Well, I've learned that the bach ants are quite different from the town ants, which are definitely Argentine ants, judging by their behaviour and what I've been learning from the Pest Man. I discovered the difference when I found my muffin in the morning, and it was not the same as the muffin it had been the night before.
It had been a raisin muffin; but in the morning it was a raisin and ant muffin (at least what was left of it). An enthusiastic ant trail, of much smaller ants, ran along the walls, across the table and into the feast. But now I knew what bait they would take: not the protein one that I'd tried before, but the liquid sweet bait. Sure enough, within minutes they had transferred allegiance from my breakfast to the new elixir (but not elixir of life; sorry ants).