In late summer little insects called picture-winged flies lay their eggs in the inflorescences of knapweed and ...
... when they hatch the larvae crawl down into the seed head, feeding on the seeds and producing a woody gall that persists through the winter. You can tell they are there just by squeezing the seed head, when you can feel the hard gall within.
Here is a seed head carefully cut open, to reveal the larvae in their woody chamber within...
.... including this one that looks well fed and ready to pupate.
I harvested some galled seed heads back in November and today the adult insects began to emerge.
The tiny flies have rather beautiful eyes but....
... their most striking feature is their wing patterns that give them their common name.
I think this species is Urophora jaceana - thanks to @SK53onOSM for correct ID
Update: Geoffrey Wilkinson (see comment below) informs me that this species is Chaetostomella cylindrica
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