Wingspan about 6 mm. The larva mines the leaves of Hazel and possibly Hornbeam.  Stigmella floslactella are generally wider, less angular and contain more scattered frass than those of S. microtheriella. The egg is underside, in a leaf-axil, and the larva mines venter (underside) down.

Stigmella microtheriella on Hazel and Hornbeam, and Stigmella carpinella on Hornbeam

Adult Leafmine+egg position

Care is needed when trying to separate from those of Stigmella microtheriella. The most reliable method of identification for this species is the egg position, S.floslactella lays an egg always on underside in a vein axil whereas S.microtheriella lays it often on or near a vein, but not in the axil. 

Areas where the larval foodplants are present.

There are two generations, with adults on the wing in May and August.

This species feeds on Hazel or Hornbeam and its larval mines are often found alongside that of Stigmella microtheriella in the same leaf. The larval mines can be found in June-July, but more often in September-October.

A widespread and fairly common species across most of the British Isles. In the Butterfly Conservation's Microlepidoptera Report 2011 this species was classified as common.

Occasional in Leicestershire and Rutland. L&R Moth Group status = C (very scarce resident or rare migrant).

http://www.leafmines.co.uk/html/Lepidoptera/S.floslactella.htm

https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/lepidoptera/monotrysia/nepticuloidea/nepticulidae/stigmella/stigmella-floslactella/