As with the Millipedes and Centipedes, woodlice belong
to the Phylum Arthropoda and I don't think that there is anybody who
doesn't know what a woodlouse is...but boy!! do they have some colloquial
names, these include; bibble-bugs, sow-bugs, cud-worms, tiggy-hogs,
shoe-laces, sink-lice, slaters, and coffin cutters.
A total of 42 species of woodlice have been recorded
from the British Isles, as far as I know, but no more than 29 of these
can be considered native to these islands. Some of the others have been
imported with plants and have become well established as Synanthropes
(this is a name that means "with man", that is, we provide
the ideal living conditions in our house, greenhouse, cafe and kitchens
etc.), but obviously there will be a restriction with distribution.
Many of our native species are confined to coastal habitats or to woodlands,
and only six species can be regarded as garden animals. All woodlice
are much more abundant on lime rich soils, than in other regions, as
they need the lime for making their shells.
Woodlice are treated harshly by gardener and housewife
alike and as soon as they appear they are squashed underfoot. In fact,
in many areas it is considered to be unlucky to have a woodlouse in
the house. They are in fact harmless creatures however, and certainly
don't deserve to be slaughtered whenever they are found. They may occasionally
nibble young seedlings, but they are generally more interested in dead
leaves and decaying material and their presence in the garden is probably
more beneficial than harmful. Those species that can roll up into a
ball were once thought to have a medicinal value and were swallowed
whole, alive, in attempts to cure digestive problems. They were also
given to cattle and this is probably why they are called cud-worms in
some areas. The practice has stopped now....I think anyway...! but we
still refer to these animals as pill bugs.
Woodlice belong to the class of arthropods known as
crustaceans, which is a predominantly aquatic group containing the crabs
and lobsters. Although they now live on land the woodlice have not completely
shaken off their aquatic habits. Their skins are not completely waterproof,
and the animals very soon desiccate in a dry environment. This is why
almost all of them are confined to damp places and why they only come
out to feed at night, when the air is cooler and damper. Many of the
species can breathe only if their bodies are covered with a thin film
of moisture, although they would soon drown if immersed in water...too
much of a good thing.!
A woodlouses body consists of three main regions, although
these are far less distinct than the three regions of an insects body.
The head is small and sunk back into the rest of the body to give a
smooth, rounded outline. It carries two pairs of antennae or feelers,
but only one pair is obvious. As in most other arthropods, the antennae
are sensory organs which help the animal to feel and smell its way about.
The tough jaws and the rest of the feeding apparatus are concealed under
the head. The central and largest part of the body is called the pereion
or thorax and it is roofed over with seven broad overlapping plates.
It bears seven pairs of legs on the underside. The hindermost region,
which is not always distinct from the perion, is called the pleon
or abdomen. It has six segments, but only four of the dorsal plates
or shields are very obvious. The last plate, which is often triangular,
is called the telson.
The first five pairs of legs on the pleon are quite
unlike normal legs. Each leg consists of two leaf like flaps lying on
top of each other. The inner flap is very thin and well supplied with
blood. It acts as a gill, enabling the woodlouse to absorb oxygen from
the air, but it will only work when surrounded by a thin film of water
in which the oxygen can dissolve. Woodlice don't just dry up in dry
atomosphere: they suffocate as well. Some species are on the way to
solving this problem however, through the development of minute breathing
tubes in the outer flaps of some of the abdominal limbs. A tiny pore
on the surface allows air to enter the tubes which spread through the
limb to a greater or lesser extent. The walls of the tubes are always
moist, and so oxygen can easily pass through the walls of the tubes
and into the bloodstream. These tubes are called pseudotracheae
and they show up as little white patches on the underside of the animal.
Woodlice possessing them can certainly tolerate dry conditions far better
than those species without them, but the animals still depend to a large
extent on their gills.
The last pair of legs on the pleon are much more like
legs than the others. They stick out from the hind end and they are
called uropods. Each one is forked, with the outer branch
usually much stouter than the inner one. The uropods probably act as
sensory organs, analogous with the hind legs of some centipedes and
they also secrete repellent fluids which protect the animals from some
of their enemies. Repellent fluids are also secreted from most segments
of the body.
Although protected to some extent by their repellent
fluids, the woodlice are eaten by a wide range of other animals. Shrews,
toads, ground beetles, centipedes and some spiders are among their most
numerous predators. The animals also suffer from attack by parasites
of several kinds. Among the commonest are several kinds of blow-fly.
Stimulated by scent, the female blow-fly seeks out the woodlouse haunts
and lay their eggs there. The maggots which hatch from the eggs bore
their way into the woodlice and remain there, feeding on the tissue
of their hosts, (just what the Cluster Fly does
to a worm). There is normally only one maggot in each woodlouse..pheww
that's enough isn't it?..and by the time the maggot is fully grown the
woodlouse is nothing more than a shell.
During the breeding season, which lasts for most of
the summer, the females develop a brood pouch under the pereion. The
pouch is formed by a number of overlapping plates which grow in from
the sides of the body, and form a false floor to the body. The space
between the body and this false floor is filled with liquid and the
female lays her eggs into it. Studies indicate that the eggs of Porcellio
scaber take about a month to hatch. The fluid then gradually
disappears from the pouch and the little woodlice, about the size of
a grain of rice, leave after a few days. They are very pale and only
have six thoracic segments, but they soon moult to reveal seven.
Treatment is the same as for any ground
dwelling insect and is fully explained on the Millipede
page. But a lot of consideration should be given to Integrated Pest
Management (IPM) and there is a company called Russell IPM who manufacture
pheromones and green sorts of treatments, some of which involves just
plain old HOUSEKEEPING, anyway click on Russellipm
to go have a look.
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