March Flies - Bibionidae

To identify flies to family level use a simple wing venation key such as A Key to the Families of British Diptera, an AIDGAP by D. Unwin, published by the FSC and available as a free download (scroll down to the titles list) or a more detailed key which includes many physical characters such as The European Families of the Diptera by P. Oosterbroek. The expert dipterists on the DipteraInfo forum are also very happy to identify, often to species level, from good photographs.

These primitive gnat like flies are not to be confused with the group of flies in Australia known as March Flies (which are Tabanidae, called Horse Flies in Europe).

European March Flies are often called St Mark's Flies if they are of the genus Bibio (because they appear around St Mark's Day, 25 April) or Fever Flies if they are of the genus Dilophus.

Female Bibio cf marci (below). 'Cf' means the species identification is not certain, but it is probably the species mentioned, or a closely related species. B. marci is a common species with completely black legs. Photographed in April.
They can be present in large numbers in the spring, drifting listlessly around in long grass, legs dangling. Most of them are black, but some of the females are red-brown. To identify them to species level you need to observe whether they have dark or light hair and whether their legs are entirely black or have some red-brown.

Saint Mark's Flies B. marci mating on a pear tree in our orchard. Male on the left, female on the right.

Female Saint Mark's Fly B. marci resting on a Ribbed Plantain Plantago lanceolata flower bud by the side of a farm track.

Saint Mark's Flies B. marci mating on a grape vine in our orchard. Female to the left, male to the right.

Male Bibio cf lanigerus, a fairly common species with pale hair and brownish legs (below). Photographed in April.
Males and females look rather different because the males have large eyes and females small eyes.