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In spite of the fact that some flies really has interesting symmetric abdominal appendices (I observed Lauxaniidae?-Sapromyza? females exposing small "hornlets" right before/at mating, I think about those pheromones...) these "bombs" may be parasitic cocoons, of course. Their symmetry may reflect specific oviposition scenario: when parasite put only 2 eggs just in these special places of this small carrier, it increase probability of larvae survival. I noticed the fact that carrier is alive (and in good health?) when larvae already reached pupa stage...

P.S. BTW I noticed also barely visible lines resembling notching on "bombs", on my first photo... And the hind view looks strange for me...Bean-shaped? :|