Wednesday, May 26, 2010

Tales from the Riverbank....


Many of the rocks along the banks of the upper reaches of the river Wear carry the empty nymphal cases of stoneflies, recognisable by the paired tail filaments. The split in the thoracic segments marks the point where the adult insect eased its way out of the nymphal exoskeleton after the nymph crawled out of the water.


Stoneflies tend to be confined to fast-flowing, well oxygenated, unpolluted rivers, which is a pretty good description of this stretch of the river Wear between Wolsingham and Black Banks, in Weardale. Currently, after a long spell without heavy rain, the water level is low and mayflies are laying eggs in the shallows and pools.


Mayflies are the only insects that moult in their adult stages. The winged adult that emerges from the nymph is the subimago (known to anglers as a 'dun') and this undergoes a final moult to become the imago ('spinner' in angling parlance). This is an imago that appears to be laying eggs, and drowning in the process. Females have shorter front legs than the males.


Mayfly nymphs have three tail filaments but the adults can have two or three tail filaments, depending on genus. The hind wings in mayflies are much shorter than the forewings....


.. and both pairs are held vertically above the body when at rest.


There are two large compound eyes and three small ocelli in between them. Adult mayflies do not feed during their short lives, so there are no functional mouthparts.


Male mayflies are notable for their extremely long front pair of legs, which are used to grasping females during mating flights.


During emergence and mating flights low over the water surface, vast numbers fall victim to fish - especially leaping trout - while those that survive fish predation ...


....become food for birds like dippers, which also feed on the nymphal stages of stoneflies and mayflies.

14 comments:

  1. Fascinating, Phil. Lovely River Wear picture too.

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  2. Beautiful wings on the Mayfly - in holding them vertically like that it has something in common with butterflies.

    The IoW is sadly lacking in that wonderfully rocky and wooded river scenery you have up there.

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  3. As ever a pleasure to read and learn, thanks Phil. My wife has also become a fan of your very educational and fascinating insights.

    Stewart

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  4. Just found your site, Phil. Thank you for a fascinating insight into a world that I would not normally see. I shall follow you site with interest!

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  5. Great detailed photographs and explanation Phil. It's many years since I last saw the fantastic sight of hundreds of Mayflies flying together over a stream in Yorkshire.

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  6. It's a lovely stretch of the river at Wolsingham, Emma .... always something intersting to see there

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  7. Hi Rob., I miss chalk streams and chalk downland, though ....

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  8. Thanks Stewart .... and thanks to your wife ... for visiting

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  9. Thanks for visiting and for your kind comments Richard

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  10. Hi John, yes, when you get a mass emergence a little later in the year, on a hot, still afternoon, and the trout are leaping it's quite a sight...

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  11. This is a really great post the pictures are all superb and the writing inspires me to get out there, thank you.

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  12. What great photos and great closeups, rivers have always fasinated me and I can never pass a stream without turning over a few stones to discover what aquatic live there is present. Dare I suggest that I think the photograph of the fish is a shoal of minnows.

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  13. Hi Adrian, Nothing ike spending an afternoon on the riverbank on a hot afternoon - which it was last week, when I took these pictures. It's a bit on the cool side this wekend.. all the best,

    Phil

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  14. Hi David, I'm sure you're right - the pools that the falling water level had left behind were full of minnows too. I used to take our kids to this part of the river to catch minnows with the time-honoured method of a bottle with string around the neck and a piece of bread inside

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