I saw this caterpillar while taking a walk in the woods near my house yesterday. I didn't have a container with me so I resorted to the ol' on-my-head-under-the-hat method of carrying insects. The caterpillar, which was a little longer than 4 cm, behaved itself for about a half an hour and didn't do anything nasty to my scalp.
This is a caterpillar of the butterfly spicebush swallowtail (Papilio troilus). According to Wagner (Caterpillars of Eastern North America, Princeton Field Guides, 2005), the caterpillars, normally green, turn yellow or orange when they are ready to pupate. The color of this particular individual was very striking. It is now back in the woods and will hopefully turn into a butterfly one day.