I encountered this tree branch on a short hike on a rainy day while Deb and I were sailing in the Virgin Islands. I'm not sure if the caterpillars were seeking shelter from the rain on the underside of the branch or if they were up to something else. Though what caterpillars might be "up to" is a bit beyond me, not that I would attempt to pass judgement on the purpose of a caterpillar's life. . .