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MTM – The St Mark’s Fly , Bibio marci

The St Mark’s fly, or ‘hawthorn fly, is a very common, long, shiny, black fly that can be found in large numbers during the spring around woodland edges, fields and wetlands. Here at the meadow the St Mark’s fly appears in large numbers and hovers about a metre above the meadow vegetation creating a huge spectacular black cloud, with its legs dangling underneath. This spectacle only lasts for seven to ten days , which is pretty much the entire life cycle of an adult fly. The common name of this species comes from the fact that the adults usually emerge in large numbers around St Mark’s Day, April 25th. They are in fact non-biting and do not sting.

St Mark’s flies will often land on any objects in their way, including fence posts and people, and are rather sluggish at rest. The larvae live in the soil feeding on roots, grasses and rotting vegetation, and are often found around compost heaps. The adults feed on nectar and are considered to be important pollinators for fruit trees and other plants.

St Mark’s flies are black and shiny; males and females look very different, however: the male has very large eyes, while the female has a small head with tiny eyes. There are several species of closely flies in the family Bibionidae that can be very difficult to tell apart. For instance, in late summer, the heather fly hatches in large numbers on moorlands; it looks very similar to the St Mark’s fly, but it has orangey-red thighs. Here at the Meadow Hide the St Mark’s fly has never really caused any problem. They normally don’t move very far and have never entered the hide itself. The flies provide a much needed protein boost for the resident birds during the early part of the nesting season.


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