The larvae of the sawfly Fenusa dohrnii mine the leaves of various species of Alder (Alnus sp.). The mine takes the form of a large brownish blotch, without an initial corridor. Usually the mine starts near a vein axil, and expands towards the leaf margin. The mine mostly remains enclosed by two thick lateral veins. The adult sawfly is largely black in colour but it is the larval mines that are more regularly recorded.

The fully grown larva is pale in colour, with fairly long and slender feet- but the young larvae may have black markings on the thorax, underneath

Heterathrus vagans leaf mines on Alder can be very similar, and to be sure of ID the larva should be examined. 

Adult Leafmine

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

Wherever Alder occurs.

When Alder is in leaf.

Widespread in Britain but infrequently recorded.

Probably quite frequent but under-recorded in Leicestershire and Rutland.

http://www.leafmines.co.uk/html/Hymenoptera/F.dohrnii.htm

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

https://www.sawflies.org.uk/fenusa-dohrnii/