St Mark’s flies – the black dangly legged ones!

Around the beginning of May, you’re very likely to see groups of fairly large, black flies with dangly legs just hanging around in the air.  Quite probably, you’re seeing St Mark’s flies: St Mark’s day is 25th April – around about the same time they usually start to appear – hence the name.

A female St Mark's fly, Bibio marci.
A female St Mark’s fly, Bibio marci.

 

The flies that hang in the air with their legs trailing are the males. If you look on the vegetation beneath them, you’ll probably see the females, doing nothing much at all, just sleepily crawling around.

Female St Mark's flies lay their eggs on the ground and the larvae feed on dead plant matter as well as plant roots.
Female St Mark’s flies lay their eggs on the ground and the larvae feed on dead plant matter as well as plant roots.

 

These flies don’t bite or sting or anything. And they only live for a very short time. But, their larvae do feed on plant roots. Generally, however, they’re not a problem and nothing to be concerned about.

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