I used a twig to place it on some walking stones, then took some pictures.  This Moth was almost as big as my hand.  It’s a Polyphemus Moth.  If I’m not mistaken, this is a female as she has smaller antennae than the male would, and also had a very large abdomen.

Female Polyphemus Moth

I thought since they were on my Sassafras tree they must be Spicebush Caterpillars?  Could these be some kind of Moth larvae?  The tree is definitely Sassafras.

Would anyone confirm these are IO Moth caterpillars?

UPDATE: Nancy Greig, Curator of Entomology at the Houston Museum of Natural Science was kind enough to reply to my email about what type of Caterpillars these might be.  She also thought these were likely IO Moth Caterpillars.

Closeup of IO Moth Caterpillars

Well I’m not having one of those days, I’m having one of those weeks.  First, the Giant Swallowtail was getting nectar from the Milkweed, and now I “accidentally” find a cluster of Spicebush Swallowtail Caterpillars feeding on my Sassafras plant.  What confused me even more is that I have two Spicebush plants, but I didn’t see any Caterpillars on those, although that is where I would expect to find them.

Another reason I realize I know so little is because I’ve never seen a Spicebush Swallowtail in my backyard, yet I guess it made a visit fairly recently.

Oh, one last reason I don’t know much.  I’ve never seen really small Spicebush Swallowtail Caterpillars, so I’m just assuming that’s what these are since they do host on Sassafras.

Update: Alex was kind enough to point out that these are IO moth caterpillars, not Spicebush

Cluster of Spicebush Swallowtail Caterpillars On Sassafras