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Though this was straight from the horse's mouth, dear Tony, I'd like to add some pics and facts. This flowering Prickly Cucumber obviously attracts small midges and mosquitoes more than bigger flies or Hymenoptera. (Eristalis intricaria and a Sphecodes species were the only bigger visitors I saw.) It was noticeable that the small flies died in numbers, their bodies lay on every leaf. Beside Dilophus febrilis (Bibionidae) and the now identified ceratopogonids, many culicids of both sexes fed on the flowers, and several ceratopogonids were present as well. The small green ones were more frequent than the bigger brownish ones, and they did not only rest on and below the leaves but worked in the flowers. There were no aphids on the plant, so honeydew could not be the attractant. I must confess I do not know whether the unisexual flowers of Echinocystis produce nectar or possibly oil (like other taxa of the family), but these small flies are certainly no pollen-eaters and so fond of the flowers; so there must be some award. Something that even Chironomidae like!
Sundew
The first pic shows the visitor spectrum.