A full-grown red slug can extend to be approximately 18 cm in length; although they usually only grow to be 7–10 cm long. Perhaps the red slug's most noticeable feature is the placement of its pneumostome just before the midpoint of its mantle. Like all slugs, the red slug moves relatively slowly, using the muscular contractions of the underside of its foot, which is mostly red with stripes. The red slug, like all other slugs, uses two pairs of tentacles to sense its environment. The darkly-coloured upper pair, called optical tentacles, is used to sense light. The lower pair provide the red slug's sense of smell. Both pairs can retract and extend themselves to avoid hazards, and, if lost to an accident or predation, can be regrown.