Animal field guide
St Mark's Fly
Bibio marci
A black, hairy fly named for its emergence around St Mark's Day, 25 April. For a few weeks in spring, males hover in slow, drifting swarms over hedges and grass with their long hind legs dangling beneath them.
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Get AnimalDexSt Mark's Fly at a glance
Invertebrate · Europe · Grassland · Relatively common
Known for: Black, Front leg tibia ending in a long, Short antennae set low on the head, Dark spot near the leading edge of each wing
Animal field guide
St Mark's Fly
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A black, hairy fly named for its emergence around St Mark's Day, 25 April. For a few weeks in spring, males hover in slow, drifting swarms over hedges and grass with their long hind legs dangling beneath them.
Native range
Natural range, not this capture location
Grassland, pasture, hedgerows, woodland edges, parks and gardens across Europe, especially where damp soil with plenty of decaying plant matter supports the larvae.
Search near meAnimalDex stats
Dominance
5
Speed
16
Size
2
Intelligence
5
Rarity
12%
Total
40
Size scale
STEP 2 · PROVE IT
STEP 3 · BATTLE & FUSE
Understand the animal
Understand the St Mark's Fly
Factual biology first: what this animal is like, where it lives, how it survives, and why those traits exist.
01
Introduction to St Mark's Fly
A black, hairy fly named for its emergence around St Mark's Day, 25 April. For a few weeks in spring, males hover in slow, drifting swarms over hedges and grass with their long hind legs dangling beneath them.
How to identify a St Mark's Fly
- Black, hairy body about a centimetre long
- Front leg tibia ending in a long, strong spur
- Short antennae set low on the head
- Dark spot near the leading edge of each wing
Why St Mark's Fly are interesting
- Mating pairs often sit joined end to end on vegetation.
- The larvae can damage the roots of grasses and vegetable crops when numerous.
- Pollen collects on the hairy bodies of feeding adults, making them useful spring pollinators.
- Anglers know it as the hawthorn fly and imitate it with artificial flies when it falls onto water.
02
Adaptations and biological abilities
03
Why does St Mark's Fly behave this way?
- Black, hairy body about a centimetre long
- Front leg tibia ending in a long, strong spur
- Short antennae set low on the head
- Dark spot near the leading edge of each wing
Active by day in warm sunshine; in cool, dull weather and at night adults rest motionless on grass stems, hedge leaves and flower heads.
04
Why Grassland?
Habitat: Grassland, pasture, hedgerows, woodland edges, parks and gardens across Europe, especially where damp soil with plenty of decaying plant matter supports the larvae.
Native range: Europe
Native range
Natural range, not this specific capture location.
Grassland, pasture, hedgerows, woodland edges, parks and gardens across Europe, especially where damp soil with plenty of decaying plant matter supports the larvae.
05
Why this diet?
Adults take nectar and pollen from spring blossom such as hawthorn, blackthorn and fruit trees. Larvae feed in the soil on decaying plant material, roots and leaf litter.
06
Activity, rest, and lifespan
Active by day in warm sunshine; in cool, dull weather and at night adults rest motionless on grass stems, hedge leaves and flower heads.
Adults live only about a week. The whole life cycle takes a year, most of it spent as a larva in the soil.
07
St Mark's Fly reproduction
After mating the female burrows into the soil and lays a batch of eggs, reportedly a few hundred, then dies. The larvae feed together in clusters, overwinter in the ground and pupate in early spring.
Males have huge eyes that meet on top of the head and clear wings; females have a small head with small eyes, a larger body and smoky brown wings.
08
Predators, threats, and defense
09
Conservation and rarity
Rarity: Relatively common · 12/100. AnimalDex canonical rarity score: 12/100, maintained by the live indexed species profile.
- Observe from a respectful distance and avoid changing the animal's behavior.
- Do not block feeding, shelter, nesting, or travel routes.
- Use a live camera capture without handling or staging wildlife.
Speak to the animal
Speak to the St Mark's Fly
It answers as itself: ask about how it lives, or bring it something of yours and hear what it would do, from its own Power.
Anonymous: 3/day. Signed-in: 10/day. Pro: 100/day.
Compare
Compare the St Mark's Fly
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Battled in the Arena
No one has taken a St Mark's Fly into an Arena battle yet. Battles players fight with this animal in the AnimalDex app show up here.
Where to find them
Where to find St Mark's Fly
To find St Mark's Fly in the wild, focus on the exact habitat patches that match its body design and daily behavior, not just the broad country where it exists. You usually do better by working one good piece of habitat inside europe than by covering too much ground.
- Forest edge, canopy gaps, fruiting trees, or shaded trails where cover and food meet
- Open grassland edges, lightly wooded plains, or raised ground where you can scan long distances
- Protected habitat blocks within europe
Scientific name
Bibio marci
Category
Invertebrate
Habitat
Grassland, pasture, hedgerows, woodland edges, parks and gardens across Europe, especially where damp soil with plenty of decaying plant matter supports the larvae.
Rarity
Relatively common · 12/100
Native range
Europe
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