6 to 8.5 mm. The hind tibia are lined with black on the anterio-dorsal face. Facial markings and saw teeth are diagnostic. Most of the stigma is pale.

Eggs are laid on Purging or Fairy Flax (Linum catharticum) and the larva develop within blisters on the leaves.  Despite apprearances, this is not a gall.

Unless identified by a recognised expert, a photo is required and the specimen should be examined with a microscope. In the comments box, state the key or ID method used and describe the size and identifying characters.

Various habitats.

May to July.

It is widely distributed in Britain with records from England, Scotland and Wales, but does not appear to be particularly common.

Rarely recorded in Leicestershire and Rutland.

https://www.sawflies.org.uk/cytisogaster-chambersi/

https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/hymenoptera/symphyta/tenthredinoidea/tenthredinidae/tenthredininae/rhogogaster/rhogogaster-chambersi/