This arthropod belongs to the order of isopods. Most of the 10,000 known species live in the ocean. Woodlice are crustaceans and more closely related to shrimp, crawfish, lobsters and crabs than insects and millipedes. They are usually dull-coloured (greyish, brownish or blackish). Unlike many crustaceans, these animals do not have a real shell. Their body is protected by an exoskeleton composed of plates. Their thorax also has seven pairs of legs called pereiopods.
Their heads have two pairs of antennae: a long pair and a very short pair, called “antennulae,” which are only visible on the woodlouse underside. Woodlouse eyes are compound and often underdeveloped, probably because of their nocturnal lifestyle and because they do not hunt to eat. When they are resting, their mouthparts are covered with a pair of thoracic appendages called “maxillipeds.” In English scientific literature, woodlice are separated into two groups: the sowbugs and the pillbugs. The sowbugs are distinguished by two appendages resembling small tails at the end of the abdomen. These appendages are absent from the pillbugs, known for their ability to roll into a tight ball that no assailant can get a grip on.
Arthropods with pale blue blood!
Woodlice have pale blue blood. The colour is caused by hemocyanin, a blue pigment with a copper core. This molecule transports oxygen through the woodlouse body, just like hemoglobin (red pigment with a copper core) does in humans. Woodlouse blood becomes colourless when it is low in oxygen. To breathe, this land-dwelling crustacean usually has pseudotracheas on its abdomen. The number of “lungs” varies according to species (zero, two or five pairs). They look like whitish spots that are visible to the naked eye on the underside of the woodlice.
Woodlice among the rare crustaceans that live on the ground
Specialists believe that woodlice evolved from ancestors that lived at the bottom of the sea. They came directly to land from the sea without transitioning through freshwater environments. Most species can survive only in humid habitats. Some woodlice are found exclusively along the seashore, where they live in the crevices of rocks or in stranded seaweed.
The earliest stages of woodlouse development take place in a brood pouch instead of aquatic larval phases. This is an important element in the group’s evolution to life on land.