Why do woodlice always scurry for the dampest, darkest corner?

Lift a damp log or an old flowerpot and the woodlice underneath scatter in a panic, all racing for the next dark, wet hiding place. Leave one out in the open on a warm, dry patio and it looks desperate to get away. For a little grey armoured bug, dryness seems almost as scary as a hungry bird. Why does a woodlouse dread a dry, sunny spot so much?