05 March 2012

Collecting dragonfly exuviae

Exuviae are the cast nymphal (or larval, depending on which term you are most familiar with) skins of the penultimate instar of Odonata. They provide important information about where species live and where they emerge. Since the larval characters are quite evident, most exuviae are identifiable to species level. Exuviae are not living, so participants not wanting to collect and kill living odonates can make a contribution just by conducting surveys for exuviae along rivers, streams, and lakes. In addition, it has been recognized by the U.S. Fish and Wildlife Service that exuviae collections are acceptable means of documenting the presence of threatened/endangered species of Odonata. Exuviae indicate the presence of larval populations and past breeding populations at a particular locale, and therefore, they are valuable records for the Atlas. Participants are encouraged to collect exuviae and note the location and conditions where they are found. The UMMZ has an outstanding collection of Odonata exuviae, and most are catalogued in a database. Exuviae should be documented with as much care as adults.

To collect an exuvia, carefully remove it from the substrate that is resting on. You may wish to photograph it in situ before removing it. Note the type of substrate that is is resting on, and the height/distance from water. Place the exuvia in a rigid container (when 35mm film was so commonly used, the plastic film cans were excellent collecting containers), such as a plastic pill vial, film can, mints tin, etc. Label it with your field number (see the page on submitting vouchers -- the documentation and submission procedure is the same). When we receive the collected exuviae, they are immersed in ethanol and placed into glass vials, labeled, and identified. All that information is recorded into our database. Variation in emergence sites says something about the behavior of the adults and when they emerge, and recording this information is very useful.

Odonata exuviae are identified the same way as preserved nymphs. We keep the exuviae in ethanol so as to keep the exoskeleton flexible and so the bits don't break off, which they would if dry. There is no all-around published guide to immature Odonata, and the best resource thus far is the still-incomplete Michigan key by Ethan Bright. It is a few years out-of-date on the distribution maps, and species found in the state, but it it still the best resource available, and is very useful.

You will usually find clubtail exuviae very close to the water's edge, often on rocks, bridge abutments, or debris. Basket-tails (Epitheca spp.) and many skimmers usually emerge in vegetation near the edge of water, often on shrubs or emergent reeds. Darners often emerge on cattails and similar plants a few inches above the water. However, you may find river cruisers and stream cruisers have emerged many yards from the water, up on the side of a structure or tree trunk. Damselfly exuviae can also be identified, but are not as robust as dragonfly exuviae, and are often missing the caudal lamellae, which are important for identification. If you are a keen observer, you can record the number of exuviae along a stretch of river or stream, which will often be useful in estimating the size of the emergence of a particular species. Exuviae surveys are often done in other states, and are used as a monitoring tool.

It is not uncommon to see adults emerging from the nympal stage. It's an amazing process that often takes just a few minutes. Of course, the adults are easier to identify than the nymphs, but photographing the adults and keeping the exuviae for vouchers is an acceptable compromise. In general, it's not recommended to collect adults immediately after they have emerged, as their bodies are too soft and lack the coloration necessary for identification. They will shrivel up to thin ghosts of themelsves. If you wish to collect them, please place the living specimen into a container large enough and rigid enough to allow them to complete their transformation into a more mature specimen. This usually requires about 4 to 8 hours for them to fully harden and not be what are called "tenerals." Then, photograph them if you wish, prepare them as specimens, or release them.

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