I grow three species of willow: Salix caroliniana, Salix babylonica and Salix matsudana 'Tortuosa'. I find these chrysalises on them frequently. Can't decide if they belong to a Viceroy or a Red-spotted Purple. Which species do you think it is, and why?
To make this interesting, both species have been recorded on the Outer Banks of North Carolina, where I live. Thanks for your input.

Even so your pupa could be Limenitis arthemis astyanax, Limenitis archippus archippus or even Limenitis arthemis arthemis.

While some lepidopterists' in locations where multiple species overlap think they may have enough long term experience with possible local species clines (and so their variations) to do so, as Angie suggested, in these species it's very hard to impossible to determine species/subspecies in the pupal stage.

An example... A very serious, over the top lepidopterist I was associated with 20-25 years ago spent many years studying a specific vast area in the west where the recorded ranges of several Limenitis species/subspecies did or could over lap. As well as for many other specifics, the research included determining if the species/subspecies could be identified from the larval and pupal stages. End result I believe was yes from larval stage (at least to species level), but only maybe from the pupal stage. It’s been a long time since I read his paper written from the study, but if I remember right the Limenitis species recorded in the location he studied included at least:
L. astyanax arizonensis (others referred to this bug as L. arthemis arizonensis)
L. archippus obsoleta
L. archippus lahontani
L. weidemeyerii latifacia
L. weidemeyerii angustifacia
L. weidemeyerii nevadae
L. lorquini lorquini
L. lorquini burrisonii

HTH,
L.