Basic Comparison of Papilio rutulus and Papilio glaucus Females

A female western tiger swallowtail (Papilio rutulus) eclosed; stock originated from a single pairing between reared female and wild male in Albany, California, Summer 2017. ​We also captured a female eastern tiger swallowtail (Papilio glaucus) at Cornell Botanic Garden. This post makes a basic comparison between these two representatives of their respective species.  

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Today, we had a medium sized Papilio rutulus female eclose from pupae originating from Albany, California. We also happened to catch a Papilio glaucus female at at the Cornell Botanic Garden. This is the first time we have had live specimen of the two similar species side by side, so we took this as an easy opportunity to compare them. Additionally, since the geographic range of both these species is quite vast and significant regional variation in phenotype is expected, this comparison may offer insight for the California Bay Area and Upstate New York type localities.

​Below, the rutulus female is shown to the left, the glaucus female in the middle, and the two species side by side to the right. 

Granted, the glaucus female has been free to rome the wild for a while and therefore not as pristine as the rutulus, it does seem that the glaucus is a paler shade of yellow with much more pronounced win venation. The rutulus is a deep, rich shade of yellow, creating an overall yellower appearance, similar to another western species, P. multicaudata. Although the venation is less pronounced in the rutulus, as well as some of the internal black stripe patterning, the black bordering the wings seems to be thicker proportional to the total wing area. Like the other rutulus females we have seen, there is a slightly visible yellow stripe running between the black border of the forewing, which contributes to making the black appear thicker. These elements make the rutulus appear "cleaner".

Interestingly, neither of the pictured individuals have much blue. This is interesting, at least for the glaucus, since glaucus females are known to have more blue coloration than other "tiger" species, with one hypothesis for this being that the dark form evolved to have more blue coloration to be better mimics of Battus philenor and that the yellow form retains this trait (reference). The dark form does not seem to be common in Ithaca and B. philenor is not present here. Additionally, the pictured female is quite small and small individuals tend to have less blue coloration proportional to total wing area, perhaps because blue is a costly color to produce and small size correlates with poor larval diet. Similarly, the rutulus female is smaller than the other females that eclosed this year from the same progeny and these other females have a bit more blue. Another thought is that the glaucus in Upstate New York are similar to P. canadensis, which is smaller than glaucus and lacks extensive blue coloration. They fly sympatric to glaucus in some parts of this region and likely hybridize, but probably not here in Ithaca. 

​A point to make is that the glaucus has more of an orange coloration at the base of the hindwings rather than a reddish color, and the uppermost discal spot is orange rather than yellow. This orange spot is present in other eastern species like P. troilus and P. polyxenes but not in any of the western species that we know of. The role of this spot is unclear, especially because the area of the wing that it takes place is usually covered by the forewing when the butterfly is at rest. One possibility could be to mimic B. philenor (troilus and polyxenes are mimics, too), which has similar orange spots on its hindwings; however, these spots are limited to the underside of the hindwing and the mimicking species do also have orange coloration on the underside of their hindwings.

One last thing to note is that the glaucus has an overall much more convex wing shape whereas the rutulus is more concave. Both the forewings and hindwings of the glaucus are shorter, rounder, and less sharp at the ends. Although the swallowtail tail area of the glaucus are a bit torn, it does seem as though that the tails are also shorter than the rutulus's. These features make the rutulus appear more triangular. In addition, the chain of yellow spots along the wings are rounder in the glaucus, although both species likely can have both extremes of this trait.  

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​Brian Liang 
Brian Liang is a student at Cornell University in Ithaca, New York pursuing an undergraduate degree in entomology. He is co-owner and a main contributor of the Liang Insects blog, insects articles, and site design.

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