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Hi Sara; I'm no expert on Chloropids, far from it, but am a trained behavioural zoologist so I notice distinctive behaviour and as I used to study pheromone communication in invertebrates, so was pretty sure I knew what I was seeing. A few months ago, around the time I posted some images of this behaviour : http://www.diptera.info/forum/viewthread.php?thread_id=29310 I did track down some online info on chloropids using eversible pheromone vesicles - check this: http://www.britannica.com/bps/additionalcontent/18/36877837/Male-flies-with-yellow-balls--New-observations-on-the-eversible-vesicles-on-the-postabdomen-of-male-Thaumatomyia-notata-Diptera-Chloropidae and think I came across some further academic references. I can't look those up again now as I have to leave home in a few minutes for a couple of days without email access, but will try to re-find them and will post them on the Chloropid wing-fanning thread for you if I find any (as we have gone off topic on this syrphid thread!).