Thursday, August 3, 2017

Golden-ringed dragonfly


Dragonflies are the subject of my Guardian Country Diary today.

This is the peak period locally to watch these spectacular gold-ringed dragonflies Cordulegaster boltonii, which have the longest body (but not the greatest wing span) of any UK species. We discovered this female in the early morning, torpid after a cool night, resting on a patch of rushes next to a footpath.
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There are several prime locations for this impressive insect here in the North Pennines. One of the best is in Hamsterley forest, near Spurleswood beck, which has the kind of crystal clear, highly oxygenated water and stony stream bed that forms a perfect breeding habitat. Another is along the river Derwent near Blanchland in Northumberland. A third is in Weardale, in a shallow stony beck that runs down the hill into the top of Tunstall reservoir near Wolsingham in Weardale.














While I was photographing this individual she began to warm up her flight muscles, first with just the merest hint of a vibration in her wings, until she was like a helicopter on the verge of take-off. Then she let go with those legs, which are of little use for walking but are used for gripping perches and catching prey in mid-air ..... and she was gone, hunting flies over the bracken.















The marvels of dragonfly vision are almost beyond our comprehension. We have trichromatic vision, with sensors that detect red, green and blue, which together define our visual spectrum. Dragonflies have sensors that detect at least five wavelengths, in addition to ultra-violet which is beyond our perception, and some species have thirty sensors that each detect a different wavelength, so their colour spectrum is vastly more complex and subtle than ours.

It's an interesting thought that those bright colours that we admire when we watch a dragonfly are not the colours that they see.

Then there are those massive compound eyes, each composed of thousands of separate facets (ommatidia), which give them highly sensitive flicker vision. When fast-moving objects cross their field of view they are tracked by a succession of ommatidia that convey the information to the brain.

Their amazing eyes, coupled with their speed of flight and extreme manoeuvrability, make them deadly aerial interceptors.



6 comments:

  1. Dragonfly vision is amazing! Teasing apart what all those different receptors are used for will be quite a challenge. I have been intending to blog about this myself sometime! Butterflies have pretty amazing vision as well!

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  2. What a wonderful encounter! Thank you for sharing it.

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    1. Has to be my favourite dragonfly species, not least because it's the easiest to ID!

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  3. I've never taken to dragonflies - and even some times find them a bit gross. Damsel flies are a different matter. But maybe I'd like this one if I met it in person.
    https://looseandleafyinhalifax.blogspot.co.uk/

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    1. Southern hawkers are very inquisitive and will hover in front of your face and have a good look at you!

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