The larvae of the sawfly Metallus lanceolatus mine the leaves of various Geum species. The mine begins with short, rather broad corridor that often is overrun by the later, large, very transparent full depth blotch. The mine begins somewhere in the centre of the leaf. Frass in many loose grains.

Adult Leafmine (when host plant is identified)

Leaf mine photographs should be backlit so that the frass patterns and larva can be seen.  The host plant must be identified in the comments

Where Geum species occur.

When Geum species are in leaf, the larvae usually occur in summer and early autumn.

The species appears to be widespread in England and Wales but is not well recorded.

Status in Leicestershire and Rutland not known.

http://www.leafmines.co.uk/html/Hymenoptera/M.lanceolatus.htm

https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/hymenoptera/symphyta/tenthredinoidea/tenthredinidae/heterarthrinae/metallus/metallus-lanceolatus/

https://www.sawflies.org.uk/metallus-lanceolatus/