Apart from the above gall, adults of other Tephritid flies have been noted over the past couple of weeks: Tephritis formosa, T. neesii and T. hyoscami all in the Yardley Chase area. All of these flies have distinctively patterned wings and are frequently referred to as "picture wing flies". The term covers members of several families, not just the Tephritids.
It is possible to find the galls of these flies in the heads of various thistles in winter. If you squeeze the dead flower head you can sometimes feel a hard capsule inside, which is the gall. It is possible to raise the adult fly from the gall to confirm which species has made it.
Other signs of the larvae of flies in winter can be leaf mines. Here is blotch mine in the leaf of hedge woundwort Stachys sylvatica. This is caused by the Agromyzid fly Amauromyza labiatarum. It was found at Yardley Chase.