St Mark's Fly Bibio marci

 
St Mark's Fly Bibio marci is a very common and widespread fly.  The males make themselves obvious as they fly in swarms with dangling legs, often landing on cars (and people).  They have large bulging eyes, unlike the females, which, with their small heads and dark wings, can look like a different species altogether.  They are often seen mating (right), when the dimorphism is especially striking.

There are other species of Bibio in Highland, but the others are always either smaller, or fly at another season.  If you see a large, all-black fly like this during May or early June, about 10mm long, drifting lazily in loose swarms, you can be confident of its identity.  If in doubt send a picture for confirmation.

More pictures of the genus can be found here.  Its red-legged cousin B. pomonae flies in late summer.

 Bibio marci in cop.

St Mark's Fly Bibio marci in cop.

Photo © Murdo Macdonald.
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