The Woodlice

(Crustacea, Isopoda, Oniscidea)

Though Woodlice are some of my favourite creatures I have not written this page, instead I am grateful and you are fortunate that Stephen Hopkin, who knows far more about woodlice than ever I will, has allowed me to use his introduction to "A Key to the Woodlice of Britain and Ireland" Aidgap Key No 204. Enjoy....

Introduction

Woodlice are among the most familiar of animals. They have featured prominently throughout history in recipes (Holt, 1885, included a recipe for 'woodlouse sauce'), in the poetry of Sir John Betjeman, the novels of Jean Paul Sartre, the paintings of Paul Klee (Chater, 1988) and have even provided the inspiration for a sermon (Thornton, 1989). In the past, woodlice were often carried in a small pouch attached to a cord around the neck. These were swallowed as a cure for stomach aches and other minor ailments. It is possible that the calcium carbonate in their exoskeletons is able to neutralise stomach acids although I confess I have not had the courage to test this theory!

 

 

The beneficial effects of woodlice far outweigh any damage that they do. However, woodlice have an ill-deserved reputation as pests, mainly because they wander into houses at night, usually to escape supersaturation with water during wet weather. Woodlice may travel considerable distances during these nocturnal rambles. I have found some specimens trapped inside a light shade of a bedroom on the first floor of a building. In the garden, woodlice rarely attack living plants, preferring to eat vegetation which has begun to decompose. In glasshouses, woodlice may occasionally nibble seedlings but they rarely do any significant economic damage. In the litter layer of deciduous woods, and on compost heaps, woodlice perform a vital role. They chew dead plants into small fragments and deposit these as faecal pellets which decompose rapidly. Woodlice may also graze fungal hyphae from leaves (Gunnarson, 1987) or their own faecal pellets (Hassall and Rushton, 1982; 1985) and this may form an important source of nutrients when the only food available is of poor quality. Deposition of faecal pellets containing fungal spores in deep moist leaf litter by woodlice may be important in stimulating decomposition in woodlands (Hassall et al., 1987). Thus, the feeding activities of woodlice speed up the decomposition process and help to return essential nutrients to the soil.

All British woodlice are predominantly vegetarian. However, there are some foreign species that have a more varied diet. In New Zealand, the main food of the sand beach woodlouse Scyphax ornatus is drowned honeybees (Quilter, 1987). In the U.S.A., woodlice are used by museum workers to clean the flesh from delicate vertebrate skeletons (Maiorana and Van Valen, 1985).

Woodlice are members of the class Crustacea, which also includes crabs, shrimps and lobsters. Most crustaceans are aquatic. Only two orders contain species which are able to live their whole life cycle away from water. These are the Isopoda, which includes the woodlice, and the Amphipoda. In the British Isles, only one species of amphipod, Arcitalitrus dorrieni, can live permanently on land. This was introduced to Britain from Australia and is now quite widespread in Cornwall and South Devon (Harding and Sutton, 1988). It has been discovered recently in several other areas of Britain including Kew Gardens.

The order Isopoda contains more than 10,000 species worldwide, most of which are marine. About 3,500 species of woodlice (Suborder Oniscidea) have been described but there are probably at least as many waiting to be discovered (Schmalfuss, personal communication), especially in tropical rain forests. Isopods evolved in the sea and are thought to have made the transition to land via the seashore. Species such as the common sea slater, Ligia oceanica, are still restricted to the coast. Successive species became increasingly resistant to the rigours of a terrestrial existence and probably colonised the land via moist leaf litter of pteridophytes (horsetails and ferns) during the Carboniferous (Piearce, 1989). Two key features 'pre-adapted' marine isopods to colonise the land. First, the dorso-ventral flattening of the body gave them an extremely stable 'squat' posture when walking. Second, retention of the young in a fluid-filled brood pouch protected them from drying out in the first few weeks after hatching from the egg. All isopods have seven pairs of legs which are usually of similar structure (hence the name iso-pod from the Greek isos meaning 'equal' and podes meaning feet).

Traditionally, woodlice have been regarded as poorly adapted to life on land. However, research conducted in recent years has shown that woodlice have evolved several sophisticated physiological and behavioural mechanisms to control water loss. These include the modification of the pleopods to form lungs and a capillary conducting system which allows nitrogenous waste to be excreted as ammonia gas, without significant loss of water (Hoese, 1984). Indeed the Dutch word for woodlice,'pissebed', is probably derived from the smell of urine that large aggregations of woodlice give off.

Woodlice have colonised some of the most extreme environments on earth including deserts in Israel and North Africa, and hypersaline pools in Australia via fully terrestrial forms (Blinn et al., 1989). The desert species Hemilepistus reaumuri occurs in burrows at densities of greater than IOO per square metre (Hoffmann, 1984). This remarkable woodlouse pairs for life and is able to relocate its burrow using the position of the sun in the sky. The juveniles help to maintain the burrow. Thus, this species is the nearest we have to true social behaviour (eusociality) in woodlice. Individual Hemilepistus are able to walk several kilometres in a few days, quite a feat in the rigorous desert environment.

