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St Mark’s fly (Bibio marci)

A long-legged fly you can tell the time by – well, the time of year. St Mark’s flies emerge en-masse around 25 April each year.

Male St Mark's flies have huge, bulbous eyes and long, dangling legs.

Credit: Kim Taylor / naturepl.com

Male and female St Mark's flies emerge within just a few days of each other and mate en-masse.

Credit: Andy Sands / naturepl.com

Quick facts

Common name: St Mark’s fly, hawthorn fly

Scientific name: Bibio marci

Family: Bibionidae

Habitat: woodland edges, hedges, rough grassland and wetlands

Diet: larvae eat rotting vegetation, adults feed on nectar

Predators: birds, fish and predatory invertebrates

Origin: native

What do St Mark’s flies look like?

Both male and female St Mark’s flies are black and shiny. Males are slightly smaller than the females at around 12mm in length, but what they lack in stature they make up for with huge, bulbous eyes. They also have longer legs and clear wings. Female St Mark’s flies average around 14mmm in length and have much smaller eyes, shorter legs and smoky-brown wings.

What do St Mark’s flies eat?

Adult St Mark’s flies are important spring pollinators with a particular penchant for the nectar of fruit tree and hawthorn blossom. St Mark’s fly larvae feed on rotting vegetation and are often found in compost heaps.

Credit: Nick Upton / naturepl.com

How do St Mark’s flies breed?

St Mark’s flies are short-lived insects, on the wing for just a week. The males emerge first – around St Mark’s Day on 25 April each year – and are usually in flight throughout May. You’ll often see them floating lazily up and down in big swarms with their long legs dangling, trying to attract a mate. Female St Mark’s flies normally appear a few days after the males emerge.

Once they’ve mated, the females lay their eggs in soil or amongst rotting vegetation and die shortly after. The young hatch into larvae in autumn and spend their days tucked away, feeding on roots, grasses and decaying organic matter until spring.

Did you know?

The male’s eyes are split into two halves, each with a separate connection to the brain. The upper half keeps an eye out for females, while the lower half checks how far the fly is from the ground during mating flights.

Where do St Mark’s flies live?

These flies are common and found right across the UK. You’re most likely to see them congregating around woodland edges, above hedges, in rough grassland and wetlands.

Credit: Kim Taylor / naturepl.com

Signs and spotting tips

You can't miss the large swarms of these flies when the males emerge around St Mark’s Day on 25 April. You can then usually see them until June. Look out for their long legs dangling as they fly, and you can pick out the huge eyes of the males when they land sluggishly on any object in their way!

Threats and conservation

The biggest threat to St Mark’s flies is habitat loss through the loss of trees and hedgerows.

It's also possible that climate change could affect their seasonal emergence, throwing their lifecycle into flux. As important pollinators, this could then have knock-on effects for the wider ecosystem.

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