The larva of the sawfly Heterarthrus microcephalus mines the leaves of willow with oviposition very near to the leaf tip. It eventually forms a large blotch mine, with little frass.  The larva pupates in the mine in a discoid cocoon

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 the host willow species are present.

The larval mines can be seen when the host plants are in leaf.

Pupation occurs within the mine in a large, transparent, discoidal cocoon.

Widespread but not well recorded in Britain.

Status in Leicestershire and Rutland not known.

http://www.leafmines.co.uk/html/Hymenoptera/H.microcephalus.htm

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

https://www.sawflies.org.uk/heterarthrus-microcephalus/