St Mark’s Fly

Description:

The St Mark’s Fly (Bibio marci) 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.  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.

The shiny black male St. Mark’s Flies are around 12mm in length with clear wings, have large eyes and long dangling hind legs. 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.  You can often see them when they congregate in big swarms flying slowly, up and down, at around head height – trying to attract females.

Female St Mark’s Flies are bigger than their male counterparts, at around 14mm in length, with smoky brown wings and much smaller eyes and legs.

• • Size: 12-14mm in length
  • Life span:  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.
  • Diet: The adults are considered important early pollinators for fruit trees and other plants as they feed on the nectar.
  • Reproduction:  The St Mark’s Fly lays eggs in the soil or rotting vegetation; shortly after this the female soon dies
  • When to see:  Adults are generally seen from April to June
  • Population Trend:  Unknown
  • Threats: Loss of habitat, as a result of destruction and degradation through human activity
  • Fun Fact: The male’s eyes are divided by a groove and have separate connections to the brain allowing them to use the upper eye part to look out for females and the lower part to monitor their position in relation to the ground.

The St Mark’s Fly is considered an important early pollinator for fruit trees and other plants as they feed on the nectar.

How you can help: 

Buglife B-Lines are an imaginative and beautiful solution to the problem of the loss of flowers and pollinators. B-Lines are a series of ‘insect pathways’ running through our countryside and towns, along which we are restoring and creating a series of wildflower-rich habitat stepping stones. Linking existing wildlife areas together, creating a network, like a railway, that will weave across the UK landscape.  More information about B-Lines and how you can help pollinators can be found on our B-Lines & Pollinator Projects pages.

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