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I'm puzzled as to why your initial photos show an all-female swarm. I can suggest two possible explanations - one is that this species is parthenogenetic, and males are absent (or very rare) - this would suggest that the males you photographed are a different species. Or that the initial photo shows a mixed swarm of males and females - some species of chironomid (e.g. Corynoneura have males that look very like females (short antennae and compact bodies). Again, it would suggest that the later photo is of a different species. Of course, John may have a simpler explanation! :)