Eastern Tiger Swallowtail Chrysalises Arrived

Today our Eastern Tiger Swallowtail (Papilio glaucus) chrysalises arrived. What we plan to do with them. . .

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We failed to successfully get our hands on some of our native Western Tiger Swallowtail (Papilio rutulus) this year, but as of today we have its Eastern counterpart -- Papilio glaucus. Obtained from an Ohio breeder, we now have thirteen Eastern Tiger chrysalises. 

We are always curious to compare swallowtail species. Upon first examination, the glaucus chrysalises are virtually identical to our rutulus. However, these glaucus are surprisingly small, significantly smaller than either of our two rutulus chrysalises that we obtained from captive raised caterpillars last year though it is widely documented that the glaucus is supposed to reach a higher wing span. The largest glaucus chrysalis, a female, is only 1.2 grams with the majority of others around 1.0 grams or less (the smallest was 0.7 grams). They are still small when put side by side with the Anise Swallowtail (Papilio zelicaon) which is supposedly smaller than either tiger; our Anise chrysalises can get up to 1.5 grams in captivity and average 1.1 grams or so. And while we never measured the largest rutulus chrysalises, it was quite a bit bigger than a 1.5 gram Anise chrysalis. Tiny sample size, regional differences, and/or field guide lies. . . ?

Brown morph chrysalises of the machaon group (black swallowtails, or the true papilios by the more conservative taxonomer), such as our Anise Swallowtail, are strikingly similar to the tiger chrysalises; both are bark mimics. However, our Anise chrysalises are much thicker and stoutier and the wing pattern is distinctly different. While the tiger chrysalises have more or less unmarked wings with the dark pattern mainly residing around the perimeter of the wing, the Anise chrysalises have a lot of veins in the middle of the wing. The black on the tiger chrysalises are also much more mottled, or messy as we see it, with the black band along the side always very pronounced. However, the black band along the back is more pronounced in the Anise. 

While they are interesting to look at, the ultimate goal of obtaining these chrysalises was, of course, to breed them. The Eastern Tiger is supposed to be multivoltine in much of its range so we suspect that our chrysalises are probably not diapausing and will eclose soon, especially because one or two of them already feel slightly soft and loose (but not dead, of course). We would have liked if they are diapausing though, because it is so late in the season to rear them. If it takes two months or more for the butterflies to emerge, to hand-pair them, to get them to lay eggs, and raise the resulting caterpillars all the way until they pupate, it will be well into October or even November, by which most of their deciduous host plants begin dropping leaves even in our mild climate. 

Whenever if we should have the chance, we are also hoping to do some hybridization experiments with these glaucus, given that it is a rare opportunity for us to have them. It is a great way to study the evolution of closely related species. While not necessarily always the case in other species complexes such as the Eastern Giant Swallowtail (Papilio cresphontes) and the Western Giant Swallowtail (Papilio rumiko) which have been hypothesized as actually being sister species diverging from a single ancestral species rather than off each other, our Western Tiger does seem to be a distinct regional counterpart to the Eastern Tiger. In older classification, the Western Tiger has actually been treated as a subspecies of the Eastern Tiger, along with the Canadian Tiger Swallowtail (Papilio canadensis) with which the Western Tiger is even more closely allied to than the Eastern. Successful hybridization between these three species is probably possible. In fact, it has been shown that the Appalachian Swallowtail (Papilio appalachiensis), which exists where glaucus and canadensis can overlap, is a species of hybrid origin between the two that can, but does not back cross with its parents naturally.  

Ironically, the Pale Tiger Swallowtail (Papilio eurymedon) and the Two-Tailed Tiger Swallowtail (Papilio multicaudata), both of which are western tiger species that overlap in some parts of their respective ranges with rutulus, according to some sources, do not show as close of a relation to rutulus and have been considered separate species from the start. One hybridization experiment between glaucus and eurymedon has been shown to yield high infertility with virtually no surviving female offspring, though other experiments have shown that both glaucus x eurymedon and glaucus x multicaudata crosses can be viable (averages of 30.2% and 44.0% respectively). In our unprofessional opinion, the three western species do seem to be more related to each other than any of the eastern ones since the caterpillars all share the double eyespots whereas the eastern ones have one eyespot.

The Mexican Tiger Swallowtail (Papilio alexiares), shown to be more closely related to the three western species  than glaucus, somehow is more compatible with glaucus -- 56% mean viability in one study and a mean viablility range of 28.4-96.9% in another.

What we eventually plan to do is attempt a crossing of a female glaucus with a wild caught male rutulus. Hybridization experiments often yield different results based on which species was the mother and which was the father, though we have no idea whether this pair will be any good. One study cites that successful hybridization is only possible with a male glaucus and a female rutulus, with the females chrysalises never emerging if obtained from a cross using the opposite sexes of the respective species. Another study showed that female glaucus x male rutulus has only 31.1% mean viability rate. But if what we do really is successful and the female offspring do not die off or are infertile, by back-crossing with additional male rutulus, we might even be able to create something that is more or less rutulus which is what we have really been seeking this whole time anyway as they are the native species (of course, a DNA analysis would probably yield still evidence of hybrid origin!).

In fact, we might even be able to shuttle some new genes into rutulus, such as those responsible for the black form (form nigra) found in virtually all fluted swallowtails (tribe Papilionini) worldwide including glaucus but, oddly, not in canadensis, alexiares or any western species. The gene controlling this black coloration is quite evidently sex-linked since it is pretty much only observed in the females (located on the Y chromosome) and daughters are the same color as their mothers. However, in hybridization experiments between form nigra glaucus and each of the various species without form nigra (rutulus, multicaudata, eurymedon, and canadensis), some suppressor genes caused there, in varying degrees, to be less black than normal in form nigra offspring. So perhaps they just doesn't like being black. After all, it has been well documented that black females are less attractive mates, probably because the males have a hard time recognizing them as members of their own species. While it seems that natural selection would quickly weed out the black form in all fluted swallowtails, the long standing belief is that the form nigra is beneficial as a Batesian mimic of the poisonous Aristolochia (pipevine) swallowtails (tribe Troidini). Too many mimics would cause the birds to learn that they are harmless and accordingly, natural selection favors nigra more where Aristolochia swallowtails are more common. And it just so happens that the only Aristolochia swallowtail that overlaps with rutulus, the Pipevine Swallowtail (Battus philenor hirsuta) probably only arrived here recently. It is easy to see, then, why the expresssion of the nigra genes are suppressed in the western swallowtails, although this "anticipatory" theory is shaky at best and has yet to be proven. 

The left shows our thirteen chrysalises; the middle compares a glaucus chrysalis (right) with a zelicaon (left); the right compares a rutulus chrysalis (left) with a glaucus chrysalis (right)

- Brian