These brown cockroach nymphs are really hard for me to ID. I would guess Parcoblatta but it would be difficult to be certain.

The clipping off of the antennae reminded me of the Cockroach Wasps, which inject mind-numbing venom into the cockroach's brain, clip off the antennae, and then lead the helpless creature to an underground lair by the stumps left from the clipping. The cockroach is eventually eaten alive by the wasp's offspring. I think I would rather be hacked apart by ants than face that fate. We have some nice pictures of those Cockroach Wasps:

I figured it might be hard to identify, was hoping the ants flipping it over might be helpful though. Speaking of cockroach wasps, I saw this one on the same log a few minutes after the ants hauled off the nymph. It even ran around the ant colony entrance in the log for a bit.

I thought it strange for a roach nymph to be out and about during the day where ants could find it so easily, and then not to scurry away quickly when attacked. The roach had probably been stung by the Cockroach Wasp already, and was taken away by the ants. Your pictures tell a very interesting story.