When I picked up a potted plant sitting on our back-deck picnic table after last week’s rain, a huge crowd of woodlice scattered for safety.
There were a few big ones in the mass — well, “big” as in more than a centimeter (about a half-inch) — but most were small to very tiny. Big and little, they quickly scrambled over each other and disappeared between the boards of the rain-soaked table.
A quick grab and I popped one of the larger animals into a clear container to examine.
These weren’t pillbugs, or “roly-polys,” as we called them when I was a child; pillbugs (Armadillidium vulgare) are rounder in cross-section and can roll up in a protective sphere when threatened. These were the flatter, non-rolling sowbugs.
Inside, with a bright light and a hand lens, and after some online searching, I guessed these were probably “common” or “garden” woodlice (Oniscus asellus). Besides a pillbug, the other common local possibility was Ligidium, but the small animal crawling around my clear container had the markings, shorter paired prongs at the end, and slightly flat edge to its sides that better fit the common woodlouse.
Although such crawlies may seem easy to overlook, there are a lot of them: over 4,000 species of “woodlice” — sowbugs, pillbugs, rock slaters, and others — call North America home.
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.
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, which look like miniature adults, spend the first couple of weeks of their lives tucked away in a special, damp pouch on the underside of the mother. The small woodlice under the plant on my deck table were clearly juveniles.
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, newts, frogs, lizards, some spiders and small birds.
While we have at many native woodlice (Ligidium gracile being the most common), the ones most commonly found in our gardens and basements — and under our plant pots — such as the sowbug and pillbug noted here, were 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. And since these detritus-eaters probably won’t do too much damage to the potted plants while they’re adding to my yard ecology, they can continue to enjoy my hospitality.