On Smooth Sowthistle. Underside of these discoloured purple. See RHoman comments on detail below
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Take a look at the Dutch leafminer website (its bilingual) where you will find keys that I have found very useful. http://www.bladmineerders.nl/plantenf/pfasteraceae/sonchus.htm and scroll down to the English key.
University of Edinburgh and Biodiversity Observatory (OU)
Many thanks R.Homan- that was my best guess just from the photos and the description so am very pleased to have guessed right. I am really impressed with the leafmines.co.uk site. A mine of information.
Jonathan, sorry, I didn't notice the English version of the keys, just saw Dutch, and it gives me a sore throat just looking at those words. I will have a go at using it backwards on these two.
Jamie from Briantspuddle
I realise that the .authorities prefer 'atricornis' but the quote marks flaw the ID panel - I may return Rob to 'correct' this.
Before posting my own I visited every post of Chromatomyia 'atricornis' to see if I could learn more - I did. But like every good iSpotter I have returned to agree where possible.
So why not look at mine http://www.ispotnature.org/node/740413
Lat/Lng: 50.7, -2.3
OS grid ref: SY8193