I saw this on the western andean slopes in Ecuador – fascinating! But what is it?
Caterpillars that move in this manner are generally referred to as Army Worms. They travel in a mass to the next source of food.
Update: March 26, 2016
We just received a comment indicating these look more like Sawfly larvae. Looking closely at this nine year old posting, and considering what we have learned since then, we are in agreement.