This spectacular fellow is the saddleback caterpillar, Acharia stimulea. It is named for that bright green “saddle” on its abdomen, but its most notable feature is the four tubercles (one on each “corner”), each featuring some serious-looking, spiky armament. The sharp spines on those tubercles are venomous, and will break off in your skin if handled, exuding irritating substances that will ensure you never touch one of these twice. (Not fatal, just extremely memorable!)
The saddleback caterpillar is a slug caterpillar, which means that its body shape is such that its legs are totally hidden behind a “fold” of “fat” hanging down from the sides of its body. This results in a really amazing looking walking style:
On their rear ends, A. stimulea have a pair of bright greenish-yellow ovals which can be taken as eyes, making its rear end a frightening face. A lot of caterpillars do this, encouraging predators to attack their back ends rather than their heads. The head is actually kept tucked down and is relatively inconspicuous.
Saddleback caterpillars are found all over eastern North America and are commonly found near human habitat, as they eat a variety of plants. They aren’t aggressive (they don’t need to be) but since they are so pretty children have been known to pick them up. They also hide under leaves and occasionally accidentally fall out of trees onto people. If you do come into contact with one, don’t panic; gently remove it using a stick or other tool. Don’t pick it up with your hands, and definitely don’t squish it!