A PROMINENT CATERPILLAR
While I chainsawed firewood the caterpillar  turned
up on a fallen oak limb. I sent the picture to Bea in
Ontario who quickly replied that it might be the
Mottled Prominent, MACRUROCAMPA MARTHESIA, and after
my own Googling I'm thinking she's right.

I knew that Bea didn't have a caterpillar field guide
so I asked her how she came up with the ID so fast.
Her reply describes a search-engine procedure that
might be useful to anyone with an unknown organism and
an Internet connection:

"I usually look for something on our specimen that
would set them apart from other insects of their type. 
After a simple search for "green caterpillar" "white
stripe" yielded way too many images and after
searching the first ten or so of those pages without
getting close I decided to try and find out the name
of those things that were sticking out his rear end. 
After finding out they were called "anal prolegs" and
the caterpillar in our picture had "long anal prolegs"
I did a search on "long anal prolegs" caterpillar ...
and after searching those images I came across a
caterpillar called Prominent, but it didn't look at
all like your picture except for the long anal prolegs
so then I searched on Prominent and eventually came
across one that looked like yours."

Once you have an organism's name you can search on the
name to find interesting stuff about it. A quick
search on "Macrurocampa marthesia" turned up a picture
of the adult moth the caterpillar eventually
metamorphoses into, after passing through its pupa
stage, as well as its distribution map.

Apparently there's not much more known about this
particular species, but the same site gives an
overview of the species' family, the Notodontidae.
There I learn that when resting the moths hold their
bodies in a way causes them to look like a stick. Most
prominents overwinter as larvae, pupating in cell in
the soil or in loose cocoons on the ground. My
caterpillar was big enough to have been looking for a
spot to metamorphose into an overwintering pupa.