Woodlice are isopod crustaceans, in the same subphylum as crabs and lobsters, and among the very few crustacean groups to colonise land fully. Gas exchange happens at the pleopods, flattened abdominal appendages derived from gills, which must be kept moist. Several genera, including Porcellio, have evolved pseudotracheae, white branching air-filled invaginations in the pleopods, visible as pale patches, which improve air breathing but do not remove the moisture requirement.
Water is the constant constraint. Woodlice lack the waxy waterproof cuticle of insects and lose water rapidly, which drives their negative phototaxis and their aggregation in humid crevices. Clustering itself reduces water loss, and experimental groups survive dry conditions longer than isolated individuals.
Other crustacean traits persist. Females brood eggs in a fluid-filled marsupium on the underside of the body, so the young develop in a small private pool. Woodlice also excrete nitrogen largely as ammonia gas rather than urine, which conserves water and accounts for the faint sharp smell of a disturbed colony.