An identification guide for larvae of the Papilio glaucus species group (Lepidoptera: Papilionidae)

The tiger swallowtails (Papilio glaucus group) consist of seven species found in North America. Papilio rutulus (Lucas, 1852), P. eurymedon (Lucas, 1852), and P. multicaudata (Kirby, 1884) occur in the western US and southwestern Canada, with the latter also found across Mexico. Papilio glaucus (Linnaeus, 1758) occurs in eastern US and southeastern Canada. Papilio canadensis (Rothschild & Jordan, 1906) occurs throughout Canada, Alaska, and northeastern US. Papilio appalachiensis (Pavulaan & Wright, 2002) occurs only in the Appalachian Mountains. Papilio alexiares (Höpffer, 1866) occurs in Mexico.

Despite being large in size and relatively well-documented, the larvae of this group are surprisingly challenging to differentiate, having diverged very little in appearance from each other. At the time of writing this, larvae are still widely un-/misidentified on crowd-sourced biodiversity databases such as iNaturalist and across the web, and many guides continue to lack diagnostic larval descriptions or contain erroneous ones. This has created a vicious cycle in which the high rate of misidentified records continues to perpetuate further misidentifications and confusion, impeding attempts at revealing diagnostic traits for each species. Here, based on a thorough examination of larval photographs and personal rearing, this paper aims to elucidate differences between tiger swallowtail larvae, discussing diagnostic traits and hostplants.

Based on larval eyespots, the species of tiger swallowtails can be split into two general groups (which are consistent phylogenetically): a western group (P. multicaudata, P. rutulus, and P. eurymedon) and an eastern group (P. glaucus, P. canadensis, P. appalachiensis, and P. alexiares). The general diagnosis for all three western species is the presence of an additional yellow spot proximal to the main spot containing the pupil, which is absent from all four members of the eastern group. Additionally, the main spot is elongated and pear-shaped in the western species whereas in the eastern species it is more rounded, with a larger pupil. These eyespot differences are enough to confidently distinguish western from eastern larvae in areas of sympatry, including: P. multicaudata from P. glaucus in central Texas; P. multicaudata from P. alexiares in Mexico; and all three western species from P. canadensis in eastern British Columbia.

Differentiating between members within the western and eastern groups, however, is more challenging, and is discussed in the following subsections. While distinguishing traits are recognized for most species, it should be noted that some are more reliable than others, and are not always sufficient to make a definitive identification. In such cases, the hostplant may help aid in diagnosis, but only if it is unique to a particular species (in the lists provided below, unique hostplants contain an asterisk). Also, due to lack of larval photos, P. appalachiensis and P. alexiares are not illustrated or discussed in detail.

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FIGS. 1-10. Illustrations of fifth instar tiger swallowtail larvae. Note that these depictions are merely representative, and do not encapsulate the full range of variation in the species. 1, P. multicaudata (lateral view). 2, P. multicaudata eyespots (dorsal view). 3, P. rutulus (lateral view). 4, P. rutulus eyespots (dorsal view). 5, P. eurymedon (lateral view). 6, P. eurymedon eyespots (dorsal view). 7, P. glaucus (lateral view). 8, P. glaucus eyespots (dorsal view). 9, P. canadensis (lateral view). 10, P. canadensis eyespots (dorsal view).

Papilio multicaudata (Figs. 1-2). Fifth instar. The larva of this species is by far the most distinctive, and the only one that is unmistakable from any other species even without location or other contextual information. The blue spot in the eyespot pupil is reduced, leaving a ring of body color within the pupil. In all other species, the blue spot is larger, completely filling the delineating black border of the pupil. The eyespots are yellow to yellow-green. The body color is lime green (becoming orange-brown in the prepupa), a distinctly more vibrant shade than in any other species, which are more glaucous. The dorsal and lateral blue spots are pale, sometimes whitish, delineated by a fine black border not seen in other species. There is a well developed yellow anal band that is faded or absent in other species except P. canadensis. The fully grown larva is larger and wider in girth than any other species, with less tapering anteriorly.

Early instars. Very glossy, dark brown (sometimes almost black), lacking the pale maculation seem in other species. The anal scoli match the body color and are not surrounded by a white patch. Transition to green occurs in the fourth or fifth instar, much later than in any other species. Hot, sunny, and dry conditions accelerate onset of green.

Hostplants. In most areas, native Prunus (especially virginiana in US and Canada and serotina in Mexico) and Fraxinus. In California and Texas, it is associated with Ptelea*, which distinguishes it from the other two western species.

Papilio rutulus (Figs. 3-4). Fifth instar. The eyespots are yellow with varying degrees of orange tinge, a color unique to this species. Occasionally, they can be pure yellow as in Papilio eurymedon, but never greenish. The body color is leaf green, becoming bark, sometimes purplish, brown in the prepupa. The countershading is especially strong in this species, with the lateral/ventral sides a pale, whitish turquoise. The dorsal and lateral blue spots are a cornflower shade, sometimes slightly purplish.

Early instars. Highly variable, displaying a wide range of transitional coloration throughout the second third instar (caramel brown, dirty green, etc.). The second through fourth instars are heavily mottled in pale spots. The anal scoli are pale and surrounded by a well developed white patch beginning in the second instar. This white patch is usually reduced or absent in P. eurymedon and Papilio multicaudata. Transition to green typically occurs anywhere between late second instar to late third instar.

