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Sunday, 29 May 2016

An emerging dragonfly - making a timelapse

The Common Clubtail dragonfly - Gomphus vulgatissimus - is a species that favours slow-moving, silty rivers.

(Can't wait? Go straight to the timelapse video!)

In Britain, the Clubtail's major locations are the Rivers Thames and Severn.
In particular a very popular place to see them, and to watch the adults emerge from their larval form, is the railway bridge over the Thames at Goring.

All dragonflies and damselflies spend the majority of their lives out of our sight as as larvae living underwater. The details are different for different species in different habitats and locations, of course, but in general flying adult damselflies live for about a month, and adult dragonflies live for about 2 months, while all larvae live for at least 10 months, many for longer.
Their life underwater (again different for different species and situations) is spent either swimming and chasing prey or hiding and ambushing prey. Whatever the lifestyle, the larva is shorter than the adult and has only small non-functioning wings but, when the time and conditions are right, it has to transform itself into a flying adult. This is called "emergence".
For a few days before emergence, the larva will be undergoing an extraordinary transformation within it's body. In effect, it will change into the adult within it's larval skin, so that when it decides to emerge, it can climb out of the water onto a stem of a plant or a hard surface and the adult can break out of the skin and fly off.
The process of emergence is probably the most dangerous part of the insect's life, because for a short period (short, but it might be an hour or more), it cannot move to escape predators.

At Goring, the bridge pillar on the north bank is built behind a concrete revetment with the path between and, while a concrete wall is certainly not the most beautiful setting for either viewing or photographing dragonflies, it is none-the-less reasonably easy for us to to get to and see.
Easy for us - but also, as the Common Clubtail likes to emerge on a hard surface, this concrete revetment is ideal for them.

On Monday (23rd May 2016), I went to Goring in the hope of finding at least one emerging dragonfly. It took an hour or so of waiting at the bridge, and I was actually thinking of abandoning the quest, but a larva did eventually climb out of the water. I leapt into action and set up my camera to capture a sequence of the dragonfly adult breaking out of it's larval skin.
I was prepared to hang the camera down from the top of the revetment, but this larva chose to climb up close to the end of the wall, so I was able to set up in a more traditional way.
So with the tripod standing in the water, over the next 70 minutes the camera captured more than 2500 still images of the dragonfly as it emerged.

I was talking to some other dragonfly-searching visitors while this was going on, when one of them exclaimed "it's gone". A few moments' panic ensued as I looked for the adult in the water (on a previous occasion, I have watched a Gomphus emerge only for it to lose it's grip a fall into the water), but couldn't find it. I stopped the camera shooting, and a quick review of the last few images revealed that it had in fact successfully flown. Excellent!

And so to the video. Creating timelapse videos on the computer takes quite a long time. Importing, adjusting and compiling 2500 stills is a big task. It's even worse when the computer is a few years old. Mine was pretty high-end in it's day, but it's day was at least 4 years ago.
It took a week of waiting for the computer to finish tasks, but finally here it is...

On this occasion I didn't see an adult, but here's a shot of one from 2010 so that you can see what they look like!

Common Clubtail - Gomphus vulgatissimus

To complete the tale, while waiting for the dragonfly to appear and emerge, I found a total of 8 other larval skins (known as "exuviae", singular exuvia). I'm told by the BDS Twitter account that this is a promising start, and bodes well for a better Gomphus season than last year.
Let's hope so!