Learning about the roly-poly
By PAUL G. JANTZEN
Contributing writer
One of the decomposers in the prairie food chain is the pillbug, often called roly-poly because of its habit of rolling into a nearly perfect ball when disturbed. The gray adults are about 14 millimeters (0.6 inches) long and covered with an outer skeleton reminding one of an armadillo. The common prairie species is even named Armadillidium vulgare. Each of its seven thoracic segments bears a pair of walking legs. It lives under rocks, logs, and other damp places.
Actually, the pillbug is not a bug — not even an insect. It is a terrestrial crustacean, a close relative of crabs, crayfishes, shrimp, waterfleas, and barnacles — mostly water creatures. It is sometimes called sowbug or woodlouse.
With its chewing mouth parts, the pillbug eats mostly dead plant matter. It also eats fungi, plant seedlings, spider eggs, fleshy fruits (usually not a serious problem), and ant droppings. In fact, about nine percent of its diet is animal wastes. Pillbugs even eat their own wastes providing a more thorough digestion of food than only one trip through its primitive digestive tract provides. The tract is a simple tube from mouth to anus with two pairs of digestive glands near the mouth. Microorganisms also aid their digestion.
Predators of pillbugs include spiders, birds, amphibians, mites, centipedes, and, during very moist conditions, molds. Some of their potential predators are repulsed by the bitter secretion produced by glands along the upper margins of their thoracic segments. But more important to their protection is the pillbug's preference for dark burrows in protected areas that keeps them hidden much of the time.
Pillbugs are common in grasslands but in some ways seem unfit for prairie life. They must interact with extremes of temperature, humidity, and evaporation rates. They lack waterproof, waxy covering of grasshoppers and therefore lose moisture through their body surface.
Pillbugs have a primitive respiratory system. About 10 percent of their oxygen is absorbed directly through the body wall. Behind the thoracic segments are six smaller abdominal segments. Attached underneath the first two of these segments are two pairs of tracheal lungs. Each of these has an opening that allows air to enter a branched tube which is bathed in blood which distributes oxygen to all of the animal's tissues. Abdominal motion encourages air to flow into the branched tube.
But these simple air passages cannot be regulated, so up to 42 percent of the pillbug's water loss is from this network of air spaces.
A pillbug's waste nitrogen is excreted as ammonia which requires water for its removal. Pillbugs also lose water with their own solid wastes.
Most methods by which pillbugs keep moisture loss to a minimum are behavioral. They select food somewhat according to water needs. They tend to spend time in cool, shaded places. They are most active during nighttime hours although they may spend mornings in the sunshine, returning to cover after noon. They are likely to keep the underside of their bodies flat to the ground.
Because of their attraction to each other's odor as they get dry, pillbugs tend to bunch together which loosely confines the humid air. And the ability to roll up like a pill reduces water loss from respiratory structures and the whole underside. They tend to crawl away from lighted, hot, dry spaces toward desired environments.
Pillbugs are able to take up water through grooves that run along the bases of their legs forward to the mouth.
Because pillbugs are plentiful and easy to catch, they are convenient subjects to study. Even with a simple magnifier one can see the structures mentioned above plus a pair of eyes, a pair of large antennae, and a pair of tiny antennules behind them.
The structures near the rear underside of a pillbug are white because of trapped air. Placing a drop of water on these structures may result in a bubble marking the openings of one or more air sacks.
You can conduct experiments to determine pillbugs' reactions to light and darkness, moisture and dryness, differences in temperature and humidity, and food preferences.
How do these reactions help pillbugs survive? What role do decomposers play in the natural community?
Raising pillbugs is easy. Up to 10 of them can be placed in a petri dish with damp filter paper on the bottom. Then add dead leaves and small twigs before placing the lid on the dish. How long does it take young ones to reach mature size?
A larger population of pillbugs can be raised in an aquarium with an inch of top soil or sand. For food, add a small piece of a fleshy fruit or vegetable such as a carrot. Include powdered chalk as a source of calcium to maintain their outside skeletons. Keep a glass cover on the aquarium to maintain humidity. If the soil dries, spray the soil or sand lightly with water. The food should be replaced when it begins to mold. Too much moisture encourages the growth of molds.
