Tuesday, September 27, 2011

Identifying Dragonfly Skins?


Dragonflies are fascinating creatures and astoundingly good predators. Often ignored is that they spend most of their lives underwater as naiads - a technical term but nymph or larva is fine, too. Naiads eat anything that swims in front of them, catching it with a protrusible lower jaw. This includes aquatic insects, other dragonfly naiads, tadpoles and even small fish. When their time in the water is done, the naiad crawls up a piece of aquatic vegetation and sheds its exoskeleton. Now referred to as a teneral, it must dry out its exoskeleton and roll out its wings. But it leaves a shed skin, which we call an "exuvia" ("exuviae" is the plural - remember your latin?).

A teneral female Sympetrum vicinum, which will turn yellow in a few days as the exoskeleton dries

These exuviae stick to the plants for several weeks after the dragonfly emerges. Because they are the last exoskeleton of the larva, they retain all the larval characteristics. Through the hard work of many scientists - and amateurs - who have collected larvae and raised them up to see what hatches out, there are great keys available to identify larvae which also work for these exuviae. 

Common Green Darner, Anax Junius
The same species, adult male
This summer, I collected over 800 exuviae from two ponds on Nantucket and identified them. While this was a maddening task at first (how many dorsal hooks does it have, and what shape are they?), I got to be quite efficient after a few days of it. It is amazing how after awhile you learn to recognize general shape and size, characters not included in the keys, and can identify them correctly just by sight - despite that the technical ID requires counting spines. My results, albeit limited, showed that some adults at the pond do not reproduce there, as their exuviae were not found. This is a common and well-documented phenomenon in dragonfly communities and may represent marginalized individuals relocating to less-than-ideal breeding sites - i.e. places where their larvae do not survive.

A few more pictures to compare exuviae and adults:


The Blue Dasher, Pachydiplax longipennis
The same, an adult male

A meadowhawk: Sympetrum sp.


A Ruby Meadowhawk, S. rubicundulum

Collecting exuviae is great as you do not have to catch and identify adults and you get a permanent specimen out of it. Also, some dragonfly species are hard to find as adults, only flying in the early morning and at dusk, and thus it is easier to find the exuviae than the adults. Sampling for endangered odonates is often done by looking at exuviae instead of adults.

Aeshna clepsydra, a species more common as exuviae than adults

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