Relative length of cerci versus paraprocts

A quote from my last blog post sets the stage for this post …

One way to differentiate Emerald from Skimmer larvae/exuvia is to examine the anal pyramid: it’s probably Corduliidae (Emeralds) if the cerci are at least half as long as the paraprocts; it’s probably Libellulidae (Skimmers) if the cerci are less than half the length of the paraprocts. More about this in a follow-up blog post. Source Credit: Ventromedial groove, by Walter Sanford.

Here are two examples that illustrate when the method works to confirm exuviae are from Family Libellulidae (Skimmers).

Eastern Amberwing (Perithemis tenera)

Look closely at the full-size version of the following annotated image of an exuvia from an Eastern Amberwing dragonfly (Perithemis tenera).

Notice the cerci are slightly less than half the length of the paraprocts, indicating this specimen is from Family Libellulidae (Skimmers).

Carolina Saddlebags (Tramea carolina)

The next specimen is an exuvia from a Carolina Saddlebags dragonfly (Tramea carolina).

Notice the cerci are slightly less than half the length of the paraprocts, indicating this specimen also is from Family Libellulidae (Skimmers).


Finally, here’s an example that illustrates when the method doesn’t work.

Spot-winged Glider (Pantala hymenaea)

The last specimen is an exuvia from a Spot-winged Glider dragonfly (Pantala hymenaea). Spot-winged Glider is a member of Family Libellulidae (Skimmers).

Notice the cerci are longer than half the length of the paraprocts, suggesting this specimen is from Family Corduliidae (Emeralds). But the exuvia isn’t an Emerald — it’s a Skimmer!

As it turns out, for genus Pantala it’s more important to look at the relative length of the superior abdominal appendage (epiproct) versus the inferior abdominal appendages (paraprocts). Who knew?

What the take-aways?

Although I haven’t identified (with certainty) many species of exuviae from Family Libellulidae (Skimmers), it didn’t take long to find an exception to the rule of thumb for differentiating Emeralds and Skimmers by examining the relative length of cerci versus paraprocts.

In contrast, the rule of thumb that looks at the presence or absence of a ventromedial groove seems to work every time, with the caveat that some species in other families feature a ventromedial groove. In those cases, the family to which the specimen belongs is fairly obvious.

I think it’s good to have more than one arrow in your quiver, so you should be familiar with both methods for differentiating Emeralds and Skimmers. That said, I recommend starting the process of identification by looking for a ventromedial groove.

Related Resources

Copyright © 2021 Walter Sanford. All rights reserved.

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