ארכיון יומן של אוקטובר 2020

אוקטובר 06, 2020

Sphinx chersis: Phenotypic Differences Between Larvae of Eastern and Western Populations


East


West

Introduction
Sphinx chersis (Hübner, 1823), the great ash sphinx, has a rather peculiar geographic distribution throughout the USA and Canada that can be roughly split into at least two distinct populations: 1) an eastern population ranging from northeast USA to southern Canada, and 2) a western population in the southwest USA. (An additional third, southern population exists in Mexico but will not be treated here due to lack of sufficient records.) The species is largely absent in the Pacific Northwest, southeast USA, and most importantly, the central USA, separating the range of the eastern and western populations by considerable distances in all directions. This naturally provokes the question of whether the two populations may actually represent distinct, geographically isolated entities. According to the Sphingidae Taxonomic Inventory, a number of subspecies have historically been described, with the eastern population treated as nominotypical chersis, the western population split into two subspecies, oreodaphne (Edwards, 1874; TL: California) and pallescens (Rothschild & Jordan, 1903; TL: Arizona), and the southern population as mexicanus (Rothschild & Jordan, 1903; TL: Mexico). The two western subspecies, however, have since been resynonymized with the nominotypical subspecies, and thus there is currently no taxonomic distinction between the eastern and western populations.

As a regular identifier of North American sphinx moths on iNaturalist, however, I noticed something unusual about the phenotypes of the mature larvae. Wagner (2005)'s description of the mature larvae is:

a large greenish or pinkish caterpillar with seven long diagonal lines that may be edged with pink. Waxy green over abdominal segments and occasionally T3; lime green below spiracles and over T1-T2 or T3.... Spiracles elongate, central black area ringed with white.... Horn blue or pink.
However, I found that the phenotype of a number of larvae I reviewed, while appearing to be Sphinx chersis, somewhat deviated from his description (if these were actually misidentifications of another similar species I am unaware of, i.e. Ceratomia sonorensis*, well then I guess this study goes down the drain and I sincerely apologize for the time wasted reading this piece). Upon closer inspection, I speculated that these differences may have geographic associations corresponding to the ranges of the eastern and western populations. This prompted me to investigate further by reviewing every larva observation on the site as well as those on BugGuide (while making sure to correct/ignore any that I believed were misidentifications that could confound the study). Through this, it became clear that there were indeed several consistent phenotypic differences between the larvae of the eastern and western populations. Descriptions of the eastern and western populations are discussed as follows:

Eastern Population (Click here for more eastern iNaturalist observations)
The phenotype of the fifth instar larva is perfectly consistent with that described and depicted in Wagner (2005), which is unsurprising given that he likely based his description on larvae taken from eastern populations. With no exceptions, the spiracles have black centers with white rings (any larva observation with orange spiracles on the east is typically a misidentified Ceratomia undulosa). The dorsal abdomen is often waxy green whilst the rest of the body is typically a lime green, as described by Wagner (2005). The oblique lines along the body are usually weakly developed, whitish, and unedged with any other colors in the pure green form. The horn is typically bright blue but occasionally pink. In some cases (14 out of 333 observations reviewed), there are pink forms (and intermediate forms) in which the oblique lines are yellow and heavily edged with pink and the horn is completely pink. In the prepupal stage, the larva usually exhibits no color change except for an occasional light amber hue dorsally (any observations of wandering prepupae that are brownish on the east is likely a misidentified Ceratomia undulosa, or simply a pink form Sphinx chersis).

Fifth Instar

Ontario, CA

Ontario, CA

Ontario, CA (pink form)

New Hamsphire, USA

Wisconsin, USA
Prepupa

Ontario, Canada

Ontario, Canada

Michigan, USA

New Jersey, USA

Western Population (Click here for more western iNaturalist observations)
The larvae of the western population differ in a number of significant ways from those of the eastern population. Perhaps most strikingly, larvae on the west tend to have spiracles with orange centers rather than black ones, especially in the desert southwest region. Black spiracles appear to be rare, except for along the coast (California and Oregon) where they seem to be more common. The dorsal abdomen often lacks the waxy green coloration and sometimes has a shiny sheen. The oblique lines along the body are usually strongly developed and solid white, sometimes thinly edged with dark teal, especially in desert larvae. No pink forms were observed in desert larvae, which suggests they may not be as common (?). There were, however, two pink form larva from Oregon. In the prepupal stage, the larva exhibits a vivid color change to an amber brown or purplish brown dorsally, the latter especially in desert individuals.

Fifth Instar

California, USA

Oregon, USA (pink form)

Arizona, USA

Arizona, USA

New Mexico, USA
Prepupa

Oregon, USA

California, USA

Arizona, USA

Arizona, USA

Colorado, USA

Conclusion
Overall, perhaps the single most striking and consistent difference between the eastern and western populations is that the prepupa becomes deep brown in the west, whereas there is little to no change in the east. The second most striking difference being the possibility of orange spiracles in the west versus consistent black spiracles on the east. While I do not necessarily suggest reinstating the former subspecies of Sphinx chersis (or other taxonomic revisions regarding eastern and western populations) based solely on phenotypic differences of the larvae, I do find these differences quite notable and intriguing. Perhaps the recognition of these larval phenotypic differences may prompt further investigation into the differences between the two populations (adult morphology, molecular sequence data, etc.). In any case, a subtle update of the larval description of the species in the field guides ought to be due, at least.

*The one major caveat of the study is that a possible systemic misidentification of Sphinx chersis larvae with that of another similar species, namely Ceratomia sonorensis in the southwest, could confound the photographic data and invalidate the phenotypic descriptions. I would say, however, that the Ceratomia sonorensis larva depicted in this image link (assuming the identification is accurate) is safely different than any of the Sphinx chersis larvae I reviewed. Ceratomia sonorensis appears to be very similar to it's eastern relative, Ceratomia undulosa and shares the same diagnostic features: unlike Sphinx chersis, there is a patch of black spots on the anal plate and the spiracles have white centers with an orange posterior and anterior edge. Oddly, however, the larvae identidfied as Ceratomia sonorensis on BugGuide appear to be identical to larvae identified as Sphinx chersis both on BugGuide itself and on iNaturalist. I suspect none of them are true Ceratomia sonorensis, and all are misidentifications of Sphinx chersis, except for possibly this one (?). Therefore, I believe most of the larvae identified as Sphinx chersis that I reviewed for this study should be accurate. I did observe quite a large number of misidentifications of other species like Ceratomia undulosa, Paratrea plebeja, Manduca rustica, just about every Smerinthine spp., and even Ceratomia amyntor, etc., but these were quite obvious and thus either corrected or ignored (a funny story is that I had originally attempted to include BAMONA data as well, but upon quick inspection, the rate of misindentifications was so astounding, to say the least, that I decided to abandon the source).

פורסם ב אוקטובר 06, 2020 05:36 לפנה"צ על־ידי alanliang alanliang | 0 comments | הוספת תגובה

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