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This looks like a Robin's Pincushion Gall (Diplolepsis Rosae) which I found quite by accident in a guide book this evening.
Apparently it is common in Britain and Ireland, found on Dog Rose, and is caused by a small (4-5mm) black wasp. If it is this, expect it to go brown in winter and for there to be up to 60 larvae inside it!!
Source: "RSPB Handbook of Garden Wildlife", Holden, P. and Abbot, G. (2011), p. 117.
Hope that's a help. :)
Yes I have another picture of a brown one I found in early spring, so I now know what's going on in it! Thanks very much.
Experts say that you can find around a dozen other species in a mature gall, either using it for shelter, or parasitising/predating on the rest.
I think this needs to be moved to invertebrates, thanks.
Chris Brooks - www.dragonfly-images.co.uk
My Flickr site - www.flickr.com/photos/ceb1298
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