Saturday, October 13, 2012

Examining Exuviae

Dragonfly exoskeleton exuviae (c) John Ashley
Transparent exoskeleton left behind after a larval dragonfly turned into an adult

 As you may know, dragonflies live double lives.

Juvenile dragonflies are long-lived, aquatic predators, while adults are short-lived aerial (terrestrial) predators. And at the exact location where these lifestyles meet, you will find a little golden-brown, dragonfly exoskeleton. These transparent exoskeletons are called "exuviae," and they are the gold-standard when using dragonflies to evaluate wetland habitat quality.

Because of their duality, dragonfly populations are good indicators for the health of both aquatic and terrestrial habitats. Most wetland projects only look at adult dragonflies because these surveys are relatively easy and inexpensive. A few projects also survey for the aquatic, juvenile dragonflies. Both of these methods, however, have a built-in bias.

Adult dragonflies are relatively easy to survey, but many pairs will lay their eggs in low-quality habitats where the larvae will not hatch and survive to adulthood. So sampling adult dragonflies can over-estimate the aquatic habitat quality. Finding dragonfly larvae without finding exuviae indicates low-quality habitat where the life cycle is not completed, so sampling for larvae only can also over-estimate habitat quality. Combining exuviae surveys with either adult or larval surveys is now the recommended dragonfly protocol.

Click to see facial details
Click to see facial details
When found and closely examined, dragonfly exuviae are amazing in their level of detail. The outer layer of everything from eyeballs to antennae is left behind in an empty shell of delicate perfection.

When preparing to emerge, the adult dragonfly body forms inside the larval exoskeleton. Even the adult wings form inside the small, larval "wing pads." The transformed larvae climbs out of the water and grabs hold of a rock or plant stem. A series of air intakes exerts pressure on the back, the old skin splits, and the new adult pulls itself out of its old exoskeleton. The crumpled wings are pumped full of fluid and allowed to harden for an hour or two. This temporary, soft-skinned adult condition is called the "teneral" period.

If no predators discover the defenseless, teneral dragonfly, then a hardened adult will soon fly away to begin their second lifestyle as acrobatic, aerial predators.

You can find a general guide to the exuviae of eastern U.S. dragonflies here

Behind the lens: photographs made with 105mm micro lens, 12mm extension tube and ring flash on Nikon D800 body. This is a great set-up for insect photography. Main lighting is sunset back-lighting on white foam core, with just a little flash fill.