St Mark’s Fly – Bibio marci

Walking around the country side, even in urban gardens, during the spring you are very likely to come across St Mark’s Fly, or ‘hawthorn fly, (iBibio marci). It is long, black and shiny, and seems to hand in the air trailing long legs beneath as it searches for a suitable mate. They can often been seen resting on leaves, fence posts, and even you if you stand still long enough.

There is a distinct difference in the shape of the face between the males and females, so much so it caused me some confusion and I did think originally I had seen two different species until I came across a mating pair. The males has very large eyes, and a classic ‘fly face’. Where as the females look very different with a long thin face and very small eyes.

A mating pair. The top one is a female showing the narrow face off well.

The adults are considered to be important pollinators, esecially fruit trees feeding on nectar. Pollen can clearly be seen on the face and thorax of the male in the photo below.

The males are the first to emerge in spring, and the females emerge a few days later. After mating the eggs are laid in the soil, and the female dies soon after. After hatching the larvae feed on roots, grasses and rotting vegetation, and it’s not unusual to find them in compost heaps. As an adult the flies have a very short life, flying for about a week before dying. The majority of their time is spent as larvae in the soil chewing on rotting vegetation with their strong mouthparts, during autumn and winter.

A male covered in pollen. The long spur on the front tibia is clearly visible.

An interesting fact about Bibio Marci is that the eyes are divided into tow main areas. The upper eyes are used exclusively for the detection of females as they fly above the males and are exposed against the sky.

There are several species of closely flies in the family Bibionidae that can a challenge to tell apart. But at about 12mm are generally larger than other species of Bibio. Mind you that’s not much help sometimes unless you have them nearby to compare. Another similar Heather Fly – Bibio pomonae emerges later in the year during summer in large numbers on moorlands; it looks very similar to the St Mark’s fly, but it has orangey-red thighs. That said another diagnostic feature of Bibio marci is the long spine at the tip of the tibia of the front legs.

A male Bibio Marci resting on a post in a characteristic pose.

St Mark’s Fly Bibio marci, is so named because it emerges around the feast day of St Mark which is celebrated on April 25. I am sure St Mark, martyr, author of the Second Gospel and patron saint of Venice never expected his name would be given to a fly though he does have links with the animal world through his symbol of a winged lion.

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