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Harpalina  - Selenophorus gagatinus

Harpalina - Selenophorus gagatinus
Toronto, Ontario, Canada
May 9, 2020
Size: 8.1mm
Smaller than the Harpalus I've been finding. I'm stumped on genus here. Under a rock in an open gravelly area with scattered Eastern Red Cedar.

Moved to secure "Selenophorus gagatinus"
based on my physical examination of this specimen that arrived just a few days ago. It is deposited in the PWMC. Thank you Owen! Your post is currently the best BugGuide photo of this relatively uncommon species.

Moved from Harpalini.

Selenophorus opalinus
is my image-based determination for this relatively common species known throughout the eastern half of North America. Note the "seriate" punctures (in a series) along elytral striae numbered 2,5, and 7. This is a leading character trait of the genus. Furthermore, the elytra in this particular species are strongly "iridescent" (rainbow reflections when tilting specimen back and forth) but of course a photo can't show that striking feature. This specimens is near the smallest known ABL (8.0 mm) for the species. Owen, please save this one for me. Selenophorus and its cousins (Discoderus, Amblygnathus, etc) are the focus of my taxonomic research. Thank you!

Thank you!
I didn't note those seriate punctures. They're very visible on elytral stria 7 in my pic. Perhaps it may be a smaller species if it seems small for that species? It is in alcohol for you to determine in maybe a month's time. :)

A classic case of an image leading the "specialist" astray.
I assumed that the elytra were iridescent based on my presumption that the overall gestalt favored S. opalinus. A routine photograph usually does not resolve the question of iridescence, but if there were iridescence pesent, the elytra ought to be perhaps shinier than what they appear in the image. S. gagatinus has duller elytra due to coarser microsculpture and therefore no such iridescence. I also note that the femora are dark brownish in the image. That favors S. gagatinus. S. opalinus has uniformly yellowish legs. The ABLs nearly overlap between the two species. S. opalinus is (8.0) 9 - 10.7 mm; S. gagatinus is (6.5) 7.0 - 7.5 mm (8.0). The parentheses contain uncommon outlier lengths. Other structural differences are more subtle which can be easily misinterpreted due to photographic trickery. For example, S. gagatinus has a notably more convex pronotum but that trait is not so convincing in the image. In gagatinus the forebody (head and pronotum) is proportionatly narrower compared to the elytra; opalinus has broader head and broader pronotum with sides less curved.

This exercise now brings me to overall favoring S. gagatinus. It is a member of the S. opalinus species group which shares the eastern half of North America with S. opalinus. There are five quite similar North American species that belong to the opalinus species group. But in Ontario these are the only two. It's frustrating that I had to convey so much uncertainty to BugGuide readers when, in fact, the two species under the scope are immediately and easily recognized. I look forward to examining this specimen. Thanks Owen.

In the meantime, I'm pleased to announce that Brian Raber and I submitted a paper last week on Nearctic Selenophorus. This comprehensive review of the genus includes 8 new species, numerous taxonomic changes, new state and country records, plates, and a detailed key to all 37 species north of Mexico.

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