This question comes up occasionally, and I thought this might be useful. The two species are very closely related, and there are probably specimens that will not be possible to identify from just a photograph, but the two do usually look different to a trained eye, and they can be told apart. These comments just relate to specimens from the United States, and may not apply equally well everywhere. First I am going to quote from the identification key in 'How to Know the Butterflies'
by Paul R. Erlich and Anne H. Erlich. Even though it is out of date, this is a very good book, and still quite useful. To use this quote, you may have to look up a diagram of wing venation, and what things like "postmedian" and "submarginal" mean. Comments in brackets I added. There will be exceptions to single characters in certain specimens, but even in those the combination of characters should make it possible to place a specimen in the proper species.
Dorsal forwing with postmedian spot in cell M2 longer than spot in cell M3 [these are the cells at the lower-outer end of the central "discoidal cell" and the one below it]; Dorsal forwing with postmedian spots in cells M3 and Cu1 [shift down one cell from previous comment] roughly spheroid [I would say broadly ovoid]; Dorsal forwing usually with only three submarginal spots (outside of the diagonal band); on SPREAD specimens the diagonal band is much narrower on inner [rear] margin of dorsal forwing than on costal [leading] margin of dorsal hindwing. --- Papilio cresphontes Cramer. Giant Swallowtail
Dorsal forwing with postmedian spot in cell M2 approximately equal to spot in cell M3; Dorsal forwing with postmedian spots in cells M3 and Cu1 nearly rectangular; Dorsal forwing usually with only four submarginal spots (outside of the diagonal band); on SPREAD specimens the diagonal band is not much narrower on inner margin of dorsal forwing than on costal margin of dorsal hindwing. --- Papilio thoas Linnaeus. Thoas Swallowtail
The three or four submarginal spots mentioned in the key above are relatively large spots. In both species there are sometimes more tiny spots in the same position in cells above these (don't count those).
Another helpful character. In most P. thoas
from North America the yellow band of spots across the hindwing commonly has the spots crescent-shaped; the lowermost/hindmost TWO large ones are usually noticeably crescent-shaped. In P. cresphontes
these are most often round or semicirclar. They are usually not strongly crescent-shaped, but if any are, it will most likely be the hind-most ONE.
There is also the fingernail test, but you have to have a male specimen in hand. It works fairly well. At the tip of the top of the abdomen, the base of the valvae (claspers) are abruptly narrowed in P. cresphontes
, leaving a little notch that your fingernail will usually drop into if you gently run it over the top surface (you can see the notch from the side too, but it may be obscured by scales/hairs). In P. thoas
, the valvae are not narrowed at this point, and there is no notch; it will feel smooth. This depends somewhat on the position of the valvae at the time, but works when they are in a "normal" closed position.
There are supposed to be pattern characters to help distinguish them on the lower surface also, but I've never learned those. Often in a photo you can see enough of the upper pattern showing through to make a good guess.
feeds on Rutaceae, probably exclusively so in the wild. P. thoas
may be restricted to Piperaceae in the wild, and I suspect that records for Rutaceae may just be assumptions, based on misidentification, or for reared caterpillars (I don't know, but it's worth checking out). I think this larval host choice may be one reason why the species is so rare in the US. P. cresphontes
can find food all the way to Canada and can produce broods in the summer. P. thoas
wanders too, but may not be able to reproduce far north of the border ???
Here are a few URL's to photos for each species. These are fairly representative of each species as they look in the United States. Also, look up these species here on BugGuide.
P. thoas autocles
[the lower photo by J.Ebner appears
to be P. cresphontes
and not P. thoas
- this certainly doesn't help with the confusion!]
There are lots of photos of South American Papilio thoas
on the internet and in catalogs of specimens for sale. Mostly these are from Peru. These are of different subspecies (usually ssp. cinyras
), and may look very different from ours (ssp. autocles
), don't judge U.S. specimens by comparison with those. However, specimens from Mexico and Central America are very like those that show up in the U.S. There is another species called Papilio paeon
in South America. It is also very like P. cresphontes
, and commonly confused with it (perhaps it is a subspecies of P. cresphontes
- I don't know). Pictures of it are often shown on web sites under the wrong name. It has more orange below than P. cresphontes
, a larger more prominent crescent mark in the middle of the hind wing below, and usually has more squared-off spots on the forwing above (it also is a very beautiful species).
Just a bit on P. ornythion
and male Papilio astyalus
here too. These species are also very similar in appearance, and turn up in the US occasionally. They are easily distinguished from the previous two, because the submarginal row of yellow spots on the upper forwing is close to the edge of the wing, and is made up of several fairly equally sized small spots (sometimes they are absent entirely).
P. astyalus bajeaensis
P. astyalus pallas
Hopefully this will help.
in central New Mexico