Wingspan 5 to 7 mm.  There are other similar species and gen det may be needed to confirm identification of adult moths. The larva mines the leaves of Alder, causing a gall within the midrib.  The mine is initially in the midrib and then along a side vein.  Then a narrow gallery is cut through to an adjacent side vein, and the larva mines down this to re-enter the midrib.  Finally it makes an oval excision in the leaf blade.  The larva inside this leaf sandwich drops to the ground to pupate.

Adult Leafmine

Photograph the backlit mine

Often fly in sunshine around Alder branches.

The adult moths fly in a single generation from late May to July.

The eggs of this species are laid on Alder (Alnus) leaves, usually in the midrib the larva then mines the leaf before pupating on the grund in an oval leaf sandwich excised from the leaf. 

The moth is widespread and locally common throughout England, Wales and parts of Southern Scotland. In the Butterfly Conservation's Microlepidoptera Report 2011 this species was classified as local.

It appears to be uncommon in Leicestershire and Rutland, where there are few records. L&R Moth Group status = D (rare or rarely recorded).

http://www.leafmines.co.uk/html/Lepidoptera/H.resplendella.htm

https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/lepidoptera/monotrysia/adeloidea/heliozelidae/heliozela/heliozela-resplendella/