Several unknown dragonfly exuviae were collected on 07 July 2016 from the Potomac River, Fairfax County, Virginia USA; two of the specimens are featured in this post. A two-step process was used to identify the genus and species of the specimens.
Family
First, determine the family of the specimens. For reference, watch the excellent Vimeo video, Identifying dragonfly larva to family (8:06).
The exuviae have a mask-like labium (not flat) with smooth crenulations, indicating these individuals are members of Family Libellulidae (Skimmers).
Genus and species
A dichotomous key was used to tentatively identify the exuviae as Eastern Amberwing dragonfly (Perithemis tenera), as indicated by the following morphological characteristics.
- The cerci (sing. cercus) are slightly less than half the length of the paraprocts.
- Dorsal hooks are clearly visible on abdominal segments four through nine (S4-9), plus a “nub” that is visible on segment three (S3).
- Lateral spines are clearly visible on abdominal segments eight and nine (S8-9).
These specimens are the first odonate exuviae that I was able to identify to the species level. Sincere thanks to Sue Gregoire, Kestrel Haven Migration Observatory, for verifying my preliminary observations and tentative identification.
No. 1
Each specimen is approximately 1.4 cm (~0.6″) in length and approximately 0.6 cm (~0.2″) in maximum width. In Photo No. 1, the specimen shown on the left is an emergent nymph that was stuck in its exuvia.
No. 1 | Eastern Amberwing (Perithemis tenera) | exuviae (lateral, dorsal)
(See a full-size version of the original photo, without annotation.)
The white filaments that extend from the split in the thorax (as shown in Photo No. 1-3) are breathing tubes, artifacts of the unique respiratory system of dragonfly nymphs.
No. 2
No. 2 | Eastern Amberwing (Perithemis tenera) | exuviae (dorsal)
No. 3
The eyes are relatively small and widely separated. Notice the mask-like labium (sometimes referred to as “spoon-shaped”) with smooth crenulations along the margins between two lateral lobes.
No. 3 | Eastern Amberwing (Perithemis tenera) | exuviae (head-on)
(See a full-size version of the original photo, without annotation.)
No. 4
One of the keys to identifying skimmer dragonflies to the species level is to carefully examine the anal pyramid (see S10, shown below), including the cerci (sing. cercus) and paraprocts. Notice the cerci are slightly less than half the length of the paraprocts.
No. 4 | Eastern Amberwing (Perithemis tenera) | exuviae (anal pyramid)
(See a full-size version of the original photo, without annotation.)
Related Resources
- Vimeo video: Identifying dragonfly larva to family (8:06)
- Anatomy of a Dragonfly Larva, annotated illustration courtesy Jay Smith
- Odonate Exuviae, by Walter Sanford
- Identification Keys to Northeastern Anisoptera Larvae, compiled by Ken Soltesz. Key to the Genera of the Family Libellulidae, p. 36.
dichotomous key: a key for the identification of organisms based on a series of choices between alternative characters. Source Credit: Merriam-Webster Dictionary.
Assuming the convention of labeling the two branches of each dichotomy as “a” and “b”, e.g. 1a, 1b, etc., a list of branches in the decision tree that I used to identify the species of the dragonfly exuviae is as follows: 1b; 4b; 5a; 6a BINGO!
In long form, the decision tree is as follows:
p. 36, Key to the Genera of the Family Libellulidae
1b – Eyes lower, more broadly rounded and more lateral in position; abdomen usually ending more bluntly. [Go to] 4
4b – These appendages [inferior abdominal appendages (paraprocts)] straight or nearly so. [Go to] 5
5a – Dorsal hooks on some abdominal segments. [Go to] 6
6a – Dorsal hook on 9. Perithemis (One species, Perithemis tenera.) BINGO!
Tech Tips
The following equipment was used to shoot the preceding photographs:
- Photo No. 1: Canon EOS 5D Mark II digital camera; Canon EF100mm f/2.8L Macro lens; Canon 580EX external flash (ETTL mode) off-camera, using a pair of Yongnuo YN622C II Wireless Flash Trigger Transceivers; and a Nissin i40 external flash (off-camera, SF mode).
- Photo No. 2-4: Canon EOS 5D Mark II digital camera; Canon EF100mm f/2.8L Macro lens; Canon 580EX II external flash (ETTL mode) off-camera, using a coiled six-foot Vello Off-Camera TTL Flash Cord for Canon Cameras; Nissin i40 external flash (off-camera, in SF mode). A Raynox DCR-250 close-up filter was used for Photo No. 3-4.
Adobe Photoshop CC 2017 was used to annotate selected images.
Copyright © 2016 Walter Sanford. All rights reserved.
Tags: anal pyramid, cerci, crenulations, dorsal hooks, Eastern Amberwing dragonfly, epiproct, exuviae, Family Libellulidae (Skimmers), gear talk, lateral spines, paraprocts, Perithemis tenera, Potomac River, wildlife photography
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