Great Blue Skimmer dragonfly terminal appendages

I spotted an unknown dragonfly on 17 May 2012 and sent the following photo (without annotation) to Matt Ryan for his take on the identity of the dragonfly. Matt and I had taken the same introductory class on dragonflies during Summer 2011 at Huntley Meadows Park (HMP). Both of us were eager to learn more about odonates in order to move to the next level of dragonfly spotting.

Matt and I narrowed the field of possible identities to either Great Blue Skimmer (Libellula vibrans) or Slaty Skimmer (Libellula incesta), but we weren’t certain of its gender or age. As I recall, Matt was the first one to mention terminal appendages — a useful field marker that turned out to be one of the key characteristics that enabled us to correctly identify the specimen as an immature male Great Blue Skimmer.

And so it began — my interest in learning more about dragonfly terminal appendages led to lots of new discoveries and remains one of the go-to field markers I look for when identifying dragonflies.

Great Blue Skimmer dragonfly (immature male)

17 MAY 2012 | HMP | Great Blue Skimmer (immature male)

Immature males appear similar to immature females of the same species (and some mature females) for many types of dragonflies that display sexual dimorphism. This is true for many members of the Skimmer Family of dragonflies, such as Great Blue Skimmer. Terminal appendages may be used to differentiate gender for many species of dragonflies.

P1180070-RW2-Ver3_Aperture-BFX32

16 AUG 2013 | ABWR | Great Blue Skimmer (mature male)

Male dragonflies have three terminal appendages, collectively called “claspers,” that are used to grab and hold female dragonflies during mating: an upper pair of cerci (“superior appendages”) and a lower unpaired epiproct (“inferior appendage”).

Female dragonflies have a pair of cerci (superior appendages) that have little or no function.

A Great Blue Skimmer dragonfly (Libellula vibrans) spotted at Accotink Bay Wildlife Refuge, Fairfax County, Virginia USA. This individual is a female.

12 SEP 2012 | ABWR | Great Blue Skimmer (mature female)

Look closely at the full-size version of the preceding image. Female Great Blue Skimmers have two flanges beneath their eighth abdominal segment (S8) that are used to scoop water when laying eggs (oviposition), hence the family name “Skimmer.” Remember that all dragonflies and damselflies have a 10-segmented abdomen, numbered from front to back.

Related Resources: Odonate Terminal Appendages.

Editor’s Note: Matt Ryan went on to become a professional naturalist who works at Huntley Meadows Park. Matt is an excellent all-around naturalist; botany is Matt’s area of specialization.

Copyright © 2015 Walter Sanford. All rights reserved.

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