It is 10 times more likely to be a moth caterpillar than butterfly.

Because it was found on the sidewalk, the host plant is unknown and they don't eat cement, making the ID difficult.

Try posting on the Butterfly Garden forum; it is very active now and moth caterpillars pictures are offered up regularly.

IT has now spun a coocoon and it is gray and silky. IF that helps any one figure out what is is

That makes it a moth; get a picture after the adult emerges, this could take days to weeks.

Then we can ID it.