This saddleback caterpillar (found in Fairfax, VA) is about an inch long, has a purplish-brown body with a green saddle shape on it's back, and has poisonous spines on four large projections (tubercles) and many smaller ones that stick out from the sides of its body. The poisonous spines are hollow and are connected to poison glands. Contact with them causes a burning sensation and inflammation that can be as painful as a bee sting. The "saddle" consists of an oval shaped, purplish-brown spot in the middle of a green patch on the back. Saddleback caterpillars feed on the leaves of basswood, chestnut, cherry, plum, oak, and other trees and shrubs. They are generally found in late summer.
Note the white cocoons on the back of this caterpillar. These are braconid wasp cocoons. The braconid wasp larvae feed on the caterpillar from inside until they're ready to emerge through small holes that they make. Credit: Thomas A. Hermann, USGS