Pillbugs, Slaters, and Woodlice: Isopoda

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Isopods come in a variety of shapes and sizes. Most species have long bodies and are somewhat flat from top to bottom. They are mostly small, ranging from 0.02 to 0.6 inches (0.5 to 15 millimeters) in length. However, the largest species, Bathynomus giganteus, is an ocean bottom-dwelling creature that measures up to 19.7 inches (500 millimeters).

All isopods have three major body regions: head, thorax, and abdomen. Both pairs of antennae are unbranched, or uniramous (YU-neh-RAY-mus). The first pair is usually small, while the second is long and well developed. Unlike most crustaceans, the compound eyes, if present, are not set on stalks. Each compound eye has multiple lenses. The jaws, or mandibles, are uniramous and quite variable. Depending on the species, they are used to grind plant tissues, slice flesh, or pierce the tissues of living animals.

The first thoracic segment is tightly joined, or fused, to the head. Its uniramous limbs are called maxillipeds (mack-SIH-leh-pehds). Maxillipeds are thoracic limbs that work together with the mouthparts. Isopods do not have a shieldlike carapace that covers the head and thorax. Most species have seven pairs of walking legs called pereopods (PAIR-ee-oh-pawds). The pereopods are usually short, but in some species they are long and spiderlike. Plates on the underside of the thorax form a brood pouch, or marsupium (mar-SOUP-ee-uhm). The marsupium is where the eggs are kept and young hatch.

The six-segmented abdomen of terrestrial (te-REH-stree-uhl) species, or isopods that live on land, have two or more pairs of appendages called pleopods (PLEE-oh-pawds). The pleopods are white, egg-shaped, and are used for breathing. These species also take oxygen into their bodies directly through their external skeleton, or exoskeleton. The abdomen of all species ends with a pair of biramous (BY-ray-mus), or branched, uropods (YUR-oh-pawds), or tail appendages. Between the uropods is a taillike segment called the telson. The telson is tightly joined, or fused, to one or more abdominal segments.

Parasitic isopods, species that spend most or all of their lives feeding and living on fish and other crustaceans in the ocean, look different from other species. The females have wide thoracic segments, while segments that make up the abdomen tend to be much narrower. Males have body segments of similar width and are more egg-shaped in outline, similar to pillbugs. Both pairs of antennae of parasitic isopods are usually very small.


Isopods are found worldwide.


Terrestrial isopods called pillbugs and woodlice are found in moist situations in a variety of habitats, including deserts. A few blind species live in ant nests. They are usually found under rocks, logs, tree barks, or other objects. Slaters are found along rocky seashores splashed by ocean waves. They also spend time in the water. Others prefer shallow waters associated with oceans, such as bays and estuaries (EHS-chew-AIR-eez). However, most species live in the ocean. There, parasitic species feed on the blood of fish and other crustaceans, such as barnacles, crabs, and shrimps.


Isopods eat plant and animal tissues. Terrestrial species search the forest floor for living and dead plants and fungi, as well as dead bodies and animal waste. At least 10% of their diet is their own waste, which they need to replenish vital nutrients to maintain normal growth rates.

Species that live in the ocean eat algae (AL-jee) and other bits of vegetation, including wood. Some isopods scavenge dead fish and other animals on the ocean bottom. Others are parasites, either all their lives, or only during their early developmental stages.


Terrestrial isopods usually come out at night to feed. During the heat of the day, they instinctively hide in dark places beneath rocks, logs, and leaf litter where there is life-sustaining moisture. Under hot, dry conditions they release odors that are attractive to other isopods of the same species. They pile on one another as a defense of against losing precious body moisture. Some desert species actually live in family groups, with adults caring for their young in burrows throughout most of the summer. Thirsty pillbugs can take up water through their uropods and channel it through grooves along the sides of their bodies to the mouth.

The brownish and gray colors help terrestrial isopods to hide from predators. They also have special glands in their thorax that produce an unpleasant odor. The European pillbug has a marking similar to that of the European black widow spider. Predators usually avoid this venomous spider. The similarly marked isopod fools potential predators into thinking that it is also a dangerous spider. Pillbugs can also protect themselves from predators and from drying out by rolling up into a ball to protect their delicate undersides.

Like all crustaceans, isopods must molt, or shed their exoskeletons, to grow. Isopods are unusual in that they molt one half at a time. The rear half of the body molts first and is followed two or three days later by the front half. It is only during this time that the eggs of the mature female can be fertilized. Both males and females are usually required for reproduction. But in some parasitic species, young adults start out as males and later become females.


The Socorro isopod, Thermosphaeroma

thermophilum, is an endangered freshwater species that resembles a pillbug. They are only found in Sedillo Spring at an abandoned spa near Socorro, New Mexico. They disappeared in the wild in 1988 when water stopped flowing to the spa. Thanks to a colony at the University of New Mexico, the species survived. Small populations exist now only under artificial conditions in four laboratories and the spa.

Males climb on the backs of females to mate. The males curl the abdomen around so that the underside comes into contact with hers. Males use their pleopods to transfer the sperm to the genital openings of the females. After mating the female releases a dozen to several hundred eggs into the marsupium. They will remain there from eight to twelve weeks. Depending on the species, one or two broods are produced each year. The pale juveniles leave the marsupium and resemble the adults. However, they lack the last pair of thoracic legs, or pereopods. The juveniles become larger and darker with each molt, eventually gaining the full number of limbs.

