saddleback caterpillar

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The brown and green larva of a moth (Sibine stimulea) of the eastern United States, having a brown saddle-shaped mark on its back and stinging spines that are mildly poisonous.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A caterpillar - the larva of a moth native to the eastern U.S., (Sibine stimulea) - which usually has a green and brown saddle-shaped mark on its back.