Woodlice are land crustaceans, not lice at all
Oniscidean isopods nicknamed woodlice haunt damp wood yet are neither parasites nor insects. Molecular clocks suggest Paleozoic landfall, though the oldest fossils are mid-Cretaceous around 100 million years ago. Mostly detritivores, they humify litter and even re-eat faeces to extract more nutrients.
The English name misleads twice over: woodlice do haunt old timber, but they are crustaceans, neither insects nor parasites. Regional names include slater, sow bug and wood pig. They make up Oniscidea, a suborder of isopods with about 3,710 validated species as of 2014, out of an estimated 5,000 to 7,000 alive today. Some, such as the sea slater Ligia, never fully left the shore, and a few have gone back to water.
Their ancestors lived on the seabed and edged ashore through intertidal forms. Molecular clocks suggest landfall in the late Paleozoic, while the oldest fossils, preserved in roughly 100-million-year-old amber from Spain, France and Myanmar, are already so widespread that the group probably predates the breakup of Pangaea. Breathing still betrays that sea origin: primitive species use gills, while fully terrestrial ones evolved paddle-shaped lungs threaded with branching air tubes. They lose water fast through the cuticle, which explains the love of dark, damp crevices and the flight from bright light, yet Hemilepistus reaumuri lives in the driest habitat any crustacean has conquered, and others reach desert slopes 4,725 m up.
Life history is unusual. Females brood eggs in a pouch of overlapping plates under the thorax, and the young emerge looking like tiny white woodlice minus their last pair of legs. Moulting happens in halves, back first and front a few days later, perhaps to stay mobile and conserve scarce calcium. Some families roll into a ball; pill bugs doing so resemble pill millipedes, a case of convergence, though the millipedes have 17 or 19 leg pairs to the woodlouse's seven.
Mostly nocturnal detritivores, woodlice chew dead leaves and then eat their own droppings once microbes have worked on them, squeezing more nutrition out of poor food and speeding humus formation. They fragment litter rather than grinding it as earthworms do. Because they store copper and other heavy metals in the hepatopancreas, they serve as pollution indicators, and they are prey for the woodlouse spider Dysdera crocata.
Source: Woodlouse
