The larvae of the sawfly Heterarthrus fiora mine the leaves of Sycamore (Acer pseudoplatanus) producing a large upper-surface (often almost full depth) blotch, without a trace of an initial corridor, beginning in the very tip of a leaf segment. The full grown larva spins itself a disc shaped cocoon within the mine having previously has made a circle of perforations in the upper epidermis with its mandibles. The cocoon is formed, attached to the upper epidermis, and the larva becomes immobile. The perforated circle of epidermis starts to dry, warps, and finally becomes detached from the surrounding tissue and drops to the ground. The resulting excision has a diameter of about 7 mm, and is best seen when the leaf is held against the light (Bladmineerders van Europa).

Heterarthrus cuneifrons makes a similar mine with a discoid cocoon, but in the centre, not the edge, of a Sycamore leaf. 

Adult, Larva and leafmine (when the 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

Wherever Sycamore occurs.

Mine: univoltine in early summer.

Not well recorded, but seems to be widespread in England and Wales.

Rare or under recorded in Leicestershire and Rutland.

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

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

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