Yeah, probably an Eastern Black, since you are east of the Rockies (west of them it's more likely an Anise Swallowtail, Papilio zelicaon. There is a tiny chance it could be an Old World Swallowtail too, Papilio machaon, though those usually only are found much further north, so any of those that show up by you are either really lost or the result of transport (say someone bringing in fennel seedlings with eggs from a nursery up north.
Besides EBS, I've also raised one tiger swalltail, one spicebrush (always like those caterpillars, they look like cartoon snakes!) a few Mourning cloaks, and one or two monarchs. I ALMOST did a few giant and silver skippers as well, but I put them out to pupate too early, and they crawled away.
The carterpillars go through five instars (molts) in thier lifetimes and change appearance DRAMTICALLY after each one. Yours are on instar 5 so they'll probably pupate soon. Assuming you are planning to keep and raise them the rest of the way, provide them some sticks or dowels to do this on (swallowtails are cantilevered pupators (they make a silk girdle and hang at an angle from the side), as opposed to things like monarchs that are danglers.
In a few weeks months the crysalides will open and the adult butterflies will emerge.
One warning, this early in the year the butteflies will probably emerge later this summer and you'll have no problems. However the next brood may be the spring brood, which overwinter as crysalides and then emerge in the spring. A good way to tell is by the color of the chrysalis; green, summer, brown winter. If you DO get a winter, it is important you put it somewhere like your garage or a patio over the winter, NOT inside. The higher temperature of your house will mess up their internal clock, and
you'll wind up with the adult butterflies emerging in the middle of January, when it's too cold for them to survive (I made this mistake a LOT and a lot of my emerged butterflies would up being passed on to members of the entomology department for a trip into the antesthetizing jar (I considered this more merciful than letting them freeze or starve.).
The chickens enjoyed them though.
