St marks fly

The St. Mark’s Fly is a species of true fly, known as Hawthorn flies. This species belongs to the Family Bibionidae and 20 species from this family are found in the UK.

Fast Facts

Latin name: Bibio marci

Rarity in the UK: Rare / Common

St. Mark’s Flies (Bibio marci) are so called because they emerge around St Mark’s Day on 25th April every year and can be seen in flight in May. They are found around woodland edges, hedges, rough grassland and wetlands and can be seen throughout the UK in spring.


Male St. Mark’s flies are around 12mm in length with clear wings, large eyes and long dangly legs. Interestingly, the male’s eyes are divided by a groove and have separate connections to the brain. This allows the males to use the upper eye part to look out for females and the lower part to monitor their position in relation to the ground, allowing them to hover in the same position. The shiny black male flies are very conspicuous, with long dangling hind legs.You can often see them when they congregate in big swarms flying slowly, up and down, at around head height – trying to attract females. Females are bigger than their male counterparts at 14mm in length with smoky brown wings and much smaller eyes and legs.

Life cycle

The St. Mark’s Fly has a very short adult life cycle, being in flight for approximately only one week. The majority of their time is spent as larva in the soil. During autumn and winter, larvae feed on rotting vegetation which they chew with their strong mouthparts. In springtime the males emerge first and the females a few days later. After mating, females lay their eggs in the soil and die soon afterwards.

Swarms of St. Mar’ks flies may be annoying in the early spring, but they are very useful creatures, they feed on nectar, making them important pollinators of fruit trees and crops.