Bright green caterpillar

Sitting at my desk the other day, I glanced outside at the tree near my window and immediately noticed this GIANT caterpillar. While I think this is the type of caterpillar that cartoons and costumes are made to look like, I don't think I've ever seen one in real life. He was quite large, slow, and stood out on the dark colored bark. I wondered why the birds hadn't eaten him, but then I noticed all the hairy spikes. Mystery solved!