Article by Abigail Pedlow
As I'm sure you all know, dragonflies and damselflies spend much of their life as aquatic larvae, gaining size and developing, whilst pretty much top of the food- chain in waterbodies and wetlands.
But did you know that when a dragonfly emerges, it leaves behind a skin or exuvia that can be identified, often to species level, and even to know what sex it was? And you can do this on cloudy days, or colder days, or just when there are no dragonflies around.
Look for exuvia clinging with their hooked feet to stems and underside of leaves of emergent plants, on vegetation around ponds, and under bridges. They are easiest to find near water, but its worth looking slightly further afield as well.
Dragonflies have an interesting life-cycle and as part of this they go through incomplete metamorphosis – unlike a butterfly, where the larval form (caterpillar) is totally different from the adult form (butterfly or imago) and the stage in between (pupa) involves a total rearrangement of all the body cells.
In dragonflies you can effectively 'see' the adult dragonfly in the body of the larva – the larvae have six legs, they have eyes similar to the adult, and they have wing cases, and dragonfly larvae have a much more robust body shape than damselfly larvae.
Metamorphosis takes place within the skin of the final growth stage (instar) of the larva, and at this stage the larva begins to breathe air, so hangs out just under the surface of the water.
The metamorphosed larvae then leave the water either by climbing plants or climbing out of the water onto terrestrial vegetation, bare rocks, or even the ground.
The skin at the back of the thorax splits open and the dragonfly or damselfly emerges headfirst. The skin – or exuvia – is left behind clinging to the plant or substrate that the creature used to emerge on. Most emerge vertically but some species can emerge on horizontal surfaces.
These exuviae are easily crushed, but if undisturbed they will stay in position until rain washes them off.
Because they are the skin of the last instar of the larvae they can be identified using the keys to larvae found in most good dragonfly books.
This is much easier for dragonflies – damselfly exuvia are very small, more difficult to find and more fragile!
If you collect exuviae over a season you can build up a much more complete picture, than snatched moments by a pond can give you: which species bred; how many dragonflies/damselflies the pond has produced; and even the sex ratio (which may vary throughout the season, and may not be 1:1 even over a complete season). It also gives you an insight into what life is like under the water surface.
They are also very useful as proof of breeding at a site.
One sunny Sunday morning I wondered up to the garden pond with my cup of coffee, and was thrilled to see some recently emerged Common Darters, amongst the forest of Common Spike-rush. As I looked harder I spotted about twenty. I began to collect the exuviae, and kept on throughout the season. I'd only ever seen two adult males flying around the pond at any one time, but I collected over 250 exuviae Common Darter that summer, and I must have missed some!
Be prepared though, once you start looking, to find the occasional incomplete emergence i.e. the dragonfly hasn't completely emerged from the larval skin for whatever reason and has died. It is a bit sad, but it will provide nutrition to other creatures!
Exuviae are also very useful for education purposes – so if anyone builds up a collection and doesn't want to keep them – let us know, and we'll find a home for them, either with ourselves, the Museum, or the British Dragonfly Society.
What emerges from the exuvia is a soft-bodied, dull coloured adult; the body and wings need to harden before they can fly, and at this stage they are very vulnerable to predation. This immature stage is known as 'teneral'.
Once they can fly they disperse away from water bodies, the adult colouration develops, and they become sexually mature over the next few days.
And why the funny name? Well according to one internet site I looked at, it means 'to slough off, or shed'.