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Reptile and Amphibian Atlas


 

Tiger Swallowtails: Making Observations in 2020

Alan Macnaughton

The article More on Ontario Tiger Swallowtails by Chris Schmidt was published in April 2020 in the TEA's seasonal summary publication, Ontario Lepidoptera 2019. Here I provide a summary of the aspects of this article that are most relevant to a butterfly watcher. At the end I draw conclusions for how to make and report 2020 tiger swallowtail observations in Ontario.


The species

Three kinds of tiger swallowtails are believed to occur in Ontario:
- Papilio canadensis (Canadian Tiger Swallowtail).
- Papilio glaucus (Eastern Tiger Swallowtail).
- Midsummer Tiger Swallowtail, a possible new species.

For many years, TEA seasonal summaries have reported tiger swallowtail specimens which show a mosaic of characters from both P. glaucus and P. canadensis. People have often thought they they were hybrids, but research has largely ruled that out. Naturally-occuring hybrids are rare, and hybrids produced in the lab from pure P. glaucus and P. canadensis specimens are different from these field specimens in several different ways. Most notably, lab-produced hybrids emerge in the spring, but the mosaic specimens seen in the field emerge in the summer.

Thus, there may be a third Tiger Swallowtail species in Ontario. Research is ongoing, but there is support for this proposition in that genetic analysis of specimens from the Ottawa area has established unique molecular markers for the Midsummer Tiger. This would be similar in concept to the US species P. appalachiensis (Appalachian Tiger), which resulted from hybridization between P. glaucus and P. canadensis 400,000 years ago.


The broods

- Papilio canadensis (Canadian Tiger Swallowtail). This has one brood, in the spring.
- Papilio glaucus (Eastern Tiger Swallowtail). This has two (or more) broods. The pupae resulting from the offspring of the spring brood do not enter diapause, and the adults emerge in the same year.
- Midsummer Tiger Swallowtail. This flies after P. canadensis and has one brood.


Typical locations and flight times

The three Tiger Swallowtails occur in different parts of Ontario. In particular, consider four locations: Sault Ste. Marie, London, Toronto and Ottawa.

* Sault Ste. Marie -- One tiger swallowtail occurs: P. canadensis. In the Sault Ste. Marie area, this species is on the wing from late May to early July.

* London -- One tiger swallowtail is known to occur: P. glaucus. This species begins flying in mid-to-late May and continues flying into mid-to-late August. However, the Midsummer Tiger has been observed in the Niagara area along the Lake Erie shore. The extent to which its range extends into the Carolinian zone is uncertain -- more data is needed.

* Toronto -- two tiger swallowtails occur: P. glaucus and Midsummer Tiger. However, there have been few observations of the Midsummer Tiger here. July is the prime time to look for it.

* Ottawa -- two tiger swallowtails occur: P. canadensis and Midsummer Tiger. P. canadensis flies from mid-May until about July 1. The Midsummer Tiger starts flying in late June and continues into mid-August. It is most common in July; hence the former name of "July Tiger".


Maps of observations

All of these maps, which are copied from the article, are based on filtered observations from the Ontario Butterfly Atlas.

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Figure 1: P. canadensis observations



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Figure 2: P. glaucus observations



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Figure 3: Midsummer Tiger observations



Conceptual division of Ontario into 3 tiger-swallowtail areas

While the maps above (Figures 1, 2 and 3) are the best indication of the ranges of the three tiger swallowtails, dividing Ontario into 3 distinct areas can provide a useful way to think about this data. Let us define areas T1, T2 and T3 (shown on the map below), which were developed by Rick Cavasin and Chris Schmidt for classifying historical tiger swallowtail observations in the Ontario Butterfly Atlas:

atlas

These areas were reported in my article Tiger Swallowtails in the Ontario Butterfly Atlas, which appeared in the TEA's seasonal summary Ontario Lepidoptera 2019.

The boundary between areas T3 and T2 is the extreme possible northern boundary for the Midsummer Tiger. In other words, a tiger swallowtail observation from area T3 can be confidently assigned to P. canadensis without having to examine the specimen for the identifying features discussed below. This boundary is 46 degrees latitude -- north of Sudbury, towards Timmins -- and is shown in the map below as the line between points 2A and 2B.

atlas


The boundary between areas T1 and T2 is the approximate northern boundary for P. glaucus. The boundary starts just north of Grand Bend on Lake Huron and moving eastward to just east of Toronto (at Pickering) -- and is shown in close-up in the map below as the line between points 1A and 1B.

atlas


Thus, in broad strokes, this boundary divides the southern part of Ontario into two areas:

- T2 -- the area in which P. canadensis and the Midsummer Tiger co-exist; and

- T1 -- the area in which P. glaucus and the Midsummer Tiger co-exist.

