Bibio marci (St Mark’s fly)

Bibio marci, St Mark’s fly, gets its name because the adults emerge around St Mark’s Day, 25^th April.

Bibio marci is a large, hairy black fly that is easily recognised by the males’ ‘dancing’ up-and-down flight and long dangling legs hanging down.

Bibio marci is also the model for a fly fishing pattern, known as the hawthorn fly.

Species detail

A mating pair of Bibio marci - the female is to the left of the picture

A mating pair of Bibio marci. The female is the larger fly with smoky wings the male is smaller with clear wings. © Laurence Livermore 2009

Biblio marci species shows strong sexual dimorphism, meaning that males and females look very different.

Males

are around 12mm long
with very large eyes
clear wings

Females

are around 14mm long
with much smaller eyes and heads
smoky brown wings

Males use their large eyes to help them spot and catch females in the mating swarm, and to see and chase off rival males.

Adult St Mark’s flies live for about a week spending most of their lives in the larval stage. After mating females lay their eggs in the soil and die shortly afterwards.

The larvae feed and grow over the autumn and winter, emerging as adults the following spring.

Taxonomy

Find out about the wider families of fly to which Bibio marci belongs.
Biology

Male and female St Mark’s flies look very different from each other. Find out more about the sexual dimorphism of this species as well as its life expectancy and growth patterns.
Behaviour

Find out the months of the year when the St Mark’s fly is most commonly spotted and about the mating dance of the males.
Distribution and ecology

Discover the areas where the St Mark’s fly is known from, the types of habitat it is found in and learn about its feeding habits.
References

Get reference material for Bibio marci.

Images

Specimens of Bibio marci from the Natural History Museum’s Entomology reference collection in the Angela Marmont Centre for UK Biodiversity. © Natural History Museum 2010