Egg The female heath fritillary lays its eggs (or ova) in batches of (15–)80–150 on the underside of leaf of a larval food plant or on a plant adjacent to the larval food plant. Eggs are oval spheroids with flattened bases, about 0.5 mm high. They are ribbed (longitudinally, i.e. from top to bottom) and striated (transversely, i.e. around the egg). Pale cream when laid, eggs darken to pale yellow within two days, and then dark grey a few days before hatching. Eggs mature in two to three weeks.
Caterpillar Upon emergence, first-
instar caterpillars (or larvae) eat their eggshells. The caterpillars from a clutch initially stay together, feeding in a small, unobtrusive web. Second or third instar caterpillars disperse into smaller groups. Then the third instars tend to feed and rest solitarily; they rest beneath dead leaves at night and during bad weather. The caterpillar
hibernates for the winter in a
hibernaculum, made from a curled dead leaf by spinning its edges together. Hibernacula are usually close to the ground. Although most caterpillars hibernate singly, they sometimes group in twos and threes, although 15 to 20 caterpillars have been found in single hibernaculum. Caterpillars re-emerge in early spring. When it is warm, they feed a little, but most of the time is spent basking in the sun. There are six instars in total. The full-grown sixth instar caterpillar is 22–25 mm long, and predominantly black; it has pale (yellow-orange) spines and (greyish-white) spots.
Pupa The pupae are 12.4–12.8 cm long and last 15–25 days (early May to late June in the UK). They are white with black and orange-brown blotches. Pupae are usually found close to the ground in or beneath dead leaves. A study in Cornwall, England, recorded mortality rates in the region of 50%, mostly from predation by small mammals, but including predation by beetles, and
parasitism.
Imago Imagines probably live for 5–10 days. Males are active on warm sunny days. Females mate once shortly after emerging; they lay their eggs only during warm weather, spending most of the time either basking or hiding in vegetation.
Flight period Across its range (see "Subspecies and variation" below), subspecies
M. a. athalia shows a protracted flight period from mid-May to mid-August. In favourable localities and/or favourable seasons, a partial second brood has been recorded from mid- or late August to September. In the UK, the flight period is from the end of May to the beginning of July (in the south-west) and early June to early August (in the south-east).
Fennoscandian subspecies
M. a. norvegica flies in June–July, precise timing being affected by the season. In southern Europe, subspecies
M. a. celadussa flies in a single brood at high altitude in June and July. Below the subalpine level, however, it is
bivoltine, flying in May–June and late July–August—except for f.
nevadensis in the
Sierra Nevada, which is univoltine. == Host plants ==