Keep in mind that the geographical transition between P. glaucus and P. canadensis is not so much a line on a map but a transition zone that is possibly tens of kilometres wide. More work is needed to define this area of transition in Ontario.


Identifying features

For any particular specimen, knowing in which area of Ontario it has been found is helpful but not conclusive evidence (other than for area T3). Instead, one needs to rely on the identifying features below, which generally require taking a picture of the butterfly's underside, both forewing and hindwing. There are three comparisons to make: canadensis vs. glaucus, canadensis vs. Midsummer Tiger, and glaucus vs. Midsummer Tiger.


1. glaucus vs. canadensis

To differentiate P. canadensis from P. glaucus, consult any butterfly field guide or the figures at the end of Xi Wang's article An Update on Tiger Swallowtails in Ontario, published in the TEA's seasonal summary Ontario Lepidoptera 2017.


2. canadensis vs. Midsummer Tiger

To differentiate P. canadensis from the Midsummer Tiger, consider the three figures from the article reproduced below.


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Figure 4: Underside of forewing of P. canadensis (top) vs. Midsummer Tiger (bottom)

Note that P. canadensis has a greater tendency for a solid yellow subterminal band and a more squared-off shape of coalescing yellow spots.



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Figure 5: Underside of hindwing of P. canadensis (left column) and Midsummer Tiger (right column)

Note that P. canadensis has: wider anal margin black band as measured at the junction of vein Cu2 (red line in top left image); narrower and sharper black V; and more extensive orange scaling.



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Figure 6: Male claspers

P. canadensis (top row), showing more interspersed black scales compared to the solid yellow scaling of the Midsummer Tiger (bottom row).


3. glaucus vs. Midsummer Tiger

Differentiating P. glaucus from the Midsummer Tiger is the most challenging identification problem. The characters shown in Figures 4 and 6 are of no help here -- both P. glaucus and the Midsummer Tiger have the pattern shown for the Midsummer Tiger in these two figures. Three other characters seem to be helpful, although their reliability remains to be worked out:

- for the underside of the hindwing, the width of the black anal margin band (as shown in Figure 5) is 10-40% for P. glaucus, but somewhat larger (30-55%) for the Midsummer Tiger. (This width is still larger (50-90%) for P. canadensis.)

- P. glaucusis generally a somewhat larger butterfly than the Midsummer Tiger (and both are larger than P. canadensis

- The abdomen lateral blank line is less wide for P. glaucus than for the Midsummer Tiger (and P. canadensis is wider still).

The peak flight period also differs: it is early to mid-July for the Midsummer Tiger, while P. glaucus has a peak in May-June, and another peak in August.


Conclusion: implications for reporting your tiger swallowtail observations

The TEA encourages people reporting tiger swallowtail observations in the T1 and T2 areas (where the vast majority of observers live) to not rely on sight records -- take pictures and upload them to eButterfly or iNaturalist. A picture showing the underside of the forewing and hindwing is a must if the observation is to be identified to species. In addition, an extreme close-up of the male genitalia would also be is also useful in distinguishing P. canadensis from the other two tiger swallowtails, but this would probably require netting the specimen and holding it in one's fingers.

Observations of tiger swallowtails from the Carolinian zone (area T1) in late June to mid-August are especially needed, with the goal of defining the southern boundary of the range of the Midsummer Tiger in Ontario.

Unfortunately, the Midsummer Tiger Swallowtail is not a species choice in either eButterfly or iNaturalist. Thus, how does one report a Midsummer Tiger Swallowtail? In iNaturalist, one can choose "Canadian Eastern Tiger Swallowtail -- Papilio canadensis x glaucus." eButterfly has a similar option, except that the species names are reversed. These classifications give the right general idea, even though the Midsummer Tiger does not appear to be the result of currently-occuring hybridization. Perhaps add a note in the "Comments" area that the specimen is a possible Midsummer Tiger, as this would indicate that the specimen has the identifying features noted above.

If it is not clear which tiger swallowtail has been observed, the best choice is to be noncommital: enter the observation as "Tiger Swallowtails and Allies" in iNaturalist and "Tiger Swallowtail complex" in eButterfly.


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