These caterpillars can give you a very serious sting with their venomous spines. If you see one, make sure you instruct any kids around that it is a very dangerous caterpillar. With my gardening gloves on, I carefully removed the leaf with the caterpillar still on it, and crushed the caterpillar on the ground with a small garden tool. I covered the smashed caterpillar with dirt because I had read that the spines can cause contact dermatitis even if they aren't attached to the caterpillar.
Apparently the treatment protocol for a saddleback caterpillar sting is to use cellophane tape to remove any spines; ice pack; then baking soda paste. See a doctor if allergic reaction occurs.
(Now, I will mention that I used meat tenderizer once when I got stung by some sort of stinging worm. I did get remarkable pain relief, but I've noticed that meat tenderizer is not a recommended protocol because it supposedly can damage skin tissues.)
Watch out for stinging insects, especially this time of year!