Walk almost anywhere where there is rough grassland on the University estate right now and you will see the air full of jet black flies with long dangling legs. This is the St Mark's fly, so called because it is supposed to appear on St Mark's day (April 25th). Take this with a pinch of salt however. This year, the long cold Spring seems to have held them back.
The dangling leg flight is thought to be all about finding a mate. This settled fly is a male - they have larger eyes that meet together on top of the head.
Phil Gates' blog - Beyond the Human Eye (click here) has some excellent close up shots and more information.