Angie, zebra hatchlings are so tiny they look more like bits of dust than caterpillars. I (unsuccessfully except for two) raised my cats myself. As I recall, when the cats started dying outside was when I brought them in to raise myself.
They ate mostly at night, and stayed on the undersides of the leaves, but, as I recall, the tiny cats moved off the leaves a LOT. I was never sure where they went when they were outside, but I do recall finding a few on the trunk of the tree. If any had lived long enough outside to get big, it would have been easier to find them.
Before I post pictures of zebra swallowtail cats, let me tell you that my pawpaws get inchworms on them that have big wide heads and narrower bodies. These are the same cats that use sassafras, and I think they're tulip tree beauty moths, but I've never ID'd them for sure. Now that I've got three tulip trees in pots, I'll have to check them to see if I find any of these strange looking cats - that would indicate that they are indeed TTB cats. My Wagner book shows a picture of a TTB cat that has the same body shape as mine, but his is mottled tan. When mine are small, they're real dark, nearly solid black, but as they grow, they change drastically. David Wagner says of TTB cats, "Ground color highly variable, ranging from nearly yellow to mauve, orange, tan, gray, chocolate brown, and black, frequently with vague pinstripes." Here's a late instar cat with what could be described as pinstripes -

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If you find these, please let me know, and we'll try to make a positive ID on them. And if anybody else knows what these cats are please let me know!

Here's a ZST hatchling - I cropped the picture a lot -

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Early instar -

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Middle Instar -

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Pale late instar -

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Dark late instar -

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Just keep looking, and if you find a caterpillar that looks like any of the above ZSTs, congrats, I think. Raising them was my biggest caterpillar disaster ever. If I had it to do over, I'd leave them outside, and if they die, they die. It was heart breaking for me to see them dying under my care! :(

If you find the wide heads, congrats on them too, since I think they're tulip tree beauties, which make a beautiful, fairly large moth.

Sherry

About the inchworms -

I found another picture of the late instar inchworm with still different colors -

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And I just found a little one on a pawpaw and made its picture -

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Sherry

Oh, thank you, thank you, thank you!!! Very very helpful, and so good of you to take the time. I will seach them over again closely tomorrow. Now, I know what I am looking for. Something is REALLy chowing down on them. That orange/brown inchworm is so cute!

You're welcome, Angie!

I looked up TTB caterpillar, and the inchworms I've referred to are indeed tulip tree beauty moths.

I can attest to the fact that they like sassafras and pawpaw as well as tulip trees!

Sherry

Here is a link that might be useful: Tulip Tree Beauty Moth Cat

Well, I seacrched them all over really well, and I didn't find any caterpillars. I will keep checking though. Thanks again.

Pawpaw sphinx is a cat I ran into once that was eating a lot of pawpaw leaves.

Thanks kc. I am going to Google it now so that I will know it if I see it.