Widely recorded across Europe, with local variation in abundance and northern or southern range limits.
Fine-art photographic collection
Bibio marci
A spring swarm whose hidden larvae turn dead leaves into soil.
About this species
St Mark's Fly
Bibio marci(Linnaeus, 1758)
Information reviewed15 July 2026
St Mark’s flies appear abruptly in spring, often hanging in the air with their long legs dangling. Their adult life is brief, but the gregarious larvae spend months in soil and leaf litter breaking down vegetation.
Taxonomy
Animalia · Arthropoda · Insecta · Diptera · Bibionidae · Bibio
Accepted species or infraspecific taxon in the current GBIF/Catalogue of Life treatment; unsupported details have been omitted rather than inferred. · checked 2026-07-15
EuropeBroad documented range and selected occurrence localities — not a complete occurrence map.
Grassland, woodland edges, gardens and other places with moist soil and decaying plant material.
Adults are most often seen in spring, traditionally around St Mark’s Day in parts of Europe.
Adults may visit flowers; larvae feed on decaying organic matter and plant material in soil.
Solitary larvae may occur in groups where habitat is suitable.
What makes this species remarkable?
Large numbers can emerge within days of one another around late April, creating conspicuous mating swarms that seem to appear from nowhere.
The larvae live in dense groups under the soil surface, shredding decaying leaves and helping convert plant debris into humus.
Adults become important early pollinators of hawthorn and fruit blossom during a flight period when relatively few other large flies are active.