In the common blue, the number of generations produced each year varies depending on the geographic location of the population. In the majority of England and Wales there are two broods per year (the populations are 'bivoltine'), with adults on the wing in May and June and again in August and September. Populations in Scotland and parts of England to the north of Yorkshire are single-brooded ('univoltine'); adults are present between June and September, with emergence occurring earlier in warmer areas. Occasionally, in warm years, double-brooded and single-brooded populations may go on to produce an extra brood in the same year, thanks to the warm weather. This phenomenon indicates that the factors governing the number of broods produced are environmental, rather than genetic (4). The eggs are laid singly on the foodplants and take around a week to hatch (3). Caterpillars of the first generation (or of the only generation in single-brooded populations) take around 6 weeks to become fully developed. Second brood caterpillars overwinter when partly grown, and complete their development during the following year (3). Pupation occurs either at the base of the foodplant or amongst litter on the ground (4) and adult butterflies emerge after around two weeks (3). In common with many other species of blue butterflies, the caterpillars of the common blue are attractive to ants when they are fully grown. The pupae are also attractive to ants and are often carried away into ant nests (4).
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