Methods of odonatological investigations
How to collect dragonflies
In most parts of Europe adult dragonflies can be recognised in the field. However sometimes it is needed to collect specimens in order to check your identification or to have proof of your record. Larva and exuviae are often more difficult to identify and collecting is often necessary. Labelling is an important part of collecting, without a correct label material becomes worthless.
Collecting exuviae
Finding exuviae
Searching exuviae (the larval skins left when a adults emerge) can be very rewarding as you often will find exuviae of species which are difficult to find as adult. This is especially true for species of the family Gomphidae of which the adults seldom come to the waterside while exuviae can be common. Exuviae are always found near water. Look for them just above the surface to about 50 cm high (figure 6). Some species may go further, even several metres far. Dragonflies prefer vertical substrates for emergence (e.g. plants, tree roots, steep banks, rock faces), but may also be found on horizontal substrates (floating plants, flat banks). When you chase up a freshly emerged adult (which is still very pale, soft and shiny) from the waterside, try to catch it and look for the exuvia. Exuviae are nothing more than dried skin, and are therefore vulnerable. The legs easily break off when the exuvia is picked up: by splashing some water on it, the exuvia becomes moist and more flexible. Notes about the emergence site (place, height, substrate) can be useful.
Preserving exuviae
Exuviae must be stored dry. For storage, it is easiest to use photo film containers, as these are small, close tightly and are easy to come by, although their availability may diminish as the digital revolution progresses. Exuviae will often be moist after collecting, and when put in a closed container may become covered with fungi. Dry wet exuviae in the sun, or make a hole in the lid, so moisture can escape through it. Never store exuviae in alcohol, as they will become soft, soggy and battered.
Collecting larvae
Finding and catching larvae
Many species are more easily found as larvae than as adult. A small metal kitchen sieve is often most suitable to find larvae in standing waters (between water weeds, small patches of leaf litter, under roots of riverside trees). In these situations it can be handy to put the leaf litter and plants in a white plastic container. This makes it easier to find the larvae among the litter. In running water it is more convenient to have a larger net with a more sturdy frame. A good method to catch larvae that live under rocks or burried in sand, is to place the net on the bottom and disturb the substrate (stones, gravel, sand) upstream from the nets opening with a rock or your feet. The disturbance will dislodge and expose the larvae, the current carries them into the net. Another good method is to put roots of riverside trees that hang in the water in the net and shake them vigorously.
Transporting larvae
If you want to keep a larva alive there are three problems: drying, drowning and overheating. It is best to put the larva in a small, closed container. Do NOT put a larva in water, but rather in moist cotton wool or toilet paper, without or with very little free water and with plenty of air. Moss or other organic material can also be used, but only shortly as it may rot. Keeping the larvae cool (i.e. in a cool-box) at 5 to 10°C will extend their lifespan. Make sure that the larva remains moist and oxygenated.
Rearing (= breeding) larvae
Basically a tray, basin or aquarium with water will suffice as habitat for the larvae. Provide some substrate for the larvae (e.g. sand, detritus, water plants) and for emergence of the adult (e.g. a stick). Feed it with small aquatic invertebrates (mosquito larvae, cladocerans etc.). Keep the basin or aquarium out of the full sun. After emergence, it is essential that adult and exuviae are marked as belonging to the same individual!
Preserving larvae
Simply put them in 70% alcohol. Refresh the alcohol a few times. The larvae may become swollen. It is often good to prick some holes in a big larva with a pin, which prevents the swelling.
Collecting adults
Catching adults
Adults are caught with a net. Generally the type used for butterflies is suitable (net opening about 50-75 cm wide, handle about 100-200 cm long). The net must be deep enough to fold closed, so the catch cannot escape. For large Anisoptera a large, light net is useful. Such a big net can be hard to manoeuvre when catching Zygoptera in dense vegetation. A net with a handle composed of aluminium tent pole segments or a telescopic rod is useful, because you can vary the length.
The easiest way to store live dragonflies is in small envelopes (like those used by philatelists for postage stamps) or folded paper triangles (papilottes) which are often used by butterfly collectors. Put pairs (tandems, copulas) together in one envelope. Carry a container (like a Tupperware box) with you to put them in. Because the colours of adults fade or change after preservation it is very valuable to take photographs of the adults, either free or in the hand.
Preserving adults
It is best to keep dragonflies alive some time after capture, so they can dispose of waste products. There are basically two preservation methods: acetone and alcohol. The first is definitely the best. Very fresh (teneral) specimens are still soft and will crumple when dried and are therefore best stored in alcohol.
1. Acetone. Throwing the individual in acetone kills it and makes it limp, so you can easily stretch the abdomen and legs a bit. Fold the wings above the body. Now leave it in acetone for 12 to 24 hours. When they are in the acetone put each individual in an envelope or between permeable paper, in order to avoid spreading of the wings. Afterwards dry it quickly. A breeze or some sun helps (but dont overheat). Keep your eyes on the material and beware of potential consumers, especially ants! Because the acetone has replaced the water in its body, the animal dries very quickly, with relatively little colour loss and odour development. Acetone also makes the material very stiff and sturdy. Putting the material in acetone is time-consuming, and acetone may be hard to find. Acetone is not allowed on aeroplanes, but should be available in chemical supply stores (for schools and laboratories) in larger towns. Using acetone gives the best material (strong, odourless, good colours), which has a lower risk of consumption by insects or fungi.
2. Alcohol. Simply put the adults in 70% alcohol. Refresh the alcohol frequently. The downside of collecting in alcohol is storage, as it requires a lot of small, not-leaking containers. These are expensive and take up a lot of room. Alternatively, put all specimens together in a large container, filled with alcohol. Keep the specimens separate by putting them in envelopes or paper triangles (but beware, the alcohol may dissolve the envelopes glue). When transporting such a container full of paper and dragonflies you can simply pour off the surplus alcohol. The paper will retain enough alcohol to keep the specimens saturated for several hours. As more studies of dragonfly DNA are undertaken, there is great demand for samples in which the DNA is preserved well: alcohol of 95% or more is best. Entire specimens can be preserved for this purpose (but refresh alcohol several times, as body moisture dilutes it) but alternatively you can removee one or two legs (with muscle). These can be preserved in alcohol, while the rest of the specimen is preserved with acetone.
3. In a very dry climate it is possible to simply dry dead specimens in their envelope, although this does not give ideal material.
For storage of dry material put the specimens in stamp envelopes or paper triangles in an airtight container. Label clearly! Add silica gel to keep the material dry (and to extract additional moisture) and perhaps some insecticide (e.g. naftaline).
Information from Naturalis webpage
