What is "bagworms"

Usage examples of "bagworms".

Near the huge gash was… tree disease… some sort of brownish lumpy sac up in the branches, such as you see in trees infested by bagworms, and there were yellowish curds on the branches around it, as if the disease had caused the sap to ooze out and fester and congeal—except that it couldn't be sap because it was streaked with blood.