April 3rd, 2006


365 Urban Species. #093: Woodlouse

Urban species #093: Wood louse Oniscus asellus

First, let's address the name problem. We can call these animals woodlice or sow bugs if they can't roll into a defensive ball. If they can roll into a defensive ball, we can call them pill bugs. We can call all of them terrestrial isopod crustaceans, or even oniscideans, if we want to be scientifically correct, but in the interest of simplicity, I'm leaving behind the following common names: roley-poley/roly-poly, potato bug, armadillo bug, slater, ball bug, chuggy pig, butcher boy, carpenter, woodbug, hardy back, doodle bug, and cheeselog. You may collect yours when we're done, and use it as you please.

There are over 3000 known species of them, but there are just a handful that are urban. Pictured is the "shiny European sow bug," identified by its relatively large size (reaching just under 2 cm) and glossy dark-gray surface, as well as a host of more difficult-to-see features. The other common species are the "rough woodlouse,"Porcellio scaber, the soft matte gray colored one that I saw so much when I turned over logs as a boy, and Armadillidium vulgare, the common pill bug, which I have never seen in New England, though I am often assured that it is here. These three species are likely to have been originally natives of Europe, but are naturalized practically everywhere, and don't seem to be bothering anybody in their new homes. Most people rarely encounter these secretive animals, unless they deliberately seek them out, or have an especially moist home.

As the only large group of fully terrestrial crustaceans (relatives of crabs and lobsters rather than insects or millipedes), they need to keep their bodies wet to some degree or another. Their breathing apparatus allows water to be easily lost from their bodies, so they tend to stay where it is always damp. They are nocturnal, and will move away from a light that is shined on them.They eat vegetation, detritus, and fungi, and are generally considered to be beneficial soil-producing decomposers, though some gardeners complain of woodlice eating their plants. Found indoors they are a symptom, not a problem: some wood in your house is probably rotting.

Woodlice are being studied as living indicators of certain pollutants. Heavy metal contamination can be detected and quantified by examining its effects on these creatures. In my research I haven't answered one question I've had for a long time: When my brother's yard is sprayed with pesticides to control black widow spiders, how come the woodlice survive?