The fly Urophora jaceana has a larva that produces galls on the flower heads of Knapweed (most often, Centaurea nigra agg.) which incorporates the receptacle and achenes. The gall is hard and woody; usually affecting most of the flower head and with several chambers, each with one larva.  The larva has a tough dark brown posterior plate which blocks the entrance to the gall.

The gall is not obvious form the outside, but can be found by squeezing the dead flowers of Knapweed in autumn and winter; the gall can be felt as a hard lump inside the flower head. 

Urophora cuspidata is very similar.  It produces similar galls, but most often in Centaurea scabiosa.  Other larvae, not gall-causers, can be found inside Knapweed flower-heads. 

Adult Gall

Wherever Common Knapweed is found.

Galls from August to April, or puparium May and June (in old flower head).

Widespread and fairly frequent in Britain.

Probably fairly frequent in Leicestershire and Rutland but not well recorded.