Caterpillars on the move

If these caterpillars hadn't been bunched up, I doubt that I would have noticed them on the cement floor of the back porch. They were that small. What really made them noticeable was that they were on the move, as one single unit, caterpillars on top of caterpillars. A pile of moving blackness. It reminded me of a similar scene once on a walking trail near the Napo River in Peru. The difference was that they were very large caterpillars and there were many more individuals. They actually took on the appearance of some medium sized animal moving across the trail.