The only animals known to prey exclusively on woodlice are spiders of the genus Dysdera. Many other animals will, however, eat them and toads can be reared to adulthood on a diet that consists entirely of woodlice. In terms of impact of predation on numbers, centipedes consume about 40",, of all woodlice that are eaten. The great majority of woodlice that fall victim to predators are immature (Sunderland and Sutton, 1980).

All woodlice have six pairs of legs when released from the brood pouch of the female. At this stage, they are called 'mancas'. After the first moult, which occurs within 24 hours of release, the 7th leg bearing segment appears and after another moult they gain their full complement of seven pairs of legs and are juveniles. Juveniles moult at regular intervals until they reach sexual maturity, usually within a year, when they become adults. The moulting process continues after maturity (although with less frequency) and involves shedding the exoskeleton in two parts. The posterior part (pleon and pereon segments 5,6 and 7: see Fig. 2) is shed a few days before the anterior part (head and pereon segments 1,2,3, and 4). Few woodlice live for more than two or three years. The life cycles of woodlice are quite variable between species, and different geographical areas. The common pill woodiouse, Armadillidium vulgare, for example, lives for 4 years in California (where it was introduced at the turn of the century) and females have 3 or 4 broods in a lifetime-, at Spurn Head in Yorkshire, however, females of the same species have only 2 broods whilst at Lakenheath in Suffolk they have only one (Sutton et al., 1984).

The common striped woodlouse, Philoscia muscorum, on dune grassland exhibits a clear example of 'year class splitting'. Individuals born in one season differentiate into two forms, one of which grows to maturity in one year, the other in two (Grundy and Sutton, 1989). Such flexibility is one of the reasons why woodlice are so successful in colonising disturbed environments.

Only 37 species of woodlice are known to breed out of doors in the British Isles (another ten or so have been recorded only in glasshouses). They range from 2 mm to 30 mm in length. Two species, Oniscus asellus and Porcellio scaber, are particularly common around buildings and are ideal animals with which to conduct simple laboratory experiments (a range of which were described by Sutton, 1972). There is even one species, Platyarthrus hoffmannseggi , which is nearly always found associated with ants.

Earlier keys to British woodlice have proved difficult for the beginner to use. Edney's (1954) Synopsis was a specialist's key which relied at many points on characters which were difficult to see with a hand lens, or required dissection. Also, some of the features that were used are not visible in specimens preserved in 70% alcohol. Sutton, Harding and Burn's key (included in Sutton, 1972) contained a number of clear and accurate line drawings, together with a series of beautiful colour paintings of a selection of species. This key was easier to use as it relied, as far as possible, on characters which could be seen with the naked eye, or a hand lens. Publication of Sutton, Harding and Burn's key stimulated an upsurge in interest in woodlice. There is now a need for a new key which incorporates up-to-date information on distribution and descriptions of the seven species discovered in Britain since 1972.

Recent discoveries on parasites (Colloff and Hopkin, 1986; 1987), the structure and function of the digestive system (Hames and Hopkin, 1989) and the accumulation of metal pollutants by woodlice (Hopkin, 1989a; Hopkin et al., 1986) have shown that there is much that remains to be discovered about the biology and distribution of these fascinating and, dare I say it, endearing creatures. It is hoped that publication of this key will encourage further interest in 'grammerzows', 'chiggy pigs' and 'cheeselogs'!

 

 


COLLECTION AND PRESERVATION OF WOODLICE

Woodlice occur in a wide range of habitats and can be searched for almost anywhere. The most productive sites are those which provide numerous crevices and microsites where the woodlice can hide. Thus, damp leaf litter, rotting wood, loose bark, piles of rocks in disused quarries, and 'mature' tips of household refuse and rubble are 'good' sites. Six species of woodlice are found only on the coast (under boulders, in rock crevices and under driftwood) and are much easier to find at night (with a torch) when they come out to feed. Harding and Sutton (1985) provide a detailed analysis of habitat preferences of British woodlice based on the results of the Isopod Survey Scheme.

Hand-searching among surface material ('grubbing about'), or even direct observation, will turn up many of the larger species of woodlice. In a Bristol garden, for example, I have observed large numbers of Armadillidium depressum walking over a brick wall in full sunlight on a hot July day with no apparent distress. However, some species are vulnerable to drying out, particularly the small soil-dwelling forms. These can be found only by digging into the soil, or by turning over deeply-embedded boulders. Remember to look very closely on the underside of such stones and in the cavities from which the boulders were dislodged (and to put the rocks back when you have finished!). Pitfall traps may be useful for sampling in areas from which it would otherwise be difficult to collect (e.g. shingle beaches) and sieving humus onto a tray may turn up some of the more inconspicuous soil species. Sutton (I972) and Chalmers and Parker (I989) contain detailed discussions of collection methods and techniques.

