Insect is about 3 1/2 inches long. Has blue/yellow suction cup type feet. 4 orange spikes on top by head, 15 yellow spikes on top down to end, blue spikes down either side. Was found crawling on teak lawn chair in Vermont on August 27, 2011.
Your large green caterpillar is a Cecropia moth caterpillar.
It is one of the large silkworm moths in the Saturniidae family. Below is a website to see the moth that your caterpillar turns into.
The caterpillars feed on foliage of many trees and shrubs including maple, oak, cherry, poplar, ash, beech, apple, boxelder, dogwood, gooseberry, elm, and willow. The moths do not feed, but survive on the energy accumulated by the caterpillar.
This insect overwinters as a pupa, so when the caterpillar is finished eating it forms a silky cocoon that is attached to twigs …then the moth emerges the next spring.
Your caterpillar may be ready to pupate and is looking for a place to do so.