Living in Texas where both species occur, I see ID’s on iNat which I think are questionable. However, even authoritative sites like butterfliesandmoths.org seem ambiguous on the distinctions. Where can we find succinct accurate descriptions?
Typically they are distinguishable only by location. In Colorado, they can be distinguished by altitude since the Papilio glaucus seems to not go far, if at all into the mountains. I am unsure that there are legitimate verifiable records of Papilio rutulus in Texas, and many guides/books do not contain Texas in their range.
The western tiger swallowtail (Papilio rutulus) doesn’t occur in Texas. Did you mean the two-tailed swallowtail (Papilio multicaudata)? I just scrolled through the first few pages of P. multicaudata and P. glaucus observations in Texas and they look pretty accurate to me. There are a many pretty reliable differences between them:
- P. multicaudata has 2-3 well developed tails on the hindwing whereas P. glaucus only has one.
- The amount of blue on P. glaucus is sexually dimorphic - males usually have little or none whereas in females it is very extensive and iridescent. In P. multicaudata it is monomorphic; both males and females have a moderate amount and it is not quite as iridescent as in P. glaucus.
- The forewing apex in P. multicaudata is more pointed whereas in P. glaucus it is more rounded.
- The ventral forewing submarginal spots in P. multicaudata are connected into a continuous band whereas in P. glaucus they are distinct spots.
- P. glaucus has thicker vein lines.
With the exception of the first one, the above points regarding P. multicaudata are more or less true for the other two western species, P. rutulus and P. eurymedon although there are several differences in wing shape and coloration between these species as well.
Thank you for your very definitive descriptions, Alan. I was mistaken in thinking the distribution maps overlap in TX.