Wingspan 9-12 mm.
Larva mines mature fern frond with well developed sporangia, usually Phyllitis scolopendrium (Hartstongue fern), and makes case (vacant when larva in frond) resembling displaced sorus (spore body). Aug.-June, most prominent April-May.
Larva (early and late instars) has black head, prothoracic shield and anal plate, which distinguish it from the other fern frond species, P. filicivora.
Pupal case, May-June, has sharp crest when unworn.
Imago, June-July, often has a violet reflection. It lacks the white tornal spot of P. filicivora
Most frequent in Wales and W. England, including Salop,Flints.,Denbighs.,Caerns.