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The surstyli are the projections at the tip of the epandrium (the external genital segment that you have tweaked downwards in these specimens). In some ephydrids the surstyli articulate with the epandrium. In some (like Setacera), the surstyli are fused with the epandrium and each other. In your final picture and in the first, you can see the small pointed projection at the tip of the surstylus, which would agree with breviventris. However in your 6th picture, tergite 5 appears to be about the same length as tergite 4. Of course, it's not unusual for two or more species of Setacera to occur together. Unfortunately I have no male trina to hand, so I cannot eliminate that species. Best to get them checked before adding to the list! :)