If you’ve been out walking in the last few days, you might have noticed clouds of small, glossy black flies drifting along paths, hedgerows, and woodland edges. I saw them this weekend, hovering in loose swarms by the canal. These are St. Mark’s flies, named for their habit of emerging around St. Mark’s Day on April 25th.

Despite their slightly alarming appearance, these leggy flies are completely harmless. They’re long, shiny, and slightly hairy, with distinctive bulbous eyes. However, their flight is their most recognisable trait: a slow, bobbing drift through the air, their long hind legs dangling beneath them.

Because they fly slowly and clumsily, they’re a welcome early-season food source for returning migrants like swallows. After their long journey from Africa, these birds rely on easy, abundant prey, and St. Mark’s flies are exactly that.

Their emergence also coincides beautifully with the blossoming of fruit trees, making St. Mark’s flies important, if often overlooked, pollinators.

Their time in the sun is brief. Adults live for little more than a week and their sole task is to mate. Once the females have laid their eggs in the soil, the season is over and they die. The larvae spend many months hidden underground, feeding on roots and decaying leaf litter, until it is their turn to rise into the light the following spring.