3rd instar of a Black Swallowtail -- Papilio polyxenes
over a period of about a week, a few somewhat similar caterpillars
appeared on the Bishopsweed in one area of my garden. Although the size
and markings differed a little, and some of the caterpillars had spines,
there were enough similarities to suggest that they were either similar species
or that they were a variety of instars of the same species.
I've been looking around for information on these caterpillats and
finally came across an excellent page with photographs of the instars
of the Black Swallowtail butterfly (Papilio polyxenes) on
the Butterfly Gardening and Conservation website.
It turns out that the caterpillars that I found were 3rd, 4th, and 5th
instars of the Black Swallowtail butterfly.