The larva of Heterarthrus wuestneii creates a blotch like mine on the leaves of Field Maple.  The mine usually includes the tip of a lobe, and the larva has dark marks on the prothorax and dark spot on the ventral side of the thorax.  As with other Heterarthrus species, the feet are short and stubby. 

The larva pupates in a disc cut out of the leaf; in the pre-pupal stage the larva loses pigmentation.

Heterarthrus fiora is similar, but mines Sycamore leaves

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 Field Maple occurs.

Larval mines may be seen during summer and autumn when Field Maple is in leaf. 

An uncommon and little recorded species in Britain.

Status in Leicestershire and Rutland not known.

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

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

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