They're a stampedin'

Recently we noticed a few caterpillars in our lime tree. Now, when the sun is out and warming the tree well, a whole herd of caterpillars arrives to congregate and warm themselves. I am close to eradicating them so they don't damage my tree, but I want to wait a little longer to see what they change into, if they do so of course. With that many caterpillars in one place you should see the pile of you know what below them. It looks like a mini cow corral.