St Mark’s flies – Bibio marci – April 2010

Visit the cemetery during late April and you are likely to be surrounded by swarms of black flies slowly moving through the air with their long back legs dangling. Although they look a bit threatening they don’t bite or disturb humans much at all.

They are called St Mark’s flies as they usually appear on or around 25th April which is St Marks Day. They normally all disappear after just a fortnight. In general they inhabit hedgerows and woodland edges and are particularly noticeable because of the sluggish manner in which they fly and how they seem to gather in dangling dance groups.

They help with the pollination of fruit trees, while their larvae feed on leaf mould.

The Transcribing Group has noticed them in abundance over the last few years when we have been working in the cemetery in late April.

The images St Marks Flies: Photo from http://www.treknature.com/gallery/photo110030.htm and drawing of dancing group from wildyorkshire.co.uk