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.
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.
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.