Isopods (Crustacea: Oniscidea) from Australia. Journal of Natural History 32(5), p. 701-732.

Pugh, P.J.A., Dartnall, H.J.G, Mcinnes, S.J. 2002. The non-marine Crustacea of Antarctica and the Islands of the Southern Ocean: biodiversity and biogeography. Journal of Natural History 36(9), p. 1047-1103.

Isopoda Resources on the Web

Woodlice Online, maintained by Greg McKenzie, includes information on taxonomy, life cycle, habitat and behaviour of Porcellio scaber in New Zealand, as well as identification keys for common European and North American species, the list of New Zealand species, and many good links.

Gordon’s Woodlice page, by Gordon Ramel, contains facsinating facts on biology of terrestrial Isopoda, including instructions on how to keep Isopoda as pets.

The Weird and Wonderful World of Woodlice, by Julie Curl, information on their history, predators, physiology, and behaviour.

Checklist and Provisional Atlas of the Terrestrial Isopoda of the Netherlands, by Matty Berg, Vrije University, check-list, taxonomy, and distribution maps.

Arthropods of Hawaii - Images, includes links to images of terrestrial isopods of Hawaii on the Internet.

Isopoda: Slaters, CSIRO, short description of morphology, life cycle, and ecology of slaters. Includes dictionary of terms.

The Tree of Life: Isopoda, description of phylogenetic relationships within Isopoda, their morphology and ecology; mostly dedicated to marine species.

Selected Literature on Terrestrial Isopods, by Martin Zimmer

Other Isopod Sites of Interest, compiled by Buz Wilson, links to various Internet sites on marine and terrestrial Isopoda.

Browse our
IMAGE GALLERY:

NZ Slaters

Links and Resources:

Woodlice Online

Gordon’s Woodlice page

The Weird and Wonderful World of Woodlice

Checklist and Provisional Atlas of the Terrestrial Isopoda of the Netherlands

Arthropods of Hawaii - Images

Isopoda: Slaters

The Tree of Life: Isopoda

Selected Literature on Terrestrial Isopods

Other Isopod Sites of Interest

 

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