Eastern tent caterpillars are reported to be among the first caterpillars to appear each spring. The adult moth lays a clutch of 200-300 eggs that take nearly a year to hatch. The caterpillars stay together as a communal unit and construct a tent of silk in the wedge of a branch, a tent that they add on to every day as they grow. The caterpillars leave the tent as a group to eat, and return as a group, using the tent to rest and to protect them from extreme temperatures.

When I was a kid, we called them 'bagworms' (which apparently is wrong, the casual name for a different caterpillar). Their structures seemed rather icky. Unfortunately this morning I had no camera with me to photograph the most beautiful caterpillar tent I've ever seen, glistening in the clouded daylight.

Most of the info included here was found in a Wkipedia article on Eastern Tent Caterpillars.