I’m a man on a mission to demystify identification of odonate exuviae, as I’m fond of saying. Easier said than done. In my experience, the process of identification can be challenging at best and impossible at worst.
For example, it’s nearly impossible to differentiate exuviae from the Genus Tramea (Saddlebags) to the species level.
The search for reliable larval characters to distinguish species of Tramea has generated considerable confusion in the literature. Source Credit: Landwer, Brett & Sites, Robert. (2006). Diagnostic efficacy of morphological characters of larval Tramea lacerata Hagen and Tramea onusta Hagen (Odonata: Libellulidae). Great Lakes Entomologist. 38. 155-163.
More recently, Tennessen cited the preceding research as well as a follow-up article by the same authors in 2010, and wrote…
…specific identification is still problematic. Source Credit: Tennessen, Kenneth. (2019). Dragonfly Nymphs of North America – An Identification Guide. Springer International Publishing. 567.
The fact of the matter is the most reliable way to identify odonate larvae to the species level is to rear them to maturity and emergence, that is, unless you’re fortunate to find a larva emerging in the field. Since an exuvia is essentially a nearly perfect shell of the last instar, it can be used to identify other specimens of the same species by pattern matching.
Rearing an unknown species of larva from genus Tramea
A larva from the Genus Tramea (Saddlebags) was collected by Andy Davidson near Richmond, Virginia USA, and reared to maturity. The following photo shows the adult dragonfly soon after emergence.
The image was rotated in order to get a better look at the shape of the “saddlebags” on the rear wings of the dragonfly.
The following composite image — created by Ed Lam, author and illustrator of Damselflies of the Northeast — clearly shows the difference in the shape of the saddlebags for Carolina- versus Red Saddlebags. Look closely at the saddlebags in the full-size version of the preceding photo and you can see the pattern perfectly matches the Carolina Saddlebags in Ed’s image, shown below.
Composite image used with permission from Ed Lam.
Look-alikes: Genus Pantala and Genus Tramea
Two genera from the Family Libellulidae (Skimmers) look similar: Genus Pantala (Rainpool Gliders); and Genus Tramea (Saddlebags). If you find an exuvia with long “tail fins,” then it might be a member of one of these two genera.
A Spot-winged Glider dragonfly (Pantala hymenaea) exuvia was collected by Andy Davidson near Richmond, Virginia USA.
A Carolina Saddlebags dragonfly (Tramea carolina) larva was collected by Andy Davidson near Richmond, Virginia USA, and reared to maturity. Andy saved the exuvia after emergence.
The following couplet from Identification Keys to Northeastern Anisoptera Larvae, compiled by Ken Soltesz, can be used to differentiate exuvia from Genus Pantala and Genus Tramea.
p. 37, Key to the Genera of the Family Libellulidae
12a – Superior abdominal appendage (epiproct) as long as, or longer than inferiors [paraprocts]. Pantala
12b – Superior abdominal appendage (epiproct) shorter than inferiors [paraprocts]. Tramea
What are the take-aways?
If you find an exuvia from the genus Tramea, then you might be unable to identify it to the species level.
I collected a Tramea sp. exuvia during Fall 2016 from a water retention pond located at a small park in Fairfax County, Virginia USA. I keyed out the specimen and identified it as Tramea carolina, in part, because Carolina Saddlebags had been observed in numbers at the same location. Am I certain of the identity? Yes and no. I’m certain the specimen is from genus Tramea; I’m reasonably sure (but not certain) it’s T. carolina. That said, my rationale is fairly good.
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Copyright © 2020 Walter Sanford. All rights reserved.
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