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October 06, 2020

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

INTRODUCTION

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 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 Larvae
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. 1a), 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. 1b) 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. 1c), 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).


a.
b.
c.

FIG. 1. Illustrations of eastern S. chersis larvae; a. fifth instar, green form; b. fifth instar, pink form; c. prepupa, green form (identical to fifth instar).

Description of Western Population Larvae
Fifth instar. The larva of western populations (Fig. 2a) 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. 2b), 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.


a.
b.

FIG. 2. Illustration of western S. chersis larvae; a. fifth instar, green form; b. 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.

Posted on October 06, 2020 05:36 by alanliang alanliang | 0 comments | Leave a comment

June 07, 2020

Tiger Swallowtail Larva Identification

Introduction
The tiger swallowtails (Papilio [Pterourus] glaucus group) are some of the most charismatic and well-known butterflies in North America. The seven closely related members of the species group include P. rutulus and P. eurymedon (northern Baja California, western US and southwest Canada), P. multicaudata (Mexico, western US, and southwest Canada), P. glaucus (eastern US and southeast Canada), P. appalachiensis (Appalachian Moutains), P. canadensis (northeast US, Canada, and Alaska), and P. alexiares (northeast Mexico and western Texas). (The latter four species are especially similar and form a species complex, which I will refer to hereafter as the eastern species complex.) Despite the abundance of photographic data for these butterflies, their larvae have long been a challenge to differentiate, even for the most avid of enthusiasts. Larvae are frequently left unidentified or misidentified on iNaturalist and the web and there have not been many distinguishing characters proposed to date. Part of the challenge in identifying character differences is because of the high rate of misidentified larvae on the web, which has likely confounded any serious attempts at revealing a correlation of traits. Here, I have proposed a tentative guide to distinguish tiger swallowtail larvae using character differences for the fifth instar and prepupal stages based on careful surveying of photos from iNaturalist and other open sources, and from personal rearing of P. rutulus and P. glaucus.

Visual Character Differences
General:
Up until the present article, the best known (and probably only) character difference used for differentiating tiger swallowtail larvae involves the inner yellow spot of the eyespot: it is present in all three western species (P. multicaudata, P. rutulus, and P. eurymedon) and absent in the eastern species complex (P. glaucus , P. canadensis , P. appalachiensis, and P. alexiares). The single spot (eastern) vs. double spot (western) character difference is immediately diagnostic on its own and has been recognized for a long time. Though useful to know, most ambiguities between the western species and eastern species complex, however, can already be resolved without visual examination simply by knowing the location of the larva (the range of P. glaucus generally does not overlap that of P. rutulus or P. eurymedon anywhere and only overlaps that of P. multicaudata in Texas and occasionally elsewhere in the Great Plains; P. canadensis meets the three western species only in southern British Columbia and occasionally Alberta). Differentiating between the three western species and members of the eastern species complex themselves, however, is much more challenging, and is discussed in the following sections.

Western Species (P. multicaudata, P. rutulus, & P. eurymedon):
Of the three western species, P. multicaudata is by far the most distinctive. Several character differences in the fifth instar (often already evident by the fourth) readily distinguish it from the other two western species whom it is often sympatric with, the single strongest one being the reduced blue spot in the eyespot pupil that often leaves a ring of ground color within the pupil (2). In addition, there are clear differences in coloration: a brighter ground color in both the fifth instar and prepupal stage (5), a generally yellow-green eyespot whilst lacking the possibility of an orange eyespot color variant (1), and lacking the possibility of having a purple color variant of the blue eyespot pupil (2). Although each of the coloration differences may not be individually diagnostic, when considered collectively, especially with the eyespot shape differences, a reliable diagnosis can be formed. In the early bird dropping instars, P. multicaudata differs from the other two western species in typically being uniformly shiny, dark brown with brown scoli at the rump. The other two species often show intermediary stages of brown/green (such as yellow-brown or dirty green) by the second or third instar and have pale, whitish scoli at the rump. Once one has familiarized themselves with these character differences, they may notice that P. multicaudata is frequently misidentified as one of the other two western species and vice versa on the web (the Wikipedia page for P. rutulus for example, blatantly shows photos of P. multicaudata larvae in my opinion). As mentioned earlier, this has likely confounded attempts at identifying diagnostic characters for the species for quite some time. However, an examination of P. multicaudata larva photos with positive adult confirmation or from localities where the two other western species are rare or absent (i.e. Mexico, Texas, parts of New Mexico and Arizona, etc.) clearly shows that the proposed P. multicaudata characters are consistently present. Conversely, the vice versa is also true when examining photos of larvae in localities where P. multicaudata is rare or absent and at least one of the other two western species predominates (i.e. southern California, most of Central Valley) – all larvae consistently lack the proposed characters.