Most isopods live one or two years, but some may live as long as five. The longest-lived species, Armadillo officinalis, is known to live up to nine years.


Wood burrowing and feeding species living in oceans and estuaries can severely damage pilings, docks, and other underwater wooden structures. Pillbugs seldom cause any damage, but large numbers may eat seedlings and other garden and greenhouse plants.


Forty-one species of isopods are listed by the World Conservation Union (IUCN). Two are listed as Data Deficient, which means there is not enough information to make a judgment about the threat of extinction. Twenty-two are considered Vulnerable, or facing a high risk of extinction in the wild. Seven are listed as Endangered, or facing very high risk of extinction in the wild. Nine are listed as Critically Endangered, or facing an extremely high risk of extinction in the wild. The last species, the Socorro isopod, Thermosphaeroma thermophilum, is Extinct in the Wild, which means it is no longer alive except in captivity or through the aid of humans.


Physical characteristics: The bodies of common pill woodlouses are egg-shaped, with adults measuring up to 0.7 inches (18 millimeters). They are dark gray, brown to red and usually have distinct rows of spots. The antennae are visible from above, but the legs are not.

Geographic range: Originally from southern Europe and North Africa, this species is now found throughout the world in temperate climates.

Habitat: Common pill woodlouses are found in many different kinds of habitats, including forests, grasslands, and even coastal and desert sand dunes. They are very common in cultivated fields, gardens, and greenhouses. Most individuals are found under rocks, logs, and other objects where there is plenty of moisture.

Diet: They eat young plant shoots, as well as dead and decaying plant matter.

Behavior and reproduction: Common pill woodlouses roll up into a ball to protect themselves. They move slowly on the ground in search of food and mates, traveling up to 43 feet (13 meters) a day under ideal summer conditions. During the winter they move only about half as far. During this time they may burrow as much as 10 inches (250 millimeters) beneath the surface of the soil.

Their reproductive cycles are triggered by rising temperatures and longer daylight hours. Mating usually occurs from late spring to early summer, sometimes even later in the year, just before the female goes through her molt. Females can store sperm in a special sac for up to one year. They produce one brood per year in the northern parts of their range, while those to the south produce two or three. Each brood has up to 100 eggs, but only half survive to become juveniles.

Common pill woodlouses and people: Large numbers of common pill woodlouses are sometimes considered pests in gardens and greenhouses because they will nibble on young plants.

Conservation status: Common pill woodlouses are not considered endangered or threatened. ∎


Physical characteristics: Adult sand isopods are flat from top to bottom and measure up to 0.6 inches (15 millimeters) in length and 0.3 inches (7 millimeters) across. Their bodies are almost paddlelike when viewed from above. The thorax is almost round and is followed by a long and pointed abdomen. The legs are thick and feathery in appearance.

Geographic range: They live in the western Atlantic Ocean, from Nova Scotia south to Florida.

Habitat: Sand isopods live along the shore and just beyond in the water, where they burrow in coarse, sandy bottoms.

Diet: Very little is known about the diet and feeding ecology of this burrowing species, but they are thought to be predators.

Behavior and reproduction: The rear pereopods are used to burrow in the sand. Mating occurs while adult sand isopods are buried in the substrate and may take up to several days. The females molt while they are in the grasp of the males. Two to six dozen eggs are produced in one brood each year. Juveniles measuring 0.1 (2.5 millimeters) long and 0.05 inches (1.25 millimeters) wide appear in spring and molt at least six times before reaching adulthood.

Sand isopods and people: This species does not impact people or their activities.

Conservation status: Sand isopods are not considered endangered or threatened. ∎


Physical characteristics: The body of the common shiny woodlouse is egg-shaped, has a shiny grayish brown back, and reaches a length of approximately 0.6 inches (16 millimeters). It has a pair of long antennae.

Geographic range: Originally from only western and northern Europe, this species is now also established in both eastern Europe and North America.

Habitat: The common shiny woodlouse lives in almost any damp habitat, particularly in forests. It is usually found under rocks and logs.

Diet: It eats plant materials, especially the leaves of lime, ash, and alder trees.

Behavior and reproduction: This species cannot roll up into a ball for protection. They avoid light and seek out areas with high levels of moisture. They tend to latch on firmly to rocks and other surfaces.

Common shiny woodlice and people: This common garden and greenhouse species does little damage to plants.

Conservation status: The common shiny woodlouse is not considered endangered or threatened. ∎



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Tavolacci, J., editor. Insects and Spiders of the World. Volume 7. Owlet Moth-Scorpion. Pill Bug. New York: Marshal Cavendish, 2003.


McDermott, J. J. "Biology of Chiridotea caeca (Say, 1818) (Isopoda: Idoteidae) in the Surf Zone of Exposed Sandy Beaches along the Coast of Southern New Jersey, U.S.A." Ophelia 55 (2001): 123-135.

Web sites:

Minibeast Profiles: Sowbugs and Pillbugs.http://members.aol.com/YESedu/MBP05.html (accessed on March 14, 2005).

"Pillbugs. Oniscidea." Critter Catalog.http://www.biokids.umich.edu/critters/information/Oniscidea.html (accessed on March 14, 2005).

Sowbugs and Pillbugs.http://www.zoo.org/educate/fact_sheets/sowbug/sowbug.htm (accessed on March 14, 2005).

World List of Marine, Freshwater and Terrestrial Isopod Crustaceans.http://www.nmnh.si.edu/iz/isopod/ (accessed on March 14, 2005).