The male has a wingspan of 29–31 millimetres (1.1–1.2 in), and the female 31–34 mm (1.2–1.3 in). The upper side of the wings are marked in a chequered pattern strongly reminiscent of a fritillary butterfly (family Nymphalidae); however, the Duke may be separated by its wing shape. H. lucina also has a distinctive underwing pattern.
H. lucina was added to the UK Biodiversity Action Plan in 2007. It is also listed in the Wildlife and Countryside Act 1981, which requires anyone wishing to trade the species to have a licence. H. lucina is listed on the German IUCN Red List, but is considered of "least concern" on a Europe-wide basis.
Two distinct habitats are used in the UK:
- Grassland on chalk or limestone
- Clearings in ancient woodland
Colonies prefer areas where the food plants grow among tussocky vegetation. The species prefers north- or west-facing slopes in downland habitats.
Eggs are typically laid in small groups (up to 8) on the underside of leaves of a host-plant; though they may also be laid singly or on foliage adjacent to the food-plant (e.g. if primulas are growing among dense vegetation, old females laying in hot weather). Particularly "good" plants or leaves may have eggs from more than one female. The egg is spherical, with a flattened base, measuring 0.6 mm in diameter. The eggs are initially glossy and opaque, turning to a uniform pale green; prior to hatching, they develop a distinct dark purple blotch. Significant, though unintentional, predators of H. lucina eggs are large snails, as they eat primulas in the spring. The eggs hatch after 7–21 days depending on weather conditions.
Newly emerged caterpillars are almost transparent, with a few long pale hairs. They move down to the base of leaf stems, where they spend the daylight hours (the caterpillars are nocturnal). For most of its first instar (before the first moult), the caterpillar is pale green. There are four instars, lasting about 4 weeks. The full-grown fourth-instar caterpillar measures 16–17 mm. It is pale brown with a dark brown dorsal stripe and numerous hairs; there is also a central black spot on each segment (inside the dorsal stripe).
Caterpillars feed mostly on the upper leaf surface, leaving leaf-veins intact, and quite unlike slug feeding signs. The caterpillars will not eat yellowed leaves, and will move on to another host-plant in search of green leaves. H. lucina caterpillars do not feed at temperatures below 11°C or when it is wet. They have no significant predators or parasites.
Pupae of H. lucina are short, just 9 mm (0.35 in) long. They are pale cream (somewhat tinged pink) with evenly spaced dark brown spots and a few pale hairs. They are found either very low in dense grass or on the ground. The pupal stage lasts 9 months, with likely high mortality rates. Predators of pupae include shrews and slugs.