Ventromedial groove

In North America (restricted to Canada and the United States of America) there are seven families of dragonflies in the suborder Anisoptera.

Identification of dragonfly larvae (nymphs)/exuviae to the Family Level is fairly straightforward for five of the seven families.

Flat labium

Three families feature a flat labium and are easy to differentiate: Aeshnidae (Darners); Gomphidae (Clubtails); and Petaluridae (Petaltails).

Darner larvae (nymphs)/exuviae have long, thin antennae; Clubtail larvae/exuviae have short, thick antennae. Petaltail larvae/exuviae feature several distinctive field marks that are easy to recognize.

Mask-like labium

Four families feature a mask-like labium: Cordulegastridae (Spiketails); Corduliidae (Emeralds); Libellulidae (Skimmers); and Macromiidae (Cruisers).

Two of these four families are easy to recognize. The distinctive jagged crenulations on the face mask of Spiketails are unmistakeable! Cruisers feature a prominent “horn” on the face-head.

In the opinion of the author, larvae/exuviae from Emeralds and Skimmers can be challenging to differentiate and identify to the family level.

Anal pyramid

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.

Ventromedial groove

Another way to differentiate Emerald from Skimmer larvae/exuvia is to look for a “ventromedial groove” in the prementum: it’s probably Corduliidae (Emeralds) if there is a ventromedial groove; it’s probably Libellulidae if there isn’t.

For example, look at the full-size version of the following annotated image of a Common Baskettail dragonfly (Epitheca cynosura) exuvia. Notice the groove down the middle of the prementum, oriented vertically in the photo — that’s a “ventromedial groove” (a compound word derived from “ventral” and “medial/midline”) — indicating this specimen probably is from Family Corduliidae (Emeralds).

Common Baskettail (Epitheca cynosura) | exuvia (face-head)

… the groove is developed on the basal half of the prementum … Source Credit: K. J. Tennessen, Dragonfly Nymphs of North America, https://doi.org/10.1007/978-3-319-97776-8_11, p. 346.

Here’s another example showing a ventromedial groove in the prementum of a Prince Baskettail dragonfly (Epitheca princeps) exuvia.

Prince Baskettail (Epitheca princeps) | exuvia (prementum)

In contrast, look closely at the full-size version of the following photo of a Spot-winged Glider dragonfly (Pantala hymenaea) exuvia. Notice there is no ventromedial groove in the prementum, indicating this specimen probably is from Family Libellulidae (Skimmers).

Spot-winged Glider (Pantala hymenaea) | exuvia (ventral)

An important caveat

Beware: Some larvae/exuviae in other families of dragonflies, such as Tiger Spiketail dragonfly (Cordulegaster erronea), feature a ventromedial groove in the prementum. Therefore the take-away from this post is the presence of a ventromedial groove is not a single definitive morphological character for Emeralds, rather it should be used in combination with other field marks.

Related Resources

• A Beginners’ Guide to Identifying the Exuviae of Wisconsin Odonata to Family, by Freda van den Broek and Walter Sanford.
• Epitheca cynosura exuvia
• Epitheca princeps exuvia
• MYN – More Pantala hymenae exuvia
• Relative length of cerci versus paraprocts
• Anal pyramid

Copyright © 2021 Walter Sanford. All rights reserved.

Tags: Family Corduliidae (Emeralds), Family Libellulidae (Skimmers), Signs of Wildlife