For the other two western species, P. rutulus and P. eurymedon, there are several character differences that can reliably differentiate them from the other tiger species. They are distinguished from P. multicaudata based on the eyespot/pupil shape differences as described earlier (2), and they are also unique among tiger species in having an orange eyespot color variant (1) and a purple pupil color variant (2). Additionally, they are often peppered in minute pale spots and exhibit stronger green-turquoise countershading, giving them a rougher, less uniform appearance; the peppering is absent in P. multicaudata and the countershading is weaker, giving it a smoother, more uniform appearance. However, differentiating between the two species themselves is of great uncertainty and it seems doubtful that a single reliable character difference exists to visually differentiate them. A discussion on BugGuide that is occasionally referenced on other identification sites (erroneously, in my opinion) suggests that P. eurymedon differs in having a "notched" eyespot. However, I am highly dubious of the validity of this character difference, as no other photos of confirmed P. eurymedon larvae support this. Additionally, there was never any positive adult confirmation of the larvae in question, and in fact, I would argue it looks like a very typical P. multicaudata larva.

Instead, I believe P. eurymedon eyespots may be narrower on average and, while possible to exhibit both green and orange color variants, are most typically pure yellow with a purple pupil. Papilio rutulus on the other hand, has a greater likelihood of exhibiting orange eyespots. In addition, the blue spots on the body might be smaller and darker on average (often purplish) in the fifth instar of P. eurymedon than in P. rutulus. However, these differences may also be dubious and would not be reliable anyways due to the overlap in variability of the examined characters. It should also be noted that the pupil often turns darker and purpler in the prepupal stage which restricts the use of this character difference to only the fifth instar. In terms of the early instar, there may exist a few possible (but probably also unreliable) differences. The shiny, raised bumps (scoli) that stud the thorax, particularly the ones in the eyespots and the horn-like ones behind the head seem on average better developed in P. eurymedon than in P. rutulus in relation to the body size. This is particularly evident in the third and fourth instars as the larvae begin turning green, but the scoli disappear entirely by fifth instar in both species. In addition, the pale saddle and other white markings are more likely to disappear by the fourth instar in P. eurymedon whereas they more likely to persist until the fifth instar in P. rutulus. In the end, however, one will likely need to resort to using non-visual clues for these two species, particularly hostplant (discussed later) and habitat (predominantly natural areas for P. eurymedon vs. generalist [i.e. natural and urban] for P. rutulus). Nonetheless, I would bet good money that the overwhelming majority of unidentified/-confirmed P. rutulus/eurymedon observations on iNaturalist are P. rutulus.

Eastern Species Complex (P. glaucus , P. canadensis , P. appalachiensis, & Papilio alexiares):
P. glaucus and the three other species in the eastern species complex, P. canadensis, P. appalachiensis, and Papilio alexiares, are unfortunately very difficult, if not impossible to differentiate. Only P. glaucus is included in the above figure as I do not believe characters 1-5 are helpful in differentiating the mature larvae of these four species. Instead, the only difference that I observed between P. glaucus and P. canadensis is that the pale markings in young larvae of P. canadensis are much better developed since birth, which translates to better developed pale stripe/scoli at the rump in the mature (≥L5) larvae (7). (One needs to make certain, however, that the larva they are examining is a fifth instar when using this character difference, and not a green fourth instar, as both species can have well developed pale markings before fifth instar. Mid to late fifth instar is most ideal for diagnosis).

Unfortunately, however, the thickness of the pale stripe exhibits clinal variation throughout the intergrade zone in which they are sympatric (northeast US to southern Ontario), and thus the full spectrum of phenotypes can be observed in P. glaucus in this region. The character difference is therefore not very useful in the only area that it is needed (sympatric populations of P. glaucus and P. canadensis are much more similar to each other than allopatric populations of either species themselves!). It may be possible however, that many of the intermediate or "canadensis-like" P. glaucus larvae observed in intergrade populations may actually be the larvae of a possible unnamed taxon in the P. glaucus/P. canadensis complex of hybrid origin. The proposed character difference would then partially maintain its integrity in distinguishing P. glaucus from P. canadensis and the unnamed taxon, but the latter two themselves would still be indistinguishable.

