St Mark’s flies are flying

Have you been assailed by clouds of black flies recently? I was introduced to St Mark’s flies two years ago in early May. I was walking near Mundford with a brilliant local naturalist, Ian Simper. Around us bird cherry trees were dripping with white blossom flowers. I noticed a large number of glossy black flies, with a distinctive longish body flying to and from the flowers and landing on our clothes. Ian told me they were St Mark’s flies, named as they emerge close to St Mark’s day, April 25th. I was delighted to see St Mark’s flies yesterday, April 27th, as I walked along a hawthorn hedgerow, near Burwell Lode.

A St Mark’s Fly on Bird cherry leaf, with blossom behind

These flies don’t get much of an adult life, only flying for around one week. Most of their life is spend as larvae, where they munch on rotten vegetation with their strong jaws.

St Mark’s flies are a distinctive glossy black, with long dangly legs.

I don’t know of any music or poems written about these flies(!) yet they are extraordinary. The males’ have large eyes which are divided by a groove. The upper and lower parts of each eye send separate signals to their brain. The male is therefore able to keep a lookout for a female with their upper eyeparts – an urgent task with only a week to mate(!) – while with their lower eyeparts they are able to assess their distance from the ground, enabling them to hover. (Information from Buglife’s website:

As they feed on nectar, these flies are important pollinators of fruit trees and crops.

As we get tidier and tidier in our gardens and out onto our verges, creatures like St Mark’s flies have fewer places to spend their time as larvae or to get protection from the cold in the winter months. Leave piles of rotting leaves in corners of your garden. Add bits of wood, anything that can provide shelter and food. Or make bug hotels out of scraps of materials, your construction can be as grand, random or quirky as you like. Buglife has some great ideas to get you started: You will be giving creatures like these St Mark’s flies, which are amazing for themselves and play such an important part in our ecosystem, a place to thrive.

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