While striking with green markings and a brown dot in the middle resembling a saddle, the saddleback caterpillar (Acharia stimulea) comes complete with a set of spiny horns fore and aft. Spines are also found along the body.
When prodded, the caterpillar arches in such a way so as to bring the maximum number of spines to bear on its attacker. And about these spines – they are filled with venom both hemolytic (destroys tissue) and vesicating (blistering) making the sting more painful than a wasp or a hornet.
Which is all to say if you accidentally grab one when putting your hand on a fence post, be prepared for a major “ouch” and a few hours with an ice pack.