English: Marsh marigolds (Caltha palustris), Lydiard Park, Swindon Marsh marigolds or kingcups are herbaceous perennial plants that grow in shady, damp areas. They are members of the buttercup family and grow widely in the northern hemisphere. The plants have long been established in the UK, surviving the harsh conditions of the last ice age. The flowers are undoubtedly attractive but excessive handling of the plants can cause skin rashes as all parts are irritant. The flies are possibly Sphegina montana but perhaps not. The name doesn't sound very British. Whatever they are they are essential to the pollination process and the flowers contain attractants to lure the flies. The relationship between plant and insect is mutually beneficial. In the UK there are two subspecies, Caltha palustris subsp. palustris, and Caltha palustris subsp. minor. Lord only knows which one this is; I've no idea. The technical information above came from wikipedia.