Another wet weather find from the woods, a tiny but distinctive fly with long, out-of-proportion legs peppered with needlessly vicious bristles and spurs, and joined to the thorax with what look like grains of rice. In comparison the thorax was tiny and so hump-backed that the head was tucked underneath it. I knew what this was, but, with an eye to propriety, I put it through the family key anyway. Does the fly have wings? – Yes; long antennae? – Yes; two anal veins on the wing and a V-shaped suture on the thorax? – No to both of those; ocelli present? These are the three (or sometimes two) primitive eyes that sit on the forehead (frons) between the larger compound eyes and they were missing. It keyed out as Ceratopogonidae, a biting midge.
Except that, that it wasn’t – it’s a fungus gnat, Mycetophilidae. Working backwards through the key to see where I might have gone wrong, it turns out that there should have been ocelli. Even on a small fly these are usually easy enough to spot since they are slightly raised up from the level of the frons on a beady-eyed triangle, often with bristles at either side. The frons was textured with little bumps, but try as I might I couldn’t convert any of them into ocelli, even on the highest magnification and with the lighting optimally adjusted.
Perhaps I was wrong about this being a fungus gnat, but the venation shown for Ceratopogonidae was all wrong, and apart from the missing ocelli, the fly had all the features expected for Mycetophilidae including the venation of the wings which is described as “characteristic (vein CuA1 and stem of veins M1 and M2 not connected or connected as far up as crossvein H).” A Royal Entomological Society key to the family from 1980 by Hutson, Ackland and Kidd says that the ocelli are sometimes in a row rather than a triangle, or can be reduced to two, either right on the midline of the head or on the very margin of the compound eyes. And indeed, I could almost persuade myself that in certain lights and angles there were glimpses of two tiny pustules at the highest and broadest point between the eyes – at least that’s how it seemed when I looked at it with my third, blind eye.