Picture-winged fly Urophora jaceana

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

Hardheads

In summer tiny picture-winged flies, similar to this one although not the same species, lay their eggs amongst the florets of knapweed Centaurea nigra, like the one in the picture above. The usual species that parasitises knapweed is Urophora jaceana - click here for a picture.

After the larvae hatch they crawl down into the base of the inflorescence and produce a hard woody gall, where they feed on the seeds and are well protected through the winter.

Parasitised seed heads tend to have thicker stems but the easy way to detect the presence of the developing larvae in winter is just to squeeze the knapweed seed heads between finger and thumb. If they are galled you can feel the hard lump inside.

Opening up the gall reveals the developing larvae down at the base. You can see how they've chewed their way down through the gall, growing fatter as they progress and finally resting in a chamber where they'll pupate and hatch as picture-winged flies in spring.

Knapweed seed heads are carried on stiff stems that often protrude through the snow in winter. Then they tend to be shredded by hungry  birds - probably blue tits and great tits. It's hard to be sure whether the birds are after some of the smaller insect larvae that sometimes colonise these seed heads or whether they can winkle out the picture-winged fly larvae from inside their woody gall - I suspect that they can.

For a closer look at the larvae, click here.

Picture-winged fly

I found this female picture-winged fly lurking on a sycamore above a patch of creeping thistle yesterday, so by dint of association and the pattern on the wings I'm guessing that it might be Urophora cardui, which lays its eggs in creeping thistle flower heads. 

That long ovipositor on the tail looks like the right kind of equipment for inserting eggs amongst thistle florets.....

             ..... like these.