Stinging Caterpillars of the United States:

http://www.arkinspace.com/2011/05/stinging-caterpillars-of-united-states.html

I've seen kids get painful stings. A friend grabbed one and his hand became painfully swollen but it got better over the next hour or two. A guy at a camp site I was at had one fall/migrate into, possibly was placed as a joke into, his boot and he stuffed his foot in without checking. He said his foot was 'on fire' and the foot was swollen up to the knee. He started to go into anaphylactic shock. The EMTs saved him.