Is anybody home?

Well, there should be. This swelling in the dead stem of a thistle is a gall - abnormal cell growth stimulated by insects or mites that forms a protective home for their larval stages. In this case, the inhabitants are larvae of a pretty little picture-winged fly called Urophora cardui. I'm used to seeing the polished green swellings on creeping thistles in the summer, but I've not noticed them before in this dried-up, woody state in the winter. There will be a number of larvae inside, each in its own compartment, waiting to pupate in the spring. The adult fly will then emerge at around the time this woody home is finally disintegrating and just before there are new fresh thistle shoots, on which it will lay its eggs.

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