Tales from the Undergrowth: Roly-polies
What is one of the first “creepy crawlies” that you remember from childhood? Roly-polies, also known as woodlice in Europe, are probably at the top of the list for many of us. But what is a roly-poly? Is it an insect or is it something else? Entomologist Dr. Art Evans and VPM radio producer Steve Clark explore the hidden lives of roly-polies and reveal how these moisture-loving animals survive on land.
Roly-polies, also known as woodlice in Europe, are omnivorous arthropods that are commonly found in gardens, parks, and schoolyards. Like all arthropods, roly-polies, also known as pillbugs and sowbugs, are protected by an external skeleton, or exoskeleton. They have neither six legs nor eight, and are neither insects or arachnids. Instead, roly-polies have fourteen legs and are classified as Crustacea, a subphylum that also includes barnacles, crabs, lobsters, and crayfish. Roly-polies belong to the order Isopoda, a name that means “equal foot,” in reference to the similar size and shape of all fourteen of their legs. Isopods are mostly marine, while a few species live in freshwater or on land.
In spite of their decidedly terrestrial existence, roly-polies must maintain a thin film of moisture on respiratory appendages called pleopods. They conserve moisture by coming out only at night when the humidity is relatively high. Their days are spent hiding in dark, moist places. Lacking a protective coating of wax on their exoskeletons that simultaneously protects them from dehydration and drowning, woodlice are forever walking an ecological tightrope to maintain proper levels of moisture. To reproduce, females carry fertilized eggs in a fluid-filled sack called the marsupium. After hatching, the young woodlice remain within the marsupium until they are strong enough to venture out on their own.
Pillbugs (written as one word, not two) are capable of rolling up defensively into a ball, a behavior known as conglobation. Their close cousins sowbugs, also known as potatobugs, can only curl their bodies slightly. Occasionally, pillbugs and sowbugs carry a lethal isopod iridovirus. Infected individuals appear iridescent blue or bluish-purple as a result of light reflecting off the crystalline-like arrangement of the viral bodies multiplying in their tissues. This virus infects roly-polies only and is absolutely harmless to people and pets.
Roly-polies are fascinating animals. There are about 100 species of roly-polies in the United States and Canada. The two most common species living in cities and suburban neighborhoods, Armadillidium vulgare and Porcellio scaber, were most likely introduced from Europe. Should they ever become a nuisance, simply eliminate the constantly damp environments that attract them. But why get rid of them? Why not simply enjoy them and marvel at how they have adapted from a life at sea to the moist environs of your garden!
Here is a story about another interesting crustacean that lives inside the shell of oysters.