For the beginner, a hand lens of at least 10 x magnification is essential for the identification of specimens in the field. With experience, it is possible to recognise most of the larger species with the naked eye. Other useful equipment includes a strong penknife for levering bark from rotting wood (try not to remove more than IO% of this from each log) and a pair of weak 'storksbill' forceps. Smaller species can be picked up with a moistened paintbrush.

Identification of larger species of woodlice can be attempted with live specimens in the field by holding them between the first finger and thumb (woodlice do not bite or sting!), or restraining specimens in a suitable holding and viewing device (see JonesWalters, 1989, for examples). The smaller 'pygmy' woodlice (Family Trichoniscidae) are more delicate and can be examined with a hand lens while still attached to a piece of substratum such as a small lump of soil or fragment of bark. Most species will survive for several days in self-seal Polythene bags on which site details can be written with an indelible pen. Some leaf litter, or moss, from the collection site should be included and the bags should be kept cool and away from direct sunlight. Tiny specimens are best placed individually in small glass or plastic tubes with tightly fitting lids. Using such methods, live woodlice are safely transported back to the laboratory/classroom for more detailed observation. Alternatively, woodlice can be placed directly in preservative in the field. However, by doing this, many characters which are extremely helpful in identification (some colours, number of lungs, behaviour etc.) will be lost.

Woodlice cannot be set and pinned in the same way as insects. Because their cuticle is heavily calcified, woodlice become very brittle when dry. Therefore, they must be immersed permanently in a preservative such as 70% alcohol (i.e. 3 parts water to 7 parts alcohol or clear industrial methylated spirit). Specimens are kept ideally in glass tubes with tightly-fitting plastic tops. It is essential to include a label (inside the tube) on which is written, in pencil or indelible ink, the Name of the species, the Collector (even if it is you), the Locality, the Ordnance Survey Grid Reference and the Date on which the specimen was collected, Remember, someone may want to consult your specimen in 100 years time; accurate labels are very important.

Positive identification of a few of the small species of woodlice may require removal and examination of the pleopods and 7th legs of a male these must be preserved as a slide, it is best to check with biological suppliers as to current recommended chemicals since new products appear on the market all the time.

WOODLICE CULTURES

Some people may not regard woodlice as the most stimulating of pets, but most species breed readily in captivity and make an interesting addition to a classroom or laboratory display. Ideal containers are clear plastic sandwich boxes containing a thin layer of soil, a few pieces of bark and some leaf litter. The addition of a few shreds of grated carrot every week, and a light spray with water, will keep the cultures healthy. Care should be taken not to make conditions too damp since woodlice are just as easily killed by conditions being too moist as being too dry.

THE ISOPOD RECORDING SCHEME

In common with many other groups of animals, an active recording scheme exists for woodlice in Britain. This is coordinated by the Biological Records Centre (BRC) of the Natural Environment Research Council's Institute of Terrestrial Ecology at Monks Wood Experimental Station. The Isopod Survey Scheme was established in 1968 and, to date, more than 30,000 individual species records have been submitted. Harding and Sutton (1985) give a detailed account of the development of the Isopod Survey Scheme, together with habitat information and distribution maps for records received until August 1982. These maps still provide an accurate picture of the general distribution of most woodlice in Britain and Ireland. However, the known ranges of some species have been extended since 1982, probably as a result of increased collecting rather than a true ecological expansion. Maps of 14 species, including records to 1987, have been published by Hopkin (I987), and more detailed surveys of local areas have been reported for Reading (Hopkin, 1988), Cornwall (Jones, 1987) and the Isles of Scilly (Jones and Pratley, 1987). Maps of the distributions of Trachelipus rathkei and Armadillidium pictum, which include records subsequent to Harding and Sutton (I 985), have also appeared recently (Whitehead, 1988; Richardson, 1989). The notes on distribution in the individual descriptions of species, includedhereafter the key, give up-to-date information based on records received to August 1991.

New recorders are welcome to join the Isopod Survey Scheme which currently has about 150 members. Those registered receive a newsletter twice a year and are encouraged to participate in the annual field meeting held jointly with the British Myriapod Group. A journal 'Isopoda' is also published which contains a variety of articles on terrestrial and freshwater isopods and amphipods. Write in the first instance to BRC (address below) who will forward your enquiry to the current scheme organiser.

Isopod Survey Scheme, Biological Records Centre, Monks Wood Experimental Station, Abbots Ripton, Huntingdon, Cambs PE17 2LS.

For more information on keeping and breeding woodlice see my page on Woodlice as pets.

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Harvard Woodlice Page Loads of info!!
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