Determining final instar the Cham way

Did you notice I added a new text label to the annotated image of an exuvia from a Comet Darner dragonfly (Anax longipes) featured in my last two blog posts? I added the label in order to make the connection between this image and related ideas discussed in two other relatively recent blog posts (See Related Resources, below).

Remember that all dragonflies and damselflies have a 10-segmented abdomen, numbered from front to back. “S4” stands for abdominal segment four.

Counting odonate abdominal segments can be challenging sometimes. A good strategy to avoid mis-numbering is to begin counting abdominal segments from S10 (located toward the posterior end of larvae (nymphs)/exuviae) and work toward the thorax.

Final instar, the Cham way

There is a simpler way to estimate final instar than calculating instar equivalent.

Larvae in the final stage can be recognized by the length of the wing buds which cover the fourth abdominal segment. Source Credit: Field Guide to the larvae and exuviae of British Dragonflies, by Steve Cham, p. 30.

Look at the preceding annotated image. Notice the tips of the wing pads reach the fourth abdominal segment (S4), indicating the dragonfly larva that emerged from this exuvia had reached final instar. And that leads to the other idea I mentioned at the outset of this blog post.

Every odonate exuvia is a cast skin of the larva at F-0, the final instar, before it emerges to become an adult.

Turns out that’s another nugget of gold paraphrased from Steve Cham’s beautiful little book.

Post Update

The beauty of the Cham way of determining final instar is it’s simplicity. That’s the upside. The downside is there’s no way to determine the actual instar when it isn’t F-0.

For example, the following composite image shows dorsal views of a Common Green Darner (Anax junius) nymph (larva) and exuvia. As expected, the exuvia is F-0 because its wing pads cover S4. On the other hand, the nymph is F-? because its wing pads only reach S2.

Image used with written permission from Freda van den Broek.

Photo Credit: Both specimens (shown above) were collected by Freda van den Broek. The nymph was collected on 06 April 2020 from the Milwaukee River in Ozaukee County, Wisconsin USA, photographed, and released unharmed. The exuvia was collected from Ozaukee County too.

Related Resources

Copyright © 2021 Walter Sanford. All rights reserved.

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