The rarest creature in Oxfordshire?

When thinking of what might be rarest creature in Oxfordshire it's easy to think of the large mammals which get a lot of media interest - the otter, once extinct from virtually all of England or the water vole - the fastest declining mammal in England and still missing from a lot of it's previous sites.
But probably the rarest animal is a creature that we wouldn't recognise even if we did actually get to see it and a contender is an insect found only in a couple of very small streams within the Cothill Fens - a series of nature reserves outside of Abingdon that feed Sandford Brook - a tributary of the Ock:
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From a distance, it looks like any of the small blue damselflies that are found around rivers and ponds at this time of year.
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And it requires a closer inspection to reveal the markings of the Southern Damslefly:

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What distinguishes it from the other small blue damselflies is the markings on the top of the abdomen.

An insect so rare that is only found in a very few sites -  shallow base-rich streams within an acidic heathland - and the UK population is estimated to be a quarter of  global population.
Yet the Oxfordshire population is so small and fragile that it doesn't even get mentioned in some field  guides.

It's scarcity lead it to be one of the insects that feature in a series of stamps released in 2008 to highlight the plight faced by many UK insects.

http://www.norphil.co.uk/2008/04b-endangered_insects.htm


References:

http://jncc.defra.gov.uk/protectedsites/sacselection/species.asp?FeatureIntCode=S1044
http://www.british-dragonflies.org.uk/species/southern-damselfly
Dragonflies & Damselflies of Great Britain & Ireland - Brooks & Lewington