Illinois Natural History Survey - University of Illinois

Species Spotlight: Terrestrial Isopods

Underneath moist logs and decaying leaves live a multitude of creatures, several of which may be terrestrial isopods, commonly known as sowbugs, woodlice, pillbugs, and slaters. These creatures are elliptical in shape, have seven equal pairs of legs, and two pairs of antennae. Isopods are primitive crustaceans related to crayfish, crabs, and lobsters.

While most crustaceans must live in or very near water, the terrestrial isopods are the only large group of crustacea to become adapted for life on land. Like all members of the crustaceans, they breathe with gills, but these gills must stay moist to operate so the terrestrial isopods are condemned to a life in damp places, usually underneath something like decaying wood or leaves. While independent of the water, they must restrict their activities to times and places where humidity is relatively high, so they are nocturnal and seasonal (encountered more frequently in damp rather than dry weather). And to conserve water, they don't expel their waste through water-based urine, but directly into the air as ammonia.

A sowbug (Trachelipus rathkei) at left and a pillbug (Armadillidium vulgare) on right with inset showing pillbug

rolled up in defense posture.

To cope with the problems of terrestrial life, the terrestrial isopods have a thick, hard exoskeleton where each body segment is topped with an armorlike plate that overlaps the plate on the section beneath. When disturbed, many of these isopods can curl themselves into a ball; this protects the softer underparts and appendages from attack and desiccation. Pillbugs are the most successful at this activity, their common name referring to the fact that when curled up they resemble tiny pills.

Unable to return to the water to breed, female isopods must carry water around with them. Fertilized eggs are deposited in a water-filled marsupium or brood pouch, located beneath the thorax. When the eggs hatch, the young isopods, which resemble adults, must remain in the pouch until they can fend for themselves. They are soon liberated from the pouch, however, and after four or five molts they reach adulthood. Molting continues throughout their lives and is rather peculiar in the isopods. First, the posterior half of the skin is shed and then two or three days later the anterior half is shed, giving rise to bicolored individuals during the molting process.

The terrestrial isopods are omnivores and scavengers, feeding mainly on vegetable or animal matter. Their food is usually already dead and decaying in the leaf litter, in crevices, or under rocks, logs, and bark. They feed wherever plant and animal refuse and detritus accumulate and moisture is present.

Two common species that may be encountered in Illinois are Trachelipus rathkei, a sowbug, that is 5/8 of an inch long and smooth brown in color, and Armadillidium vulgare, a pillbug, which is the same size as the sowbug but dark gray or black with rows of distinct spots.

Susan Post, Center for Economic Entomology


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