An exuvia from an unknown species of dragonfly was collected on 04 October 2016 at Mason Neck West Park (MNWP), Fairfax County, Virginia USA. A two-step process was used to identify the genus and species of the specimen.
Family
First, determine the family of the specimen. For reference, watch the excellent Vimeo video, Identifying dragonfly larva to family (8:06).
The exuviae has a mask-like labium (not flat) with evenly-toothed crenulations, indicating this individual is a member of Family Libellulidae (Skimmers). [See Photo No. 2.]
Genus and species
A dichotomous key was used to tentatively identify the exuvia as Carolina Saddlebags dragonfly (Tramea carolina), as indicated by the following morphological characteristics.
- No dorsal hooks on any abdominal segments.
- Lateral spines of abdominal segment nine (S9) are much longer than its mid-dorsal length. Lateral spines on segment eight (S8) are nearly as long as on segment nine (S9).[See Photo No. 3.]
- Superior abdominal appendage (epiproct) is shorter than inferior abdominal appendages (paraprocts). [See Photo No. 3.]
Sincere thanks to Sue Gregoire, Kestrel Haven Migration Observatory, for verifying my preliminary observations and tentative identification!
No. 1
The specimen is approximately 2.4 cm (~0.9″) in length. Notice there are no dorsal hooks on any abdominal segments.
The white filaments that extend from the split in the thorax (as shown in Photo No. 1-2, 4-6) are breathing tubes, artifacts of the unique respiratory system of dragonfly nymphs.
No. 2
The eyes are rounded and widely separated. Notice the mask-like labium (sometimes referred to as “spoon-shaped”) with evenly-toothed crenulations along the margins between two lateral lobes.
(See a full-size version of the original photo, without annotation.)
No. 3
A vertical white line marks the mid-dorsal length of abdominal segment nine (S9), as shown in the following annotated image; the vertical black line labeled “mid-dorsal length” is the same length as the white line. Notice the lateral spines of abdominal segment nine (S9) are much longer than its mid-dorsal length.
(See a full-size version of the original photo, without annotation.)
One of the keys to identifying skimmer dragonflies to the species level is to carefully examine the anal pyramid (S10), including the cerci (sing. cercus), epiproct, and paraprocts. Notice the epiproct is shorter than the paraprocts.
There is a lot of “seaweed” (aquatic vegetation) clinging to the exuvia, especially noticeable at the posterior end. Some collectors like to clean their specimens; I prefer to photograph them “as is.”
More photos of the exuvia are shown below.
No. 4
No. 5
No. 6
Related Resources
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 genus of the dragonfly exuviae is as follows: 1b; 4b; 5b; 10a; 11b; 12b Tramea. A supplemental key featuring one dichotomy was used to identify the species: 1a carolina 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
5b – No dorsal hooks on any abdominal segments. [Go to] 10
10a – Lateral spines of segment 9 much longer than its mid-dorsal length. [Go to] 11
11b – Lateral spines on 8 nearly as long as on 9. [Go to] 12
12b – Superior abdominal appendage (epiproct) shorter than inferiors [inferior abdominal appendages (paraprocts)]. Tramea BINGO!
p. 41, Key to the species of the genus Tramea
1a – Lateral spines of segment 8 directed straight to rearward; paraprocts longer than epiproct; two rows of spinules on upper surface of epiproct. carolina BINGO!
Tech Tips
The following equipment was used to shoot the preceding photographs:
Adobe Photoshop CC 2017 was used to annotate selected images.
Copyright © 2016 Walter Sanford. All rights reserved.
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