The Pipevine that Polydamas was on wasn't very big to begin with and the two big caterpillars I found had eaten a lot. So I think he just happened to settle where there was enough room. I'm trying to remember if that was the one I suspected had gut purged. There was one I picked up that was too squishy for that size so I was thinking he may have been looking for a place to make his chrysalis. They generally leave the eggs alone, but since those had hatched, maybe they didn't "count".