For P. appalachiensis, no reference photos could be found so it is assumed to be identical to P. glaucus, whom it is largely sympatric with. The logic for the assumption being that photos that depict P. appalachiensis larvae do exist on the web but the species is so cryptic in regards to P. glaucus that thus far all photos have (unintentionally) been identified as P. glaucus, hence the reason why there is a lack of photos under a P. appalachiensis identification. For Papilio alexiares, only a single image of a prepupal larva could be found and it appears identical to P. glaucus, at least from what can be seen at the angle. Fortunately, however, Papilio alexiares is not sympatric with any of the other members of the eastern species complex, so no character differences are needed to identify it. (It is, however, fully sympatric with P. multicaudata, but the "single spot vs. double spot" and other character differences easily distinguishes them). Like for P. rutulus and P. eurymedon, one may need to resort to using non-visual clues to differentiate members of the eastern species complex, such as date of observation (time of year, uni- vs. bi-/multivoltine), the specific locality, and hostplant (discussed later) in areas of sympatry.

Diagnostic Hostplants
The tiger swallowtail species are unique among Papilio in being polyphagous on a large range of host plant families. Though there is considerable overlap in the host plants utilized between one or more species (ex. Rosaceae, Oleaceae), some families may be locally (or even completely) unique to a species and can thus be diagnostic for identification (6). From reviewing observations on iNaturalist, a more comprehensive (but not exhaustive) list of what I believe to be the more specific preferred hostplants of each species is given below, with the locally unique/diagnostic hostplants in bold (regional preferences discussed are mostly guesswork):

P. multicaudata - appears to have a preference for Ptelea (Rutaceae) in the southwest US (California to Texas) which is supported by the strong range association between the species and the host in this region; further north where Ptelea is absent (Pacific Northwest, Great Basin, Rocky Mountain states), largely Prunus (Rosaceae) and Fraxinus (Oleaceae); in Mexico, all three commonly utilized, perhaps with preference for Fraxinus (?).
P. rutulus - largely Salix and Populus (Salicaceae) in most of its range, especially inland populations (?). Seems more variable along the coast, often utilizing Prunus, Fraxinus, and Platanus (Platanaceae) as well, especially in urban settings (?). Alnus (Betulaceae) also seems to be somewhat common in the Pacific Northwest (?).
P. eurymedon - largely Ceanothus and Frangula californica (Rhamnaceae), the former almost exclusively utilized in inland populations (Great Basin, Rocky Mountain states), the latter is a common alternative in California; coastal populations additionally utilize Prunus. Alnus also seems to be somewhat common in the Pacific Northwest (?).
P. glaucus - overall the most polyphagous of all tiger swallowtail species; Liriodendron tulipifera (Magnoliacaeae) and probably related trees (i.e. Magnolia virginiana) preferred in much of its range (except far inland populations where it is absent); Prunus and Fraxinus, and perhaps also Ptelea in some places, also commonly used.
P. canadensis - Populus (and probably also Salix) preferred but may often utilize Prunus and Fraxinus as well.
P. appalachiensis - unknown; only recorded host is Prunus.
P. alexiares - unknown.

Though I am not completely certain on the accuracy of the entire list, what is important are the diagnostic hostplants which can be used for identification. For example, Magnoliaceae and Rhamnaceae are only utilized by P. glaucus and P. eurymedon, respectively, and thus a larva found on such hostplants can often be reliably identified. Other diagnostic hostplants such as Ptelea for P. multicaudata and Salicaceae for P. rutulus can also be used for identification but need to be combined with location context as they are not necessarily unique to the species. For example, a larva on Ptelea in California is almost certainly a P. multicaudata, while in Texas, it could also be P. glaucus; fortunately, however, the visual character differences (single spot vs. double spot) are already easily sufficient to distinguish the two. A parallel reasoning can also be made for identifying P. rutulus, the predominant utilizer of Salicaceae in much of its range, except where it meets P. canadensis in southwestern Canada; again, however, the visual character differences (single spot vs. double spot) are already sufficient to distinguish these two species.

Posted on June 07, 2020 09:16 by alanliang alanliang | 0 comments | Leave a comment

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