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At my observations it is typical for any Chrysomya, but for bright enough illustration we need a common species like Ch.megacephala or Ch.albiceps.
It may be also intra-species conglomeration as in:
http://www.diptera.info/forum/viewthread.php?forum_id=5&thread_id=5057
2 - Ch.albiceps, 1 - Ch.megacephala
I think that idea of such a gathering is: after laying egg(s) or larva(e) fameles wait for another eggs maturing, males wait to copulate with females. Why the hell should they to fly out?
Nikita