Q: My son found this in the wood chips. It latched onto him when he carried it, and had small little spines on its back.

A: A: It’s a luna moth caterpillar.

Note the white stripe down the side, which distinguishes this caterpillar from a prometheus moth caterpillar.

It is harmless. Put it back in the wood chip pile and allow it to form a pupa for the winter so it can hatch into a beautiful moth next spring.

see
http://www.hiltonpond.org/ThisWeek020801.html