The gnats like to lay their eggs in the destruction and rotten remains of the aphids dinner so they appear to be co morbid. They all bring infection, bac,vir,fung
Looking closer at those photos I can clearly see a difference between the two like you said.
The bottom bugs have one particular "feature" that makes me think they are indeed the same bugs as the top pics. The translucent nature of the rear end and the legs. It is also evident in the rather "new" and undeveloped appearance of the wings compared to the first bug.
those in the pick a page or two back are most certainly fungus gnats not root aphids water with bt a few times and be done with it. your lucky there only gnats.
And finally, here are the tiny "eggs" I'm seeing all over the lower surfaces of the "infected" plants. I know, I know, they look like spider mite eggs but I don't think they are. I have NO adult mites, no stippling on leaves, and I've been down that road before. Also can't seem to find any of these hatching or ANYTHING crawling on my leaves.
like you suggested i think a second very similar species to fungus gnats but quicker and fast to develop none the less not an aphid species. the first pictures on the first page are the dreaded root aphids i have had pear shaped long round wings and green or black coloring also almost always on the move. i know the imid is the best way but i was able to eliminate them with repeated soil/foliage drenches of pyrethrum and canola some plants were badly damaged and were culled. the biggest mistake i made was letting them get out of hand and also overwatering which they love.I will acknowledge that I am still not 100% certain that the bottom image is actually a root aphid. Reservations are based mostly on the appearance of the front and rear ends of the bug (the head seems much more detached than in other aphids (where you get the "pear" shape and the head seemed to be a continuation of the body) - also the eyes (the multi-eye, fly-look) does not remind me of aphid eyes at all. Further, the thing on its abdomen (little forked "tail") looks more like the classic fungus gnat tail-end than it does like aphid cornicles (parallel tailpipes emerging from opposite sides).
While it makes sense that they would develop a different, downward-facing head (to see better as a flier) and that the cornicles would no longer be necessary, I do not know that this is what happens - I will admit that it still seems uncertain why there would be two different winged forms/why they would still morph after developing wings.
That said, I am nearly certain that the bottom thing is NOT a fungus gnat (from ALL the differences pointed out), and that it IS the fast-running/flying, more-aware bug that emerges in droves far greater than fungus gnats ever do. Perhaps this is a third species that is also capitalizing on damaged plants/decayed root, but because of their similarities (size, reproduction/life cycle, areas of interest [roots]) to RAs and the fact that they are associated with RA people/problems and that they are pretty clearly not fungus gnats leads me to conclude that they are a form of RA rather than something else entirely that no one has ever heard of before that acts just like RAs, is the same size and comes in after an RA occupation. This is just the simplest explanation, though I will admit I do not have enough evidence to be certain that the bug is an RA.
The only thing I felt that I displayed clearly with the image analysis was how different the two bugs we were looking at actually are; I stand by that and still feel it is plain as day.
-WB