These galls develop a hard inner shell & contain 4-6 larvae of the Urophora cardui fly. I have not seen the fly but is described as "picture-winged" with a black line resembling the letter M. The larvae over winter in the gall & hope the case breaks down in the spring to let them out. Mainly in southern England but spreading northwards. Was introduced into Canada to try to control creeping thistle but without success
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