Geoscience Reference
In-Depth Information
Woodlice are the only discrete crustacean group to have become widespread in ter-
restrial habitats. Their closest relatives are all aquatic, and they are structurally less
well adapted to life on land than millipedes, spiders or insects. They resemble milli-
pedes in having a calcified exoskeleton which affords protection from enemies, but
this is not so well water-proofed; in the more primitive species, up to 80% of water
loss from the body takes place through the skin or cuticle. The smallest British species
Trichoniscus pygmaeus
is only 2.5mm in length and lives deep in soil or litter where
the humidity is at or near saturation. It loses water so quickly if kept in the open that it
only survives a few minutes. The newly hatched young of all species are particularly
vulnerable to desiccation and are carried within a brood pouch under the abdomen of
the female until they are ready to emerge and fend for themselves. As in their aquatic
relatives, breathing takes place through a series of gill-like respiratory plates called
pleopods formed by modified abdominal limbs. The genus
Porcellio
and its close re-
latives have a rudimentary system of branching tubes in the outer pleopods called
pleopodal lungs or pseudotracheae, by analogy with the more sophisticated breathing
apparatus of insects.
To supplement these physiological features, woodlice have a highly developed
behaviour pattern which causes them to seek out places having a suitably high humid-
ity. A few species, including
Cylisticus convexus
and the pillbugs
Armadillidium vul-
gare
and
A. nasatum
, can tolerate lower humidities and are found in dry habitats such
as chalk and limestone scree;
A. vulgare
is, indeed, quite commonly seen wandering
about on chalk turf in the sun. Both genera can roll up into a ball; the name of the lat-
ter suggests a likeness to the armour-plated armadillos of Argentina, though only the
three-banded armadillo can roll up into a perfect sphere (armadillo is a Spanish word
meaning, literally, little armoured thing).
Woodlice feed predominantly on dead plant material, and they can play a signi-
ficant part in the primary breakdown of leaf litter and decaying wood, especially in
deciduous woodland. The contribution is mainly in the mechanical comminution of
material since as little as ten percent of the food eaten may be assimilated. It seems
that digestion is dependent on some preliminary microbial attack having taken place.
Without this, a woodlouse surrounded by leaf litter may be in a comparable predica-
ment to the Ancient Mariner with “water, water every where nor any drop to drink”.
The absorption of copper, which is needed in particularly large amounts, cannot, in
fact, be absorbed adequately from fresh food. Woodlice therefore have the unusual
habit of eating their own faeces in which bacteria have converted the copper - and
possibly other essential salts - into a more digestible form. Rabbits likewise practise
coprophagy, and for very similar reasons, while ruminants have perfected the art of
double digestion internally.
