Huge swarms of these flies drift over plants and vegetation in spring normally around St Mark Day (25 of April) which is where they get there name from.
There legs dangle in a wierd way that may cause alarm, but they are quite harmless.
The females have much smaller eyes then the male, the female has smokey grey wings but the males wings are clear!
Here are some mating, the female is the is the bigger one and the male is the smaller one.
This is an example of how they swarm.