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OK the females began to arrive and the full behaviour of these aggregating males seems clearer. They mass on leaves in big groups 30-40 at times now and then suddenly a lot take off at once, buzz noisily around and land again. I think the take offs probably coincide with females flying nearby as the day this new take off behaviour became obvious, I found pairs of mating M. autumnalis on nearby leaves. Maybe the massed males attract the females who fly by and encourage force them to compete in their air for females (with some prallalels to "lek" mating systems in some birds, mammals and other insects). Not sure if they go into copula in the air (the paired flies can fly in tandem for sure) or more likely together and then court further before mating. As a behaviourist by training, I'd be interested to know if anyone knows if this has been described before in M. autumnalis. I can find a few references to close quarters pheromone use in courtship, but not to this massing behaviour of males. Here is a mating pair and then one of the male take off flurrries.