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> I wonder if undina-bird could post a closer crop of the pic viewing from the top? The angle from underneath might be giving a false impression.

This one the most "from the top" view I've got. Here is original size. Hope this helps.:)
There are black lines, though very thin ones?

Unfortunately, there is no S.dignota in the gallery on this site to compare, only S.selenitica http://www.diptera.info/photogallery.php?album_id=49&rowstart=780
Identified as Scaeva dignota — http://www.diptera.info/forum/viewthread.php?forum_id=7&thread_id=12115