St. Mark's flies have amazing eyes

There were swarms of these little black St. Mark's flies Bibio marci hovering over the hedges around Tunstall reservoir in Weardale earlier this week. Their long dangling legs are a distinctive feature and they have extraordinary eyes.

During their brief one week life male  St. Mark's flies spend much of their time either hovering in swarms, attempting to attract a mate, or resting on leaves on the sunny side of hedges and woodland edges.

These remarkable eyes belong to a male. This is the black fly that dances just above the grass on spring days, dangling its long hind legs, on the lookout for females which they approach from below. It may be that those long, fine hairs in between each individual compound eye lens (ommatidium) somehow help in the fly's detection of movement above and help with precise positioning when he grabs a female.

Both sexes have one pair of exceptionally long legs, that dangle below in flight, although the extent of the difference in leg length is over-emphasised in this picture of a male because the front two pairs of legs have curled up in death.

This head-on view of the male reveals a distinctive horizontal groove across each eye, just below the mid-point. In fact upper and lower eyes are quite separate on each side and have separate connections to the brain, where the images they produce are processed separately - the upper eye on the look out for females, the lower monitoring ground position. 

The female's heavier body, smoky wings and narrow head are evident in this side view, while ....

... a close-up view of the female's head reveals that her eyes are smaller and hairless - but then she doesn't have to worry about finding males; they find her with their strange two-tier visual system.

In this mating pair the larger female is above, with the more slender head, heavier body and smoky-coloured wings.

St. Mark's fly gets its common name from the belief that it always emerges on St. Mark's Day April 25th. The cold spring seems to have delayed their emergence this year.

St. Mark's Fly

St. Mark's fly Bibio marci gets its common name from the belief that it always emerges on St. Mark's Day April 25th., which isn't too wide of the mark because swarms of these flies have emerged over the last few days.

They usually hover up and down just above the grass but in this morning's overcast weather literally thousands were resting on the sycamore leaves on the edge of a woodland just south of Durham city.

These flies have remarkable eyes - four in males, which you can read about here and you can find an interesting post by Nyctalus on these common insects, showing one in flight, here.