Nature on our doorsteps: St Mark’s Fly...

By Rosaleen Dwyer

Rosaleen Dwyer is the County Heritage Officer at South Dublin County Council – every week she gives us an insight into the natural heritage around us and the beautiful biodiversity of the plants and creatures

Every year at the end of April and beginning of May, many people notice an usual black fly in their gardens or parks. This fly is large and robust, with a black hairy head and body. 

What makes them even more noticeable, however, are their long black legs that dangle beneath their bodies as they fly.

The long hind legs of St. Marks Fly are very distinctive 1

The long hind legs of St Mark's Fly are very distinctive

This insect is known as St. Mark’s Fly, simply because it emerges around St. Mark’s Day, on 25th April.  It is also sometimes called the Hawthorn Fly.

It has a distinctive ‘lazy’ flight, drifting slowly by in large numbers. 

This is often at the level of our heads and faces, and as they will land gently on anything that they encounter, this can cause consternation when they land on us.

The adult fly has a very short life, living for just one or two weeks. Males emerge before the females in springtime and gather in large numbers for one purpose only, to mate with a female. 

The harmless St. Marks Fly feeds only on nectar 1

The harmless St Mark's Fly feeds only on nectar

Eggs are laid in the soil where the hatched larvae chew on rotting organic matter.  

St. Mark’s Fly is a completely harmless insect. It does not bite or sting. 

Adults feed only on nectar, meaning they are important pollinators. 

Also, because they fly so slowly, they are easy prey for hungry birds in springtime.

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