Aggregation of caterpillars in the process of skeletonizing a leaf.
Aggregation of caterpillars in the process of skeletonizing a leaf. (Black and white version)
Hunger strips the leaf bare, and then it’s over, all over again on the next leaf.
BugPhoto.net 
It’s interesting in that it appears they’ve eaten a dark green lay of the leaf leaving a light green layer. I think I’ve only seen insects eating a leaf and going all the way through leaving a hole or an edge showing the bite mark(s).
I could be wrong on this, but I believe this eating the top layer is known as “skeletonizing” the leaf; ergo such caterpillars would be “leaf skeletonizers.”