Hi there,
I probably brought in the aphids on some plant or other that I brought into my sunroom, "closed system" that it is in this season.
Thanks so much for the advice about using soapy water! I found some insecticidal soap spray at the plant nursery and sprayed the seedlings well, underside and top of leaves. I also removed them from their original trays and repotted them in clean pots with new soil. I THINK that the little green blighters are all gone--I don't see any more of them. The pansy- seedlings' leaves also seem to be OK with the spray--no signs of burning or other harm. Except to aphids, mwahahhahhha!
Anyhow, you all were very helpful and I thank you. My pansies thank you as well!