Hostplants. Most universally, Salix* and Populus* (Salicaceae), which distinguish it from sympatric species except P. canadensis. Acer macrophyllum* (coastal populations) and Platanus* are also common and unique hostplants among sympatric species. Other non-diagnostic hostplants include Fraxinus and various Rosaceae such as Prunus, Malus, Amelanchier.

Papilio eurymedon (Figs. 5-6). Fifth instar. Strongly resembles its closest relative, P. rutulus, and cannot be reliably distinguished using morphological traits alone. Diagnostic traits discussed in other guides (such as “notched” eyespots) have proven unreliable or erroneous. To add to the confusion, this species shows significant geographic variability, and diagnostic traits from one population may not hold true for others. The traits discussed here are based only on coastal California populations and should not be generalized for all populations (not even inland California populations). All P. eurymedon that I have reared in coastal California have possessed pale yellow eyespots, sometimes with a green tinge. Never have I encountered one with an orange tinge, as in P. rutulus (other populations of P. eurymedon may very well possess the orange tinge, however). The eyespots also tend to be smaller/narrower on average, with the inner spot often triangular. The yellow band on the thorax is paler, sometimes whitish. The body's countershading tends to be less extreme, with the lateral/ventral sides not as pale or whitish as in P. rutulus.

Early instars. Very similar to P. rutulus, differing only in a combination of fine details that require a trained eye to discern. The first instar is slightly darker, with the white patch at the rear end much smaller or nearly absent. This white patch also tends to be reduced or absent in the second and third instars, whereas it is usually well developed in P. rutulus. The pair of thoracic scoli behind the head are slightly longer proportionally. The second through fourth instars tend to be slightly less mottled in pale spots compared P. rutulus, and this maculation is usually much fainter. In the third and fourth instar, there is a possible form in which the eyespots contain prominent, reddish raised bumps. This form has not been observed in P. rutulus, or at least not to such an extreme degree. Transition to green occurs either in the third or fourth instar.

Habitat. While morphological similar, there are differences in habitat preference between P. eurymedon and P. rutulus that can be useful for identification. Along the coast, P. rutulus is a very ubiquitous species, breeding in both urban residential areas and natural habitat alike. Inland populations tend to be more restricted to natural, riparian habitats. P. eurymedon on the other hand, breeds almost exclusively in natural habitat just about anywhere in its range, and rarely strays into residential areas even if host plant is abundant. For this reason, there is likely a strong bias for unidentified larval records of P. eurymedon and P. rutulus on the web to be the latter.

Hostplants. Largely Ceanothus*, Rhamnus*, and Frangula californica* (all Rhamnaceae), which are unique to this species. In coastal California, I have only had luck collecting immatures on Frangula californica (even when other host plants are available) which suggests it is perhaps preferred. Certain Fabaceae, including Pickeringia montana* and Melilotus albus*, have also been recorded, at least for oviposition. These plants presumably share chemical similarities with the usual Rhamnaceae host plants. Other non-diagnostic hostplants include various Rosaceae such as Prunus (especially ilicifolia in California), Malus, and Amelanchier.

Papilio glaucus (Figs. 7-8). Fifth instar. The eyespots are pale yellow, sometimes with a greenish tinge. The body color is leaf green, sometimes slightly yellow-green. In the prepupa, the larva becomes bark brown and, unlike any of the western species, is heavily freckled in pale spots, producing a “rough” appearance. The dorsal and lateral blue spots are pale to cornflower in shade. In northern populations, the yellow anal band is retained throughout the final instar and the anal scoli are more prominent, as in P. canadensis. In the majority of its range, however, the yellow anal band is greatly faded or absent by the mid to late fifth instar, with the anal scoli reduced.

Early instars. Glossy, dark brown, with anal scoli consistently more prominent and yellow in the north, elsewhere more variable. Transition to green typically occurs throughout the third instar, earlier in northern populations.

Hostplants. Liriodendron tulipifera* and Magnolia virginiana* (Magnoliaceae) preferred by southern populations, and are unique to this species. Other non-diagnostic hostplants include Ptelea, Fraxinus, and various Rosaceae such as Prunus, Malus, Amelanchier.

Papilio canadensis (Figs. 9-10). Fifth instar. Identical to P. glaucus except smaller with prominent yellow anal band and scoli. Because northern populations of P. glaucus also possess this trait, the two cannot be distinguished in areas of sympatry despite this being a fairly reliable distinction elsewhere.

Early instars. In the first instar, the larva is marked with three distinct white saddles on the thorax, mid-abdomen, and rear. The thorax and rear saddles are reduced or absent in P. glaucus and other species. Beginning in the second instar, the anal scoli and band are highly prominent and yellow, much more so compared to in P. glaucus, and the body more mottled in pale spots. Transition to green begins throughout the second instar, earlier on average than in any other species.

Hostplants. Salix* and Populus* distinguish it from sympatric species except P. rutulus. Other non-diagnostic hostplants include Betula, Alnus, Fraxinus, and various Rosaceae such as Prunus, Malus, Amelanchier.

Papilio appalachiensis. The larva of this species is largely unknown, but likely identical to P. glaucus. Photos of the larva have surely been taken from areas where it breeds, but are under the default identification of P. glaucus, with no way of knowing the true identity.

Papilio alexiares. From the small sample of photos available, it would appear this species is identical to its closest relative, P. glaucus. However, this is not an issue for identification as long as the location is known, since it is not sympatric with any of the other members of the eastern group and is easily distinguished from P. multicaudata.

Publicado el junio 7, 2020 09:16 MAÑANA por alanliang