Nature Notes # 42
Last Saturday I was stood with Bill at the viewing banks overlooking No.6 tank looking for an odd-looking aythya duck that had turned up earlier in the week (the duck was not present). It was a really warm day and the intake of refreshments was necessary so, Bill had brought one of his essential and most cherished possessions along in the form of his Sigg water bottle which he perched precariously on top of the nearest fence post top. The fact that the flask had many dents in its aluminium shell indicated that it had fallen in the line of duty on more than one occasion in the past and each dent had a story to tell.
However fascinating this may be it was about to have a new chapter in its illustrious history. The interesting thing was that both the fence and water bottle were soon covered in literally hundreds of tiny black flies. All of these flies were engaged in some form of mating ritual and whenever Bill wanted a drink he had to disturb them from their activity, a quick swig and the bottle was placed back in position for their ritual to commence.
When disturbed, they would fly about us in a swarm before returning to the fence post and Sigg. I took a couple of photos and sent them to the Dipterists forum for their experts to pass an opinion on. One knowledgeable member supplied the following information.
The family Scatopsidae , is commonly known as Scavenger Flies or Dung Midges. The former name is better I think as the first is somewhat of a misnomer based on a mis-spelt scientific name. The ones on the fence post could have been Anapausis soluta, which, like other Scatopsidae, form swarms which include an equal number of males and females. This is different to the Chironomids, or true midges where the males assemble and wait for the females to arrive. In the swarms of Scavenger Flies there are usually mating couples, indicating that it may be a mass emergence event we witnessed, which would end in mating and dispersal, somewhat like St Mark’s Fly Bibio marci.
Written by Tony Broome and image 2. Additional images & video, 1&3: WSM.