06 de octubre de 2020

Differences in larval phenotype between eastern and western populations of Sphinx chersis (Lepidoptera: Sphingidae)

Sphinx chersis (Hübner, 1823), the great ash sphinx, has an unusually disjunct geographic distribution throughout North America, with populations largely concentrated in two regions: 1) the northeastern USA and southeastern Canada in the east and 2) the southwestern USA in the west. The species is scarce to absent in the southeastern USA, Great Plains, and Pacific Northwest, separating eastern and western populations by considerable distances. Albeit, its current eastern range has apparently recessed in recent decades due to the destruction of Fraxinus host plants by the invasive emerald ash borer, Agrilus planipennis (Wagner & Todd 2016). Still, even historically the species was probably only weakly contiguous across the Great Plains.

The unusual distribution of S. chersis has naturally provoked the question of whether it represents multiple cryptic, allopatric entities. Populations in the west are sometimes split into subspecies oreodaphne (Edwards, 1874; TL: California) and pallescens (Rothschild & Jordan, 1903; TL: Arizona), and those in Mexico treated as subspecies mexicanus (Rothschild & Jordan, 1903; not treated in this study due to insufficient records). Geographical variation in size and wing shape have been noted, with western adults being smaller in size and having a sharper forewing apex compared to eastern adults (Tuttle 2007; Kesting-Handly & Koiber 2018). Recently, it was also realized based on Arizona specimen that the larval phenotype and DNA barcoding of western populations differs significantly from that of the eastern, providing increasing evidence that the former may represent a separate species (Wagner & Todd 2016).

Given this possibility, it would be greatly beneficial to have a better understanding of the phenotypic differences between eastern and western larvae, prompting the current study. Previous descriptions found in the literature, such as those given in Wagner (2005), Tuttle (2007) and Powell & Opler (2009), only treat one or the other population without comparative context of the other (eastern for the former and western for the latter two, respectively), unbeknownst that they may be describing distinct taxonomic units. Through an extensive examination of larval photographs, I present detailed descriptions and illustrations of the (fifth instar and prepupal) larval phenotype and geographic distribution of eastern and western populations of S. chersis and discuss key diagnostic differences.

MATERIALS AND METHODS

Photographs of S. chersis larvae available on the citizen science platform iNaturalist (https://www.inaturalist.org/observations?place_id=any&taxon_id=127169) were examined. As this platform is prone to misidentifications, I was careful to ignore any larvae that were not S. chersis.

RESULTS AND DISCUSSION

Description of eastern population larva. Fifth instar. The phenotype of the fifth instar larva is perfectly consistent with that described and depicted in Wagner (2005), which is unsurprising given that he based his description on larvae taken from eastern populations. In the green form (Fig. 1), the dorsal abdomen is often whitish green whilst the rest of the body is typically a lime green. There is moderate to heavy speckling of white granules in the thoracic region and below the spiracles. With no exceptions, the spiracles have black centers with white rings (eastern larval records with orange spiracles are almost certainly misidentifications, typically of the similar Ceratomia undulosa). The oblique stripes along the body are usually weakly to moderately well developed, whitish, and unedged with any other colors in the pure green form. The horn is typically bright blue but occasionally pink.

A pink form (Fig. 2) and their intermediates occur at an uncommon but appreciable rate (23/492 = 4.67% of records examined); in the most extreme manifestations, the ground color is bright yellow dorsally and pink below the spiracles, the oblique stripes are heavily edged with pink, and all sclerotized parts (head, horn, etc.) are completely pink. This form enhances crypsis on the unique red autumn foliage of white ash (Fraxinus americana), the most common ash species on the east.

Prepupa. In the prepupal stage (Fig. 3), the larva usually exhibits no color change unlike many sphinx species (records of wandering prepupae that are brownish on the east are usually misidentified C. undulosa, or a pink form S. chersis).


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FIGS. 1-3. Illustrations of eastern S. chersis larvae. 1, Fifth instar, green form. 2, Fifth instar, pink form. 3, Prepupa, green form (identical to fifth instar).

Description of western population larva. Fifth instar. The larva of western populations (Fig. 4) differs in several significant ways from that of eastern populations. The dorsal abdomen has a bright green cast rather than a whitish cast and the rest of the body is a pale, turquoise green: essentially the reverse ground coloration scheme as eastern larvae. There is much lighter speckling of white granules, largely limited to the thoracic region. Strikingly, larvae on the west generally have spiracles with orange centers rather than black ones, especially in the desert regions (Arizona, New Mexico, and adjacent regions), although exceptions were found. The oblique stripes along the body are usually more strongly developed and solid white, sometimes thinly edged with dark teal or purple, especially in desert larvae.

Records of pink forms are lacking and likely less common than on the east, though large-scale larval rearing may be needed to quantitively verify this. The common ash species on the west, velvet ash (F. velutina) and Oregon ash (F. latifolia), do not have red autumn foliage.

Prepupa. During the prepupal stage (Fig. 5), western larvae display a vivid color change to a deep amber to purplish brown dorsally. This life history trait objectively unifies and distinguished all western larvae from eastern ones.


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FIGS. 4-5. Illustrations of western S. chersis larvae. 4, Fifth instar, green form. 5, Prepupa, green form, displaying color change.

Distribution. Examined S. chersis larval records with either the eastern or western phenotype formed well defined, allopatric distributions (Fig. 3). Those with the eastern phenotype occurred predominantly throughout New England, the Midwest, and southern Canada. A few surprising records occurred in Montana, Nebraska, Colorado, and New Mexico at the western extremities. Records from the latter two states were the sole exceptions that were in proximity with those with the western phenotype and suggest that the two entities may be sympatric along the eastern foothills of the Rocky Mountains. It is noteworthy, however, that all records from Canada, even far west to Alberta, displayed the eastern phenotype; populations in western Canada are considerably further north than most populations with the western phenotype.

Those with the western phenotype occurred predominantly throughout the desert Southwest and Great Basin, with somewhat spottier records along the northern California coast up to extreme southern Washington. A few Mexican records occurred in the immediate vicinity of the southwestern USA records, which were assumed to belong to the western entity that has been described here rather than the mexicanus subspecies, if distinct. Although larval photographs are missing from the heart of Mexico, based on geography, one would imagine that mexicanus is more closely allied to the western entity than it is to the eastern one.



FIG. 3. Distribution of examined larval records of S. chersis from iNaturalist, which displayed either the eastern phenotype (blue) or the western phenotype (red).

Conclusion. Overall, the single most objective and diagnostic difference between the eastern and western populations of S. chersis is that the prepupa becomes deep brown in the west, whereas there is little to no change in the east. There are also notable differences in final instar ground color, spiracle color, speckling, oblique stripes, and occurrence of pink forms. The two entities do not appear to be significantly sympatric in any of their range, except along the eastern foothills of the Rocky Mountains.

The degree of larval and distributional differences seen in eastern and western populations of S. chersis is comparable to that of other eastern/western sphingid sister pairs such as Pachysphinx modesta/occidentalis and Hemaris diffinis/thetis. Likewise, this stark geographic dichotomy in larval phenotype would be highly unusual for a single species to exhibit. Thus, based on larval phenotype and distribution alone, a compelling argument could be made for a taxonomic distinction between eastern and western populations. A more extensive molecular survey of these populations is needed to confirm whether the magnitude of their phenotypic differences is reflected at the molecular level.

LITERATURE CITED

Kesting-Handly T., S. Kloiber. 2018. Sphinx chersis, Sphingidae of the United States. Available from: https://www.sphingidae.us/sphinx-chersis.html (October 28, 2021).
Powell J.A., P.A. Opler. 2009. Moths of Western North America. University of California Press. Berkeley, CA. 243 pp.
Tuttle J.P. 2007. The hawk moths of North America: A natural history study of the Sphingidae of the United States and Canada. The Wedge Entomological Research Foundation. Washington, D.C. 75 pp.
Wagner D.L. 2005. Caterpillars of Eastern North America. Princeton University Press. Princeton, New Jersey. 256 pp.
Wagner D.L., K.J. Todd . 2016. New ecological assessment for the emerald ash borer: a cautionary tale about unvetted host-plant literature. Am. Entomol. 62:26–35.

Publicado el octubre 6, 2020 05:36 MAÑANA por alanliang alanliang | 0 comentarios | Deja un comentario

07 de junio de 2020

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. 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 is extremely delayed as compared to any other species, not occurring until the fourth or fifth instar.

Hostplants. In most areas, 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, typically infused with varying degrees of orange, a diagnostic trait among other tiger swallowtail larvae outside of P. eurymedon. From personal large-scale rearing of coastal California populations, larvae with completely pure yellow eyespots are a small minority. Although orange-infused eyespots are also observed in P. eurymedon, they seem to be much less frequent, with the pure yellow eyespots more typical (however, this is based on a relatively small sample so far). Unlike any other species, P. rutulus eyespots are never infused with green. The body color is leaf green, becoming bark, sometimes purplish, brown in the prepupa. 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 and third instar (caramel brown, dirty green, etc.) and heavily maculated compared to other species. 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 always be reliably distinguished from it from visual characteristics alone. Any methods to separate the two that have been proposed in guides (such as the notion that P. eurymedon has “notched” eyespots) are unreliable or erroneous. From my personal rearing of coastal California populations, P. eurymedon eyespots are narrower on average and are most often a pure pale yellow to yellow-green, lacking the high degree of variation seen in P. rutulus. The body shape is more compact and stouter in proportion. Again, none of these characteristics are definitive pending further study and should not be used to reliably distinguish the two species.

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. The second and third instar are less heavily maculated than in P. rutulus (this maculation also tends to be less contrasting), and the white patch at the rear end is reduced or absent, whereas it is usually well developed in P. rutulus. By the fourth instar, larvae usually have little to no maculation and often have a reduced or absent saddle. The thoracic scoli behind the head tend to be slightly larger proportionally and redder than in P. rutulus. Transition to green occurs either in the third or fourth instar.

Habitat. While visual characteristics are usually insufficient to distinguish the two, there do exist differences in habitat, which can aid in identification. While P. rutulus is a ubiquitous species, breeding in both residential areas and natural habitat alike, P. eurymedon has a much more localized distribution, breeding almost exclusively in natural habitat, rarely in 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 completely unique to and diagnostic of this species. In coastal California, I have only had luck collecting immatures on Frangula californica, which 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 infused with green. 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, Prunus, Malus, and 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 maculated. 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, Fraxinus, Prunus, Malus, and 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 alanliang | 0 comentarios | Deja un comentario

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