The marsh fritillary is yet another butterfly that is absent from much of Britain but that can be found in a number of places in Dorset. It is certainly not common here, nor is it anywhere, but careful habitat management means its seems to be holding its own and not suffering the declines of elsewhere in the country. Marsh fritillary is a bit of an odd name for this butterfly; it has the chequered wing patterns that fritillaries have but it is not found in marshes. It does like damp, flower rich meadows where devil's-bit scabious grows but it also can be found on dry downland slopes that are far from being 'marshy'.
The marsh fritillary is single brooded and the books say it emerges in Mid-May and can be seen until mid-July. The records we have so far in the Nature of Dorset database would indicate a much shorter season starting in week 19 which is mid-May but there are no reports after week 23 in mid-June. The most prolific weeks are 21 and 22 at the end of May and beginning of June.
There are records from fourteen locations in Dorset which are split roughly equally between damp grasslands sites and chalk downland. Alners Gorse, Giant Hill and Powerstock Common all hold reasonably sized colonies and a visit at the right time of year to any one of these should give you a chance to see them.