Common Woodlouse (Oniscus asellus)

KINGDOM Animalia
PHYLUM Arthropoda
CLASS Malacostraca
ORDER Isopoda
FAMILY Armadillidiidae


This species is European in origin but has been introduced widely in North America, where it is quite common now. It occurs in a wide range of habitats, usually moist, and can be found by turning over rocks and looking in decaying wood. Up to 16 mm in length, it is somewhat flattened and is characterized by large numbers of legs visible as numerous individuals run about after their shelter is removed.

Most crustaceans are aquatic and marine, but a lesser number are common in fresh water. A still smaller number have escaped from the aquatic realm over evolutionary time, and they include terrestrial isopods, of which the woodlouse is one.

Aquatic crustaceans have rows of gills for aquatic respiration, but the terrestrial ones respire by using some of these same appendages modified for aerial respiration. Branched “air trees” have evolved on modified hind legs, associated with an abundant blood supply for transporting respiratory gases. These structures must be kept moist, as the gases diffuse through water to reach the air trees. Thus terrestrial isopods must live in damp places, although this species is able to exist in relatively dry areas. Many adaptations serve more than one function; rolling up the body probably reduces water loss.

Pale patches on the exoskeleton of some woodlice are storage areas for calcium, an important mineral to animals that must construct a new “shell” every time they molt. Woodlice molt their rear half, then their front half, and sometimes consume the cast exoskeleton (exuvia).

Woodlice are in the same group as pillbugs (sowbugs), among the animals that generate delight and curiosity when encountered. People are often put off by little crawly critters, but when one curls up into a ball each time it is disturbed, presumably an anti-predator adaptation, it crosses the line into cuteness. Because of this, these tiny invertebrates are sometimes kept as pets! To add a new word to your vocabulary, this ball-curling behavior is called conglobation.

Like other terrestrial isopods, woodlice feed on detritus (decaying plant matter), but they also graze on algae and lichens growing on surfaces such as tree trunks and walls. Their predators are mostly invertebrates such as beetles, centipedes, and a specialized type of spider that can penetrate their exoskeleton with its narrowly pointed chelicerae.

A male woodlouse courts a female by waving his antennae and licking and tapping his potential mate. After fertilizing her from one side, he will move over to her other side to copulate again. The fertilized eggs are held in a pouch under the body until they hatch, then they emerge from the pouch to appear as if the female is giving live birth. Females can store sperm from a single fertilization for up to a year; thus, they can determine when their young will be released into the world, presumably at an optimal time for survival.