This was a fascinating caterpillar found during a night walk. Years later, I learned that this is referred to as a "Shag-carpet Caterpillar," which is actually a mimic. Those white hairs on the bag emulate the eggs of a parasitic wasp, whose young invade and kill their caterpillar hosts. This disguise keeps the caterpillar from actually be attacked by the wasp. Thanks to Chien Lee for the information and ID.