Here's a thread over at ICMag that has some pics:

Fungus gnats or WINGED ROOT APHIDS??? - International Cannagraphic Magazine Forums

They are really easy to mistake for fungus gnats. I thought they were fungus gnats until I noticed they were kinda hanging out in my buds. And I took a microscope to them one day and noticed they were laying eggs or giving birth to little spider-like things...

The symptoms for me were basically a phantom nitrogen deficiency. The plant was slowly yellowing... although it was the entire plant... not just lower leaves. It was not helped at all by high-nitrogen nutes. By then it's pretty much too late.

Nematodes supposedly work well as prevention. But it appears there are a few different kinds of root aphids.