St Mark's Fly

If you've been out enjoying a walk in our beautiful countryside over the last few weeks, chances are at some point along the way you will have been surrounded by black flies hanging in the air, drifting by with long dangly legs. St Mark's flies (Bibio marci), can be found around woodland edges, hedgerows, rough grassland and wetlands and are named after St Mark's day on 25th April which is roughly when they emerge from spending the previous autumn and winter as a larvae feeding on rotting vegetation in the soil. Individuals are on the wing for only a week and after mating and laying eggs in the soil the females soon perish.


St Mark's flies feed on nectar and are considered important pollinations of fruit trees and crops. The males are around 12mm long, have clear wings and large eyes which are divided by a groove and have separate connections to the brains allowing males to use the upper eye part to look for females and the lower part to monitor their position relative to the ground enabling them to hover. Females are slightly larger at 14mm, have brown wings and smaller eyes and legs.




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