Monday, May 10, 2010

Four-eyed Fly

These remarkable eyes belong to a male St. Mark's fly Bibio marci - which has not one pair of compound eyes, but two. This is the black fly that dances just above the grass on spring days, dangling its long hind legs. You can find a picture of this behaviour on Nyctalus's Stand and Stare blog. These dancing males are on the lookout for females, which they approach from below, and the conjecture is that those long, fine hairs in between each individual compound eye lens (ommatidium) somehow help in the fly's detection of movement above and precise positioning when he grabs a female.

Remarkably, male and female St.Mark's flies have quite different eyes and are also distinctive in other respects. In this mating pair the larger female is above, with the more slender head, heavier body and smokey-coloured wings.

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. This fly has four eyes (as do whirligig beetles which swim on the surface of ponds and simultaneous look towards the sky and down into the water).

The female's heavier body, smokey 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.


  1. This is great Phil - very intriguing. Its good to know that there is a Bibiophile out there somewhere who has worked out all that visual system stuff. So, both males and females dangle their legs in flight. But why the long pair of legs in the first place I wonder? Some kind of visual display perhaps....

  2. It's intriguing to think just what does a compound eye see. Presumably it can't focus so I guess it just senses changes in light level and direction of movement. Are they a decling species? I don't seem to see as many about as there used to be years ago.

  3. I love your taking us way beyond everyday life!

    Keep it up!!

  4. Hi Nyctalus, interesting question. could be a visual display, or for gripping during mating in flight, or for clinging onto grass stems in dense vegetation....maybe?

  5. Hi John, there have been a lot of St. Mark's flies around here this year. It's though that compund eyes produce a pyxelated image whose resolution depends on the number of facets in the eye - but it's impossible to really know what a flie sees. But compound eyes do have very sensitive flicker vision - the ability to detect movement as the image flickers across the eye facets - which is why it's hard to swat a fly!

  6. Will do Ron - thanks for your encouragement!


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