St Mark's Fly (Bibio marci)
The St. Mark’s fly (Bibio marci) has a one-year life cycle, spending most of it as a soil-dwelling larva before emerging as an adult for only about one week in late April. Larvae feed on decaying vegetation, pupate, and emerge as adults to mate, with females laying eggs in the soil before dying.
Key Life Cycle Stages:
Eggs (Spring): Female flies lay eggs in the soil shortly after mating, typically around St. Mark's Day (April 25th).
Larvae (Summer–Winter): Larvae hatch and live in the soil or compost, feeding on rotting vegetation, roots, and grasses. They overwinter as larvae.
Pupae (Early Spring): The larvae transform into pupae in the soil.
Adult (Late April/May): Adult flies emerge, often in large, slow-moving, swarms with dangling legs.
Lifespan: Adults live for approximately one week.
Behavior: They are important pollinators of fruit trees and hawthorn, often seen around woodland edges.
Adult males usually appear first, followed by females a few days later. They are characterized by their slow, clumsy flight, shiny black bodies, and, in males, very large eyes.