Well, scorpions don't leave such a mark and are unlikely to show up in such numbers. Catepillars, however, do show up in groups. I once rubbed up against one of the saddlebacks on my arm, wrist to elbow, and thought that I had run up against a hot ember, unlikely while just walking through the woods. Welts, scabs and sloughing about a week later. Sounds like the critter, but visual evidence is better. Photos, anyone?