Like most bibionid larvae, they grow up in grassy areas and are herbivores and scavengers feeding on dead vegetation or living plant roots. Bibio marci larvae are known to be root damage pests of celery, asparagus, roses, saxifrages, lawn grass, lettuce and Polyanthus. They also feed on a very large number of plant species that are commercially unimportant.
Bibio marci, St Mark’s fly, gets its name because the adults emerge around St Mark’s Day, 25th 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.
These flies are common during May and June when adult males form mating swarms to attract females. They fly slowly in an up-and-down dancing pattern at about head height with their long back legs hanging down.Females can often be found sitting on vegetation near swarms of males.
This species shows strong sexual dimorphism, meaning that males and females look very different.Males are around 12mm long with very large eyes and clear wings, whereas females are around 14mm long with much smaller eyes and heads and 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 only live for about a week so they spend 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 before emerging as adults the following spring.
Habitat Bibio marci can be found in rough grassland, hedgerows and around the edges of woodland.
Trophic strategy Adult St Mark’s flies are thought to be important pollinators of fruit trees and other crops. Larvae feed on decaying organic matter and the roots of grasses, and can occasionally be pests of lawns and cereal crops.
St Mark’s flies are members of the Diptera (true flies) and they belong to the family Bibionidae.In the UK alone there are over 7,000 species of Diptera, including 20 in the family Bibionidae and 16 in the genus Bibio.