Small Fever Fly Bibio lanigerus
Despite their name St. Mark's Flies (also known as Fever Flies) are mosquitoes and not flies. The females usually have small eyes, but the males have very big eyes indeed and that's why they're called flies. In springtime you often find numerous small fly looking mosquitoes of the Bibio generus. Dilophus species also appear in spring, but their small eyes give them away easily. And then there are many other species of small black flies and mosquitoes around in spring, most of which are simply called Black Flies.
There is a huge difference between males and females, as you can tell from the pictures: the male is in the top picture, the female in the bottom picture. The same difference exists in all Fever Flies.
Bibio lanigerus or the Small Fever Fly is a rather small and hairy species. It is a very common species in most of Britain, but you have to look carefully if you want to identify it. It looks like the Hawthorn Fly a lot, but it has reddish legs, at least partially, while the Hawthorn Fly has black legs. And there is no black dot in the wings, like in some other species. The vein running parallel to the fringe of the wing may be highlighted in black though. The Small Fever Fly is smaller than the the real March Fly reaching a length of 5 to 8 millimeters.
