This Genus is just maddening...
No interactions present.
I have been wondering if there is something vigorous about Eristalis hoverflies e.g. in their feeding rate and being active earlier in the day (MM*). If so, perhaps some form of hybrid vigour got installed during their evolutionary development – which might bear on your point.
[ or perhaps the numbers we are seeing get kick-started at the larval stage in “organic-rich” puddles]
Lat/Lng: 53.4054, -2.8785
OS grid ref: SJ416902