27. Fanniidae – lost and found

16th May 2019

The day started off, as it seems to more often these days, with a hunt for a pair of missing spectacles. The degree of difficulty was much higher than usual however, as the search area was an acre or two of woodland with an understory of heather and bilberry and not much idea of where they might have fallen. There was the hope of finding green hairstreak butterflies, which would have been a notable first for me, but an easterly wind kept the temperature below their comfort zone and they didn’t appear. There weren’t even that many flies to distract me from the searching which was eventually brought to an unsatisfactory close.

Tantallon castle from Seacliff harbour

Hoping for better things we decamped to the coast where there was strong sunshine but an even brisker wind off the sea. A wall brown butterfly patrolled the sunny side of a stone dyke and hogweed flowers attracted a few flies, some of which were missed, but two at least were empids. Heading down a steep track through a knee-high blockade of butterbur leaves brought us down to a quiet sandy cove. As we ate our lunch, perched on a table-sized grassy knoll, a kestrel was equally poised upon an even smaller patch of wind, fluting and flicking as the eddies tumbled against the cliff above, then dropping down hopefully, but fruitlessly before a twist of the wing took it away elsewhere. There were plenty of coleopids on the wrack, and I found an interesting looking acalypterate on a rock which I caught and put in a tube. But at some stage the tube lost its lid and so too the fly. Along the way I also had managed to lose half of my pooter, invisible now somewhere underneath the butterbur despite the second search of the day.

Returning home we climbed up through woodland and along the outside of the high wind-protecting perimeter wall of Seacliff House, now a Manderley-like roofless shell, just needing one of the ravens of the day to fly over and croak for complete gothic authenticity. I swiped at a dance of flies enjoying a beam of sunshine poking through the sycamore canopy and caught one, expecting it to be another muscid. However, two features of its wings mark it out as belonging to the sister family Fanniidae. First, the vein nearest the hind edge of the wing, just by the tip of the thorax, is sharply bowed so that it would meet the faint short vein before it were they both extended to the wing margin. Secondly, if you look at the cluster of three veins that flank the wing tip, the next one along the front of the wing is thicker, and the thinner one after meets the wing at the end of a straight run, rather than being kinked as for muscids.

The males of this family (which this one was, as you can see from its almost touching eyes) are notorious sun dancers, poetically described in Coyler and Hammond’s Flies of the British Isles as “indulging in sheltered spots under trees, and seem particularly fond of flying in and out of shafts of sunlight striking through the foliage; at one moment they are brilliantly illuminated, and the next, they have apparently vanished. I know the feeling.

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