My young daughter laughed as the slate-gray sowbug tickled across her
hand.
This was a sowbug (probably Oniscus aselus), since it didn't
roll into a protective sphere as do the rounder pillbugs, or
"roly-polys" (Armadillidium vulgare). As a group,
sowbugs, pillbugs, and their some 3,500 relatives are often called
"woodlice."
Count the legs of a woodlouse and you'll know they're not insects or spiders—or
even millipedes. Juvenile and adult woodlice have seven pairs of
legs on their many-segmented bodies. (Insects have six pairs of
legs, spiders have four pairs.)
Woodlice are land-going crustaceans. As you might expect from
terrestrial relatives of crab and shrimp,
keeping moist is a high priority for woodlice.
Lacking the heavy, waxy, water-retaining surface of other land
arthropods, woodlice seek cool, dark, and damp locations. They take
up precious water several ways: by sipping water through the mouth,
through the food eaten, and by using the small tubes at the tail end to
draw water into the anus.
All animals (even marine crustaceans) require oxygen. Many
species of woodlice draw their oxygen from moist air using lung-like
structures on the underside, near the tail. Look for light-colored
spots, protected from drying by flaps.
Like other arthropods, from crabs to spiders, woodlice must molt to
grow. Adult woodlice molt about every two months, shedding the
exoskeleton on the back half of the body a few days before shedding the
front half, leaving the animal bi-colored for a short time. Why
don't we see more shed skins? The frail cast exoskeletons are often
eaten by their producer or by other woodlice.
Although such small animals may seem to live fast-paced lives, some
larger species of woodlice don't breed until they're two years old.
They're surprisingly maternal, too: Pale, newly hatched woodlice
spend the first couple of weeks of their lives tucked away in a special,
damp pouch on the underside of the mother.
Woodlice feed primarily on dead and decaying material they encounter as
they travel through leaf litter and other hidden places.
Occasionally, they turn to tender living plant material when their
populations get crowded. Woodlice process bits of dead plants and
animals into nutrients more readily used by the plants in your garden
ecosystem. In turn, woodlice are eaten by shrews, salamanders
and newts, frogs,
lizards, some spiders and small birds.
While Oregon has at least one native woodlouse (Ligidium gracile),
the ones most commonly found in our gardens and basements, such as the
sowbug and pillbug, are introduced from Europe.
If you've ever accidentally squashed a handful of woodlice, you may
have noticed the smell of ammonia. Woodlice don't process their
nitrogen-based waste into urea (which we excrete as urine), but pass the
gaseous ammonia off through their thin shells.
Woodlice don't bite and don't damage our buildings or
possessions. In fact, in their native Great Britain, our garden
woodlice are sometimes kept as pets.
These fascinating land-living crustaceans are much more fun to watch
than squash.
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Visit our pages on related topics:
Dungeness crab
spiders
salamanders
frogs
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Wavecrest Discoveries can craft your personal discovery of this
delightful part of our world by customizing one of our distinctive guided
excursions. Our walks, tours,
and special activities are wonderful ways to
explore this fascinating region—and are the
perfect entertainment for